the chemical behavior of active nitrogen

ANGEWANDTE CHEMIE V O L U M E 9 * N U M B E R 3 M A R C H 1970 PAGES 1 8 1 - 2 5 4

The Chemical Behavior of Active Nitrogen

By R. Brown and C. A. Winkler[*]

Active nitrogen, which is formed on passage of an electric discharge through a stream of nitrogen, is detected by a characteristic yellow afterglow. It can be determined quanti- tatively, e.g. by gas phase “titration” with nitric oxide. Active nitrogen reacts with many chemical elements, as well as with organic and inorganic compounds. Although nitrogen atoms are responsibe for many of the reactions of active nitrogen, electronically excited molecules also play a role.

1. Introduction

Morren, in 1865, first reported the now well-known yellow nitrogen afterglow that results when an electri- cal discharge is passed through molecular nitrogen (11.

Subsequently, Lewis confirmed its relatively long lifetime, and identified the banded spectrum of the emission with that of molecular nitrogen L2-41. From 1911 to 1913, Strutt published several papers in which he showed that the glowing gas possesses a marked chemical reactivity, as a consequence of which he referred to it as active nitrogen c5-81. He recognized that many of its properties could be attributed to nitrogen atoms. Since that time, however, the chemical activity has been ascribed also to such species as molecular nitrogen ions, and various electronically or vibrationally excited nitrogen molecules. Progress in resolving the resulting controversies was hindered for many years by uncer- tainties in the experimental values for the dissociation energy of ground state molecular nitrogen, and proba-

[*] R. Brown and C. A. Winkler McGill University Montreal (Canada)

I11 M . A. Morren, Ann. Chem. Phys. 4, 293 (1865) [2] E. P . Lewis, Ann. Physik [4] 2,459 (1900) [3] E. P. Lewis, Astrophysic. J. 12, 8 (1900) [4] E . P . Lewis, Physic. Rev. 18, 125 (1904). IS] R. J. Strutt, Proc. Roy. SOC. (London), Ser. A 85, 219 (1911). [6] R. J . Strutt and A . Fowler, Proc. Roy. SOC. (London), Ser. A 86, 105 (1912). [7] R. J . Strutt, Proc. Roy. Soc. (London), Ser. A 86, 56 (1912). [8] R. J . Strutt, Proc. Roy. SOC. (London), Ser. A 88, 539 (1913).

ly continues to be hindered by a greater sensitivity of active nitrogen systems to experimental conditions than has generally been recognized.

Mitra made a comprehensive review of the work on active nitrogen prior to 1945r9l. The available information led him to suggest that the chemical activity of active nitrogen was due to N;, but he later recognized that this was inconsistent with experimental evidence L1OJ. Evans, Freeman, and Winkler proposed a “unified” mechanism to account for the more significant experimental observations [Ill. Data available at that time suggested that the lifetimes of electronicalZyex- cited molecular species were too short for them to be important participants in the reactions. Consequently, it was concluded that, while nitrogen atoms are the pre- dominant chemically active species in the yellow (Le- wis-Rayleigh) afterglow, vibrationally excited ground state N2(X 1ci) molecules might be responsible for certain reactions in which nitrogen atoms seemingly do not participate[121. However, recent data on the lifetimes of electronically excited nitrogen molecules suggest that their participation could be significant [12a, 12bJ.

[9] S. K . Mitra: Active Nitrogen - A New Theory. Association for the Cultivation of Science, Calcutta, India, 1945.

[lo] S. K . Mitm, Physic. Rev. 90, 516 (1953). [ l l ] H. G. V. Evans, G. R. Freeman, and C. A . Winkler, Canad. J. Chem. 34, 1271 (1956). 1121 H . G. V. Evans and C. A . Winkler, Canad. J. Chem. 34,1217 (1956). [12a] D . E. Siiemansky and N . P . Carleton, J. chem. Physics 51, 682 (1969). [12b] A. L. Broadfoot and S. P. Maran, J. chem. Physics 51, 678 (1969).

Angew. Chem. internat. Edit. / Vol. 9 (1970) No. 3 181

Subsequent reviews have examined the main spectroscopic and chemical features of active nitrogen I13-171.

The present discussion will, therefore, be directed largely to the information that has become available during the past few years. The discussion will be fur- ter limited to reactions involving uncharged nitrogen species in the region of the Lewis-Rayleigh afterglow. No attempt will be made to discuss more transient nitrogen afterglows and reactions of atomic and molecular nitrogen ions.

2. Experimental Methods

Most studies of the reactions of active nitrogen have been made in conventional, fast flow, low pressure (< 10 torr) systems of the type shown schematically in Figure 1. Comprehensive surveys of the methods ap- plicable to such systems, and their limitations, have been made 118-201. Generally, the active nitrogen is produced by either a microwave or a condensed DC discharge, although other methods, such as shock tube and vacuum ultraviolet photolysis [17al, have been tried with varying degrees of success[171. I t has been suggested that the reactions of N atoms can also be studied by y-irradiation of dilute solutions of reactant in liquid nitrogen [20a, 20bl.

rn LJ Fig. 1. Apparatus for production and investigation of active nitrogen. 1 , storage. 2, calibrated volume. 3, flow control. 4, pressure gauge. 5, microwave source. 6 and 7, condensed discharge. 8, analysis. 9, flow control. 10, copper furnace. 1 1 , vacuum.

[I31 K. R . Jennings and J . W. Linnett, Quart. Rev. (chem. SOC. London) 12, 116 (1958). [14] G. G. Mannella, Chem. Reviews 63, 1 (1963). [15] B. Brocklehurst and K. R . Jennings, Progr. Reaction Kinet- ics 4, 1 (1967). [16] 0. K. Fomin, Usp. Chim. 36, 1701 (1967); Russ. chem. Reviews (English translation) 36, 725 (1967). [17] A . N. Wright and C. A . Winkler: ActiveNitrogen. Academic Press, New York 1967. [17a] K. D . Bayer and K . H . Welge, J. chem. Physics 51, 5323 (1969). [18] F. Kaufman, Progr. Reaction Kinetics I, 1 (1961). [19] B. A. Thrush, Science 156, 470 (1967). [20] M. Venugopalan and R. A . Jones: Chemistry of Dissociated Water Vapor and Related Systems. Wiley, New York 1968.

The active nitrogen concentration is generally assumed to be given by the concentration of atomic nitrogen, and may be estimated by either physical or chemical methods. Such methods as ESR, mass spectrometry, and photometric measurements have the advantage of minimum interference with the actual system, but they generally do not give an absolute measurement of m]. The measuring equipment is usually calibrated with ref- erence to some chemical reaction, such as the gasphase “titration” with NO (see Section 4.2.1.) [15-17J. The method depends upon the very high rate of reaction of NO with N and certain well-defined color changes that occur essentially at the equivalence point. It has also been proposed that the N atom concentration may be given by the maximum yield of HCN from the reaction of active nitrogen with C2H4 and some other hydrocarbons [21-281. However, the value of [N] measured by the NO “titration” is usually considerably greater than that by the maximum HCN production from the hydrocarbons, i.e. a ratio NO/HCN >1 [29-311. This discrepancy is probably due to complications in the hydrocarbon reactions [15,32,331.

A recent report that maximal HCN production might, under certain conditions, be greater than the amount of NO destroyed (NO/HCN <1) has now been shown to be due to back diffusionrl7,341. It has been found that, with a system presumed to be “unpoisoned”, back diffusion of reactants or products into the discharge zone, in amounts as little as one part in 5 million of N2, may treble the production of N atoms. The problems associated with the estimation of the active nitrogen concentration have been discussed in detail elsewhere[*5.17].

3. The Nitrogen Afterglow

The Lewis-Rayleigh afterglow consists predominantly of certain bands of the first positive [*I system (B 3 n g

+ A3C:), together with some of the “Y” bands (B‘ 3 X ; -+ B 3 n g ) . Despite many investigations, the

[20a] T. Oka and S. Sato, Bull. chem. SOC. Japan 42, 582(1969). [20b] T. Oka, Y. Suda, and S. Sato, Bull. chem. SOC. Japan 42, 3083 (1969). [21] R . Kelly and C. A . Winkler, Canad. J. Chem. 37, 62 (1959). [22] W. E. Jones and C. A . Winkler, Canad. J. Chem. 42, 1948 (1964). [23] J . Versteegand C. A . Winkler, Canad. J. Chem. 31, l (1953). [24] W . Forst, H . G . V. Evans, and C. A . Winkler, J. physic. Chem. 61, 320 (1957). [25] D. M. Wiles and C. A . Winkler, Canad. J. Chem. 35, 1298 (1957). [26] R . A . Back, W. Dutton, and C. A . Winkler, Canad. J. Chem. 37, 2059 (1959). [27] H. B. Dunford, H. G . V . Evans, and C. A . Winkler, Canad. J. Chem. 34, 1074 (1956). [28] W . Forst and C. A . Winkler, J . physic. Chem. 60,1424 (1956). [29] G. J . Verbeke and C. A . Winkler, J. physic. Chem. 64, 319 (1 960). [30] A . N. Wright, R . L. Nelson, and C. A. Winkler, Canad. J. Chem. 40, 1082 (1962). [31] J. T. Herron, J. chem. Physics 33,1273 (1960). [32] D. R . Safrany and W. Jaster, J. physic. Chem. 72,518 (1968). [33] J . T. Herron, J. physic. Chem. 69, 2736 (1965). [34] B. T. Yo and C. A . Winkler, unpublished. [*I The sign + or - as superscrlpt describes the symmetry behavior on reflection in a plane through the symmetry axis of the molecule.

182 Angew. Chem. internat. Edir. / Vol. 9 (1970) NO. 3

mechanism by which the afterglow is produced is not fully understood (c.f. earlier theories [14,171). This is due, in part, to conflicting experimental evidence.

There seems to be general agreement that the intensity of the first positive bands is second order in [N], but the effect of total pressure is not as well understood 135-381. “Pressure jump” experiments have shown that, at about 0.04 torr, the intensity is pressure dependent [39,39a, 39bl. Recent evidence seems to indicate, however, that the intensity is independent of pressure at higher pressures, at least in the range 1 to 10 torr [35,36,383. As the pressure is increased, there appears also to be a change from first to zero order dependence of the intensity on [MI, the third body concentration 1391. Gross used a glow discharge shock tube to study the dependence of the intensity ( I ) on pressure (0.25-1.5 torr) and on temperature in the range 300” to 2400 “K 1361. He found

I = 0.60 x 10-17 “12 photons cm-3 sec-1

Berkowitz, Chupka, and Kistiakowsky 1403 proposed that the afterglow is produced by

The N2(B 3&) state is assumed to be populated by predissociation from the ( 5 x 3 state in a collision- induced radiationless transition.

Benson [411 has suggested that the production of the first positive emission depends upon the primary reaction

N(4S) t N(4S) Z N;(5C;) Z N;(B 311g, v = 12) (4)

Assuming that the curve-crossing transition has a probability between 10-1 and 10-2, this reaction rep- resents an equilibrium between free N(4S) atoms and rotationally “hot” molecules in the twelfth vibrational level of the B state. No third body is required. The vibrationally “hot” B state molecules can decay to the A3Ct state by radiation, or undergo some form of relaxation - rotational, vibrational, or electronic. Ben- son showed that, for the intensity to be independent of pressure, it is kinetically required that the relaxation

[35] R. J . McNeal, Bull. Amer. physic. SOC. 12, 542 (1967). [36] R . W . F. Gross, J. chem. Physics 48, 1302 (1968). [37] K . D . Bayes and G. B. Kistiakowsky, J. chem. Physics 32, 992 (1960). [38] I . M . Campbelland B. A . Thrush, Proc. Roy. SOC. (London), Ser. A 296, 201 (1967). 1391 W. Brennen and E. C . Sliane, Chem. Physics Letters 2, 143 (1968). [39a] E . C. Sham and W. Brennan, Chem. Physics Letters 4 , 31 (1969). [39b] N . Jonathon and R . Perry, J. chem. Physics 50, 3804 (1969). 1401 J. Berkowitz, W. A . Chupka, and G. B. Kistiakowsky, J. chem. Physics 25, 457 (1956). [41] S . W. Benson, J. chem. Physics 48. 1765 (1968).

processes be much faster than the radiation process. It seems reasonable that rotational relaxation should be rapid, especially at pressures >, 1 torr [421. Vibra- tional relaxation would occur by a stepwise process, and might be relatively inefficient 143-451. On the other hand, recent studies seem to indicate rapid electronic quenching of the B state molecules by ground state N2 molecules ( k = 6.2 x 10-11 cm3 molec-1 s-1) [39,46-481.

Young and co-workers have determined the quenching efficiencies of various gases for N2(A 3xi) and N2(B 3ng) [48a1. The Stern-Volmer type quenching of the B state by added gases can also be explained by this

A somewhat similar mechanism for the production of the nitrogen after-glow, based on a collision induced population of the B 3ng state from the A 32;: state, was proposed by Campbell and Thrush 1381. Benson pointed out, however, that the proposed population of B 3ng levels by collision from the A 3 X i level is inconsistent with the kinetic aspects of afterglow production 1411. This conclusion has been disputed by Jonathon and PettyL39bl in their study of the pressure dependence of the nitrogen afterglow. It has also been suggested that oxygen interacts with vibrationally excited N2(A 3C:) to produce N2(B 3&) [52al.

The absence of the Vegard-Kaplan bands (i.e. A 3c: + X 1 Xi) in the nitrogen afterglow has been of considerable interest, since the A 3xi state is continually populated by the first positive system. The Vegard- Kaplan bands have recently been detected in systems containing a large concentration of A 3 X i molecules in the absence of significant quantities of nitrogen atoms 153,541. They have also been detected in the absorption spectrum of N2154aI. The emission was

mechanism 135,38,49-521.

[42] D . R. Miller and R . P. Andres, J. chem. Physics 46, 3418 (1967). [43] R. L. Brown and S. Dittman, Chem. Commun. 1967, 1144.

[44] S. J. Colgan and B. P . Levitt, Trans. Faraday SOC. 63, 2898 (1 967). 1451 R . E. W. Jansson, L. A . Middleton, and J. Lewis, Nature (London) 214, 589 (1967); J . W. D. Breshears and J. F. Bird, J. chem. Physics 48, 4768 (1968). [461 R. A . Young and G. Black, J. chem. Physics 44, 3741 (1966). [47] H . J . Hartfuss and A . Schmillan, Z. Naturforsch. 23a, 722 (1968). I481 M . Jeunehomme and A . B. F. Duncan, J. chem. Physics 41, 1692 (1964). [48a] R . A . Young, G. Black, and T. G. Slanger, J. chem. Physics 50, 303 (1969). [49] W. R . Brennan and G . B. Kistiakowsky, J. chem. Physics 44, 2695 (1966). [50] S. Miyazakiand S. Takahashi, Mem. Defense Acad., Mathe- matics, Physics, Chemistry, and Engineering (Yokosuka, Japan) 6, 469 (1967). [51] S. Miyazaki and S. Takahashi, Mem. Defense Acad., Mathe- matics, Physics, Chemistry, and Engineering (Yokosuka, Japan) 7, 1155 (1967). [52] B. Brocklehurst and R . M . Duckworth, J. Physics, B (Proc. physic. SOC.) [2] I , 990 (1968). [52a] M . P . Weinreb and G. G. Mannella, J. chem. Physics 51, 4973 (1969). 1531 R. A . Young and G. A . St. John, J. chem. Physics 48, 895 (1 968). [54] D . H . Stedman and D. W. Setser, Chem. Physics Letters 2, 542 (1968). [54a] D . E. Shemansky, J. chem. Physics 51, 689 (1969).

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destroyed by addition of nitrogen atorns1531. This is consistent with a rapid reaction

N2(A 3C:) 4- N(4S) + Nz(X 1x9’) + N(4S) ( 5 )

The rate constant at room temperature was estimated to be k5 = 5 x 10-11 cm3 particle-1 s-1. Indirect proof for the occurrence of this reaction has been obtained from beam studies 155J and from mercury emission in an active nitrogen stream 1561.

It has been pointed out that the reverse reaction can also occur[571, with a rate constant estimated from shock tube studies to be k-5 = 1.9 x 10-6 T-312 exp (-142 kcal/RT) cm3 particle-1 s-1 [57al. By combining these results with data from an earlier study by Nuxon[581, a value has been calculated for k5 that is in excellent agreement with the experimentally determined value 1591.

4. Chemical Reactions of Active Nitrogen

Since studies of the reactions of active nitrogen prior to 1967 have already been extensively reviewed [171, the present discussion will be concerned primarily with progress since that time.

4.1. Recombination of Nitrogen Atoms

Of the reactions that may involve atoms produced by the gas phase dissociation of a stable diatomic molecule, the simplest is their recombination, either heterogeneously or by a three-body homogeneous process.

4.1 .I. H e t e r o g e n e o u s R e c o m b i n a t i o n

At low pressures (< 1 torr) in glass vessels, the heterogeneous recombination of nitrogen atoms becomes a significant factor in active nitrogen reactions 115,601.

Its extent may be largely reduced by a strong “poison- ing” by trace impurities in the nitrogen stream, or by coating the walls with a suitable “poison”, such as phosphoric acid. Early studies indicated that the heterogeneous recombination process was probably first order in [N] [61-631.

The values obtained for the first order rate constant

[55] C. H. Dugan, J. chem. Physics 47, 1512 (1967). 1561 D . H . Stedman, J . A . Meyer and D . W. Setser, J. chem. Physics 48, 4320 (1968). [57] L . F. PhiNips, Canad. J. Chem. 46, 1429 (1968). [57a] K . L . Wray, J. chem. Physics 44, 623 (1966). [581 J . F. Noxon, J. chem. Physics 36, 926 (1962). 1591 B. A . Thrush, J. chem. Physics 47, 3691 (1967). [60] R. E . Lund and H . J . Oskam, J. chem.Physics 48,109 (1968). [61] Lord Rayleigh, Proc. Roy. SOC. (London), Ser. A 151, 567 (1935). 1621 N. Buben and A . Schecter, Acta physicochim. URSS 10, 371 (1939). [63] A . Schecter, Acta physicochim. URSS 10, 379 (1939).

varied from 4.0 s-1 to 4.2 x 10-2 s-1 for an unheated, clean glass surface [30,641. The corresponding value of y, the recombination coefficient, varied between 10-4 and 10-5 for glass surfaces.

More recently, Campbell and Thrush have applied photometric methods to a flow system and found that the heterogeneous recombination is a second order process, with k = 7.3 x 10-16 cm3 molec-1 s-1[3sI. Their observations have been confirmed b y Clyne and Stedman, who found that two heterogeneous recombination processes may occur[651. One of these is first order and predominates for low [N]. The other is second order in [N], and predominates in an unheated, clean pyrex vessel. Under such conditions, however, the first order process was appreciable at 90” and 611 OK, and the second order wall reaction could be virtually eliminated in an unheated vessel by coating the wall with phosphoric acid.

It is found, in general, that metal surfaces are more efficient than glass in promoting heterogeneous recombination [171. The value o fy for metals can be fitted to the expression y = a/(bP + c) , where P is the pressure and b and c depend on the surface [661. Thus, for clean Mo sputtered on a glass surface, y decreases from 0.22 at 0.09 torr to 0.04 a t 1.35 torr[601. The high efficiency of metals or their oxides, especially “spongy” cobalt and copper oxide, in causing heterogeneous N atom recombination has made them useful in the prep- aration of surfaces to terminate gas phase reactions [671.

4.1.2. H o m o g e n e o u s R e c o m b i n a t i o n

It is generally accepted that the homogeneous recombination of nitrogen atoms is a termolecular process

Homogeneous recombination has special importance as a major source of electronically excited nitrogen molecules, the precursors of the nitrogen afterglow. At pressures greater than about 3 torr, the rate of homogeneous recombination largely determines the rate of nitrogen atom disappearance.

Clyne and Stedman have recently followed the disappearance of N atoms by gas phase “titration” with NO, making an appropriate correction for the heterogeneous recombination 1651. They estimated a termolecular rate constant of k6 = 0.96 x lO-32cm6 molec-2 s-1. By combining their results with earlier data, they showed that the temperature variation of k6 was given by the simple Arrhenius expression, with A = 10-32.81 cm6 molec-2 s-1 and E = 1000 cal mole-1 [38,68,693.

1641 R. A. Young, J. chem. Physics 34, 1292 (1961). [65] M . A . A . Clyne and D . H . Stedman, I. physic. Chem. 71, 3071 (1967). 1661 R . A . Young, J. chem. Physics 34, 1295 (1961). [67] E. M . Levy and C. A. Winkler, Canad. J. Chem. 40, 686 (1962). [68] S. Byron, J. chem. Physics 44, 1378 (1966); Erratum (with D . L . Matthews), ibid. 45, 3165 (1966). 1691 B. Cary, Physics of Fluids 8, 26 (1965).

184 Angew. Chem. internat. Edit. 1 Vol. 9 (1970) No. 3

It is interesting that the mechanism proposed by Ben- son for the first positive emission is not consistent with the third order kinetics observed for the homogeneous recombination of N atoms. This seems to suggest that reactions leading to first positive emission are of minor importance relative to the total extent of homogeneous recombination. This observation is supported by calcu- lation of the relative rates of photon emission[361 and termolecular recombination [651 which show only one photon emitted as a consequence of 51 recombina- tions at 1 torr and 300 OK. The variation in ra te of recombination with change in M, the third body, has not yet been reliably determined. This is partly due t o the fact that in such studies the intensity of afterglow emission has been used as a measure of N a tom concentration. This is probably not a reliable method because a change in M can cause a change in intensity due t o a shift in relative importance of the relaxation processes and the radiation process in afterglow production. I t is well known that NH, and CH4, for example, strongly quench first positive emission, presumably by rapid electronic quenching of one of the precursors of the afterglowr35,38,51, 701. On the other hand, Ar and H e increase t h e emission from the vibrational levels (v i 1 2 ) in the B state because of fast vibrational quenching[35,431. With Ar as a third body, the recombination rate appears t o be unchanged [38], although a recent shock tube study of the reverse reaction [711

N z + M + 2 N i - M (7)

showed that t h e relativeefficiencies of N, Nz, and Ar a s third bodies a r e in the ratio 1 : 4.4 : 12. T h e rate of N a tom production was obtained from vacuum UV absorption.

4.2. Inorganic Reactants

4.2.1. N i t r i c Ox ide

Of all the reactions of active nitrogen with either inorganic or organic reactants, that with NO is perhaps best understood. The primary reaction of active nitrogen with NO almost certainly involves N atoms:

This reaction probably occurs on an attractive potential energy surface since the heat of reaction seems to appear mainly as vibrational energy in the nitrogen produced 172-751. It has been suggested that the infrared chemiluminescence that accompanies this reaction comes from NO excited by vibrationally excited Nz [75aJ. Reaction (8) has little or no activation energy,

[70] S . Takahashi, Mem. Defense Acad., Mathematics, Physics, Chemistry, and Engineering (Yokosuka, Japan) 7, 475 (1967). [71] J . P. Appleton, M . Steinberg, and D. J . Liquornik, J. chem. Physics 48, 599 (1968). 1721 J. E. Morgan, L. F. Phillips, and H. I. SchiK Discuss. Fara- day SOC. 33, 118 (1962). [73] J . E. Morgan and H . I . Schiff; Canad. J . Chem. 41, 903 (1 963). [741 L. F. PhiNips and H . I . Schif, J. chem. Physics 36, 3283 (1 962). [751 N. Basco and R . G. W. Norrish, Canad. J. Chem. 38, 1769 (1 960). [75a] R . Joeckle and M . Peyron, C. R. hebd. Seances Acad. Sci. 268, 2133 (1969).

and proceeds at close to collision rates (ks = 2 x 10-11 cm3 molec-1 s-1)176-791. Because of its simplicity, this reaction has been used as a source of 0 atoms, especially when the presence of excess 0 2 may cause un- desirable complications 1801.

When [NO] < [N], there is complete reaction of the available NO. Unreacted N atoms then react with 0 atoms produced in reaction (8 ) , with emission of the i j. and y bands of NO:

N -t 0 + M -> NO* + M (9)

NO* --f NO -t hv (1 0)

The emission may range in color from purple to blue as the concentration of N atoms decreases relative to that of 0 atoms. The rate constant of reaction (9) at room temperature has been estimated to be kg = 1.1 x 10-32 cm6 molec-2 s-1[81,821. It has been reported that a sensitive measure of the N atom concentration can be obtained from the intensity of the p bands [38,811. Direct estimates of [N] have recently been made possible from a determina- tion of the variation of the absolute intensity of the 9, y, and 8 bands with temperature and pressureLs31.

When [NO] is greater than [N], the N atoms are rapidIy and completely consumed. The resulting 0 atoms then react with the excess NO:

The greenish-yellow NO2 bands of the “air afterglow” are then emitted by a mechanism which is not yet fully understood 1841. At the equivalence point, i.e. [NO] = [N], reactions (9) and (11) are not possible, and there is no emission. Accordingly, a gas phase “titration” to this dark end- point makes is possible to estimate [N] by visual or photometric methods. The results agree well with those obtained from maximum destruction of NO 1291.

Fontijn and Vree have observed that chemi-ionization and formation of electronically excited species (such as NO(A *C, B 2n) and O(1S)) is catalyzed by addition of certain carbon-containing compounds to systems containing N and 0 atoms [851. As a result, the intensity of certain bands in the NO system is increased,

[76] Y. Takezakiand S. Mori, Bull. Inst. chem. Res., Kyoto Univ. (Kagaku Kenkyusho Hokoku) 45, 388 (1967). [77] J . T . Herron, J. Res. nat. Bur. Standards, Sect. A 65, 411 (1961). [78] L. F. PhiNipsand H.I.Schifl, J.chem. Physics36,1509(1962). 1791 J. T. Herron, J. chem. Physics 35, 1138 (1961). [SO] J . T. Herron and R . D. Penzhorn, J . physic. Chem. 73, 191 (1 969). [Sl] I . M . Campbelland B. A . Thrush, Proc. Roy. SOC. (London), Ser. A 296, 222 (1967). [82] S. Takahashi, Mem. Defense Acad., Mathematics, Physics, Chemistry, and Engineering (Yokosuka, Japan) 8, 61 1 (1968). [83] R . W. F. Gross and N . Cohen, J. chem. Physics 48, 2582 (1 968). [84] A . MacKenzie and B. A . Thrush, Chem. Physics Letters I , 681 (1968). [85] A. FontQn and P. H. Vree, 11th Int. Sympos. Combustion, Pittsburgh, 1967.

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and the authors warn that “caution should be exer- cised in the use of the NO afterglow intensity for atom concentration measurements in the presence of a third reactant” 1861.

4.2.2. N i t r o g e n Diox ide

To explain the experimental features of the fast reaction of active nitrogen with NO*, it has been postulated that four primary reactions occur simultaneously [87J:

N + NO2 + NzO + 0 (43’1) (1 2)

N + NO2 + 2 N 0 (33’;) (1 3)

N +NO2 + N 2 + 2 0 (13’4) (1 4)

N + NO2 + N2 + 0 2 (10%) (1 5)

The rate constant for removal of NO2 by N atoms at room temperature was estimated to be k = 1.85 x 10-11 cm3 molec-1 s-1. A study of the emission from the N20 formed by reaction (12) showed that the bending mode v2 is not excited but the stretching modes v1 and v3 are vibrationally excited [@I. This seems to suggest that the reaction occurs on a repulsive potential energy surface.

4.2.3. O t h e r Oxides of N i t rogen

Unlike the reactions of active nitrogen with NO and NO2, the corresponding reactions with N20 and with N2O5 are extremely slow. Mass spectrometric studies indicate that the rate constant for the reaction of NzOs is < 10-15 cm3 particle-1 s-1 [891.

The reaction of active nitrogen with N2O does not appear to involve N atoms. The slight reaction that is observed has been attributed to reaction with electronically excited N2 molecules, either in the B 3& or the A 3Zu state [gol 911. N20 seems to be particularly effective as a third body for relaxation of vibrationally excited N2 molecules [72,73,92,92a]. It has been pointed out by Rubin that the cross-section for vibrational de- excitation of Nz by NzO, measured by Morgan and Schif, was actually a cross-section for two processes [73,931

N>(., = 1) + M + N2 + M + kineticenergy (1 6)

Nz(v = 1) + M + Nz + M” (1 7)

[86] A . Font& and R . Ellison, J. physic. Chem. 72, 3701 (1968). 1871 L. F. Phillips and H . I . Schif, J. chem. Physics 42, 3171 (1965); G. Liuti. S . Dondes, and P. Harteck, Atti. Accad. naz. Lincei, Rend., Cl. Sci. fisiche, mat. natur. 45, 364 (1968). [88] P . N. Clough and B. A. Thrush, Discuss. Faraday SOC. 44, 205 (1967); P . N . Clough and B. A . Thrush, Proc. Roy. SOC. (London), Ser. A 309, 419 (1969). [891 G . Liufi, S. Dondes, and P . Harteck, J. physic. Chem. 72, 1081 (1968). [90] I . M . Campbell and B. A . Thrush, Trans. Faraday SOC. 62, 3366 (1966). [911 I . M. Campbell and B. A . Thrush, Trans. Faraday SOC. 64, 1275 (1968). [92] F. Legay, J. Chim. physique 64, 9 (1967). [92a] J . F . Roach and W. R . Smith, J. chem. Physics 50, 4114 (1 969). [93] P. L . Rubin, Canad. J. Chem. 45, 2858 (1967).

followed by

M* + N2 + M + N2 -I- kineticenergy (18)

A rapid reaction of N atoms with NOCl has been reported recently [93a].

4.2.4. Hydrogen

Campbell and Thrush have studied the reaction of H2 in a system containing both N and 0 atoms, produced by a partial “titration” of active nitrogen with NO c941.

Since no primary reaction is observed when H2 is added to active nitrogen, the initial reaction in this system can be attributed to the reaction of 0 atoms with H2 1951:

O + H 2 + O H + H (19)

From photometric measurements of the rate of consumption of 0 and N atoms, they were able to show that the rate of

N t O H + N O + H (20)

is 1.4 times the rate of the corresponding reaction of 0 atoms.

4.2.5. Oxygen a n d Ozone

N atoms react with 0 2 by[97,981

NO2 and N2O were also obtained as products1961. The rate of reaction (21) at room temperature has been estimated to be k21 el x 10-16cm3 molec-1 s-1 and has a n activation energy of about 7 kcal mole-1 [97-101,10laJ. It is interesting to note that N atoms react much more rapidly with O(‘&) than with ground state 0 2 f lolbl .

The overall reaction of N atoms with O3 has been postulated to be [781

[93a] J . C . Biordi, J. physic. Chem. 73, 3163 (1969). [941 I . M . CampbelI and 8. A . Thrush, Trans. Faraday SOC. 64, 1265 (1968). [95] C. Mavroyannis and C. A. Winkler, Canad. J. Chem. 40, 240 (1 962). 1961 C. Mavroyannis and C. A. Winkler, Proc. int. Sympos. chem. Reactions Lower Upper Atmosphere, 1961, p. 287, Wiley, New York 1961. 1971 T. Vlastaras and C. A. Winkler, Canad. J . Chem. 45, 2837 (1967). [98] W. E. Wilson, J. chem. Physics 46, 2017 (1967). [99] M . A . A . Clyne and E . A. Thrush, Proc. Roy. SOC. (London), Ser. A 261, 259 (1961). [loo] M. A . A. Clyne and B. A . Thrush, Nature (London) 189, 56 (1961). [loll S . Miyazaki and S. Takahashi, Mem. Defense Acad., Math- ematics, Physics, Chemistry, and Engineering (Yokosuka, Japan) 8, 469 (1968). [lola] K . H . Becker, W . Groth, and D. Kley, Z. Naturforsch. 24a, 280 (1969). [IOlbl I . D. Clark and R . P . Wayne, Chem. Physics Letters 3 , 405 (1969).

186 Angew. Chem. internat. Edit. / Vol. 9 (1970) / No. 3

The rate of this reaction at room temperature has been estimated as kz2 = 5.7 x 10-13 cm3 molec-1 s-11781. Al- though upstream addition of N20 does not affect the 0 3 reaction, about 75 % of the N; formed by reacting N atoms with NO are capable of destroying 0 3 [74,781.

4.2.6. P h o s p h o r u s

The reaction of active nitrogen with white phosphorus is fast ( k 2 2 x 10-13 01113 molec-1 s-1) and produces nitrogen-phosphorus polymers [1021. The P/N ratio of the polymeric product was found to vary with the relative concentrations of reactants. The N atom flow rate inferred from the amount of nitrogen incorporated into the polymer agreed well with that estimated by the NO “titration”.

4.2.7. A m m o n i a a n d Hydraz ine

The high activation energy (2 17 kcal mole-1) thermodynamically required for hydrogen abstraction from NH3 by N atoms makes its occurrence highly improb- able. This is supported by mass spectrometric studies, which indicate that N atoms are not consumed in the presence of NH3 [1031. It has been suggested, therefore, that the observed decomposition of NH3 by active nitrogen ‘15,17J is due to energy transfer from electronically excited nitrogen molecules.

Because of its relatively long lifetime, N2(A 3x3 has frequently been postulated as the species responsible for the destruction of ammonia 1104-1061. However, since the nitrogen afterglow may be quenched by NH3, it is possible that a precursor of the afterglow, N2(5Ci) or N2(B 3&), might be involved [15J, and that destruction of NH3 might be represented as

N a + N H 3 + N H + 2 H t N z ( 2 3 )

followed perhaps by

Apparently, the reaction

does not occur since *4N15N is produced exclusively during the reaction of 14N active nitrogen with 15NH3 [105J. Reaction (26) has been postulated to account for the rapid disappearance of N H radicals dur-

[I021 S. Khanna, Sharon Furnival, and C . A. Winkler, J. physic. Chem. 73, 2062 (1969). [I031 G. B. Kistiakowsky and G. G. Volpi. J. chem. Physics 28, 665 (1958). [lo41 H. B. Dunford, J. physic. Chem. 67, 258 (1963). [I051 R. A . Back and D. R . Salahub, Canad. J. Chem. 45, 851 (1967). [lo61 I . M . Campbell and B. A . Thrush, Chem. Commun. 1967, 932.

ing gas phase radiolysis of NH3, but the possible con- tribution of reaction (24) was not considered [107J.

The fact that the N atom concentration remains virtually unchanged when NH3 is added to active nitrogen might imply that reactions (24) and (25) occur to approximately the same extent. Since [HI < [N], it would follow that k24 > k25.

When H atoms are added to excess active nitrogen, NH3 is produced in amounts directly proportional t o [HI 11081. Moreover, NH(A 311 + X 3x -) emission occurs during this reaction, which shows that NH radicals are present as an intermediate [109,1101. Hence, the reaction

M N + H -+ N H (27)

must compete favorably with reactions (24) and (25). Further evidence for the importance of such a reaction is provided by the production of NH3 during the reaction of active nitrogen with C2H4, where it is appreciable only when [CZH~] < [NJ[321.

Kinetic studies of the active nitrogen-NH3 reaction indicate that it is not truly first order in [NH3] and tends to zero order as [NH3] is increased1104,1111. This might be a “pseudo” zero order behavior with NH3 in large excess if N atoms are continually regenerated as in reaction (24). It is also possible, however, that the reaction is partially heterogeneous, comparable with that observed at -196 “C [112J.

The rate constant for the reaction of N atoms with N2H4, to produce NH3 and H2[112aJ, has been estimated as 1.2 x 10-13 cm3 molec-1 s-1 at 298 OK [34,112bl.

4.2.8. P h o s p h i n e

The reaction of active nitrogen with the phosphorus analog of NH3, i.e. PH3, is probably a reaction of N atoms 11131. The strength of the H2P-H bond is such that H atom abstraction is thermodynamically possible, i.e.,

N+I’H3 --t N H + P H z (281

The main products of the reaction are H2 and the a-form of (PN)n. No red phosphorus is formed.

11071 G. M . Meaburn and S. Gordon, J. physic. Chem. 72, 1592 (1 968). [lo81 J . K . Dixon and W . Seiner, Z . physik Chem., Abt. B 17, 327 (1932). [lo91 H. Guenebaut, G. Pannetier, and P. Goudmand, C. R. hebd. Seances Acad. Sci. 251, 1480 (1960).

[110] G. G. Mannella, J. chem. Physics 37, 678 (1962). [111] A . N . Wright and C. A. U’inkler, Canad. J. Chem. 40, 5 (1962). [112] E. R. Zabolotny and H. Gesser, J. physic. Chem. 66, 408 (1962). [112a] G. R. Freeman and C . A. Winkler, Canad. J. Chem. 33, 692 (1955). [112bl M . Gehring, K . Hoyermann, H. Gg. Wagner, and J . Wolfrurn, Ber. Bunsenges. physik. Chem. 73, 921 (1969). I1131 D . M . Wiles and C. A. Winkler, J. physic. Chem. 61, 902 (1957).

Angew. Chem. internut. Edit. 1 V G l . 9 (1970) 1 No. 3 187

4.2.9. S u l f u r a n d C o m p o u n d s of Su l fu r

Although the reactions of active nitrogen with sulfur and with the various compounds of sulfur have been rather extensively studied, the formation of solid, polymeric products has tended to make interpretation of the results difficult. A great deal of spectroscopic work has been done on the reactions of active nitrogen with sulfur [5,6,114-1241, CS2 [6,114,125-1281, H2S [6,123,127, 129-1321, SClz [133-135,135al, and S2CI2 [S, l32,135a,1361

(and also on SeC14[134,135.137,1381).

Spectroscopic studies clearly show that the NS radical is formed as an intermediate during the reaction of active nitrogen with sulfur. Various nitrides of sulfur, including (NS), polymer, have also been identified as reaction products [136,1391.

Since s8 is one of the products of the reaction of active nitrogen with H 2 S it is not surprising that (NS), polymer is also found as a product of this reaction [127,1361. Other products identified were N H 3 , S7NH 11361, and H2.

11141 A. Fowler and M. W. Vaidya, Proc. Roy. SOC. (London), Ser. A 132, 310 (1931). [115] B. D . Chhabra and H . R . Luthra, J. Indian chern. SOC. 9,21 (1 932). [116] A . Fowler and C. J . Bakker, Proc. Roy. SOC. (London), Ser. A 136, 28 (1932). [117] P. B. Zeeman, Canad. J. Physics 29, 174 (1951). 11181 R . F. Barrow, A . R . Downie, and R. K . Laird, Proc. physic. SOC. (London) 65, 70 (1952). [119] R. F. Barrow, G . Drummond, and P. B. Zeeman, Proc. physic. SOC. (London) 67, 365 (1954). [120] K . Dressler, Helv. physica Acta 28, 563 (1955). [lZl] N. A . Narasimham and K . Srikameswaran, Nature (London) 197, 370 (1963). 11221 M. M . Patel, Z. Physik 173, 347 (1963). 11231 J . A . S . Bett and C. A. Winkler, J. physic. Chem. 68, 2735 (1964). [124] K. C. Joshi, Z. Physik 191, 126 (1966). [125] C. K . N . Patel, Appl. Physics Letters 7, 273 (1965). [126] G. Pannetier, P . Goudmand, 0. Dessaux, and I . Rebejkow, Bull. SOC. chim. France 1963, 2811. [127] R . A . Westbury and C. A . Winkler, Canad. J. Chern. 38, 334 (1960). [128] P . Harteck and R . R . Reeves j r . , Bull. SOC. chim. Belgique 71, 682 (1962). [129] G . Pannetier, P . Goudmand, 0. Dessaux, and N . Tavernier, C. R. hebd. Seances Acad. Sci. 255,91 (1962). [130] G. Pannetier, P . Goudmand, 0. Dessaux, and N. Tavernier, J. Chim. physique 61, 395 (1964). [131] P. Goudmand, G . Pametier, 0. Dessaux, and L . Marsigny, C. R. hebd. Seances Acad. Sci. 256,422 (1963). [132] J . J . Smith and B. Meyer, J. molecular Spectroscopy 14, 160 (1964). [133] G. Pannetier, 0. Dessaux, I . Arditi, and P. Goudmand, C. R. hebd. Seances Acad. Sci. 259,2198 (1964). f1341 G . Pannetier, 0. Dessaux, I . Arditi, and P . Goudmand, Bulk. SOC. chim. France 1966, 313. [135] P . Goudmand and 0. Dessaux, J. Chim. physique 64, 135 (1 967). [135a] B. Vidal, 0. Dessaux, J . P . Marteel, and P. Goudmand, C. R. hebd. Seances Acad. Sci. 268, 2140 (1969). 11361 J. J . Smith and W. L. Jolly, Inorg. Chern. 4 , 1006 (1965). [137] 0. Dessaux and P. Goudmand, C. R. hebd. Seances Acad. Sci. 267, 1198 (1968). (1381 G. Pannetier, P . Goudmand, 0. Dessaux, and I . Arditi, C.R. hebd. Seances Acad. Sci. 260, 2155 (1965). [1391 J . A . S . Bett and C. A . Winkler, J. physic. Chem. 68, 2501 (1964).

Polymeric (NS), is the only nitrogen-containing product that has been identified among the products of the reaction of active nitrogen with CS2 and with COS 11363.

A black polymer, resembling paracyanogen, was also formed during the CS2 reactionr1271, in accord with the spectroscopic observation of certain emissions of the CN systemC1261.

It has been found that N atoms react rapidly with SO, (k = 0.8 x 10-12 cm3 molec-1 s-1 at room temperature), to form NO and S[140,140al. A similar, but slower 0 atom abstraction, (k = 5.4 x 10-16 cm3 molec-1 s-1 at room temperature), occurs when N atoms react with SO3 to form SO2 and NO [*401. The S O 2 molecule appears to be too stable to be directly attacked by N atoms. However, a limited decomposition of S 0 2 , somewhat analogous to the NH3 reaction, has been observed under appropriate conditions 1140,1411. It has been suggested that this is due to energy transfer from an electronically excited nitrogen molecule, since ESR measurements have indicated no decrease in [N] when SO2 is added to active nitrogen r142,1431.

The reaction of active nitrogen with SOCl2 was found to yield NOCl, N 2 0 , SOzC12, C12, and small amounts of sulfur 11361. No sulfur nitrides could be detected. The main products of the reaction with the chlorides of sulfur, SC12 and S2C12, were NSCl and Cl2 [1361.

4.2.10. Ox ides of C a r b o n

The interaction of active nitrogen with CO and C 0 2 has received special attention recently because of the observed laser action in these systems. (The exchange of energy from vibrationally excited N 2 ( X 1x3 molecules to CO, C 0 2 , N 2 O and similar molecules has been considered in detail elsewhere [92,93,144-1501).

Avramenko and Krasnen’kov have reported a rate constant, k~~ = 3.2 x 10-13 exp (-3.4 kcal/RT) cm3 molec-1 s-1, for the reaction [I511

N + C 0 2 + N O + C O (29)

11401 A . Jacob and C. A . Winkler, unpublished results; Ph. D. Thesis by A . Jacob, McGill University, Montreal 1968. [140a] G. Liuti, Atti Accad. naz. Lincei Rend., C1. Sci. fisiche, mat. natur. 45, 358 (1968). [141] A. Jacob, R. A . Westbury, and C. A . Winkler, J. physic. Chem. 70, 4066 (1966). [142] K . D . Bayes, D. Kivelsun, and S. C. Wong, J. chern. Physics 37, 1217 (1962). [143] D . D . Stedman, J . A . Meyer, and D . W. Setser, J. Amer. chem. SOC. 90, 6856 (1968). [144] M. H. Bruce, A . T . Stair jr . , and J . P. Kennealy, J. Chim. physique 64, 36 (1967). 11451 J . P . Kennealy, A . T . Stair j r . , and M. H . Bruce, J. Chim. physique 64, 43 (1967). [746] A. T . Stairjr., J . P . Kennedy, and R. E. Murphy, J. Chim. physique 64, 52 (1967). (1471 C. K . N . Patel, J. Chim. physique 64, 82 (1967). 11481 C. Rossetti, R. Farrenq, and P. Barchewitz, J. Chim. physique 64, 93 (1967). [149] P . Laures and X . Ziegler, J. Chim. physique 64,100 (1967). [lSOl W. J . Witternan, J. Chim. physique 64, 107 (1967). [151] L. I . Avramenko and V. M. Krasnen’kov, Izvest. Akad. Nauk SSSR. Ser. Chim. 1967, 516; Bull. Acad. Sci. USSR, Div. chem. Sci. (English translation) 1967, 501.

188 Angew. Chem. internat. Edit. 1 Vol. 9 (1970) / NO. 3

The production of NO, as well as *3N-N and N20, was also reported to occur as a result of the reaction of recoil 13N with COzr1521. However, this is in disagreement with the observed laser action, which is not accompanied by COz decomposition. Moreover, earlier studies reported only slight decomposition of COz that appeared to be a result of a reaction with electronically excited nitrogen moleculesr90,911. In an attempt to resolve this discrepancy, Herron and Huie have re- investigated this reaction, using a mass spectrometer to follow reactant concentrations 11531. They were unable to detect any reaction at room temperature and concluded that k29 < 1.7 x 10-16 cm3 molec-1 s-1. It has been suggested that the disagreement might be due t o considerable heterogeneous reaction under the conditions used by the Russian workers“8l. This is perhaps also indicated by their report that the rate of the reaction of active nitrogen with H2 t o produce N H 3 is independent of temperature “541. As mentioned earlier, other workers have been unable to detect any reaction with H21Q4. 1081.

A mass spectrometric study indicated only relatively small decomposition of C3Oz by active nitrogen (at room temperature k 7 : 2.3 x 10-15 cm3 molec-1 s-1) to produce “dicyanogen” as the only detected gas phase product 11551. Intense emissions from the CN red and violet systems accompanied the reaction.

4.2.11. H a l o g e n s a n d H a l o g e n A c i d s

Much of the present knowledge of the reactions of active nitrogen with halogens can be attributed to studies by Phillips and co-workers. In general, no stable products are formed, although N atoms do dis- appear, possibly by the sequence [57,156-160]

N f X z + N X + X

N X f N + N ; - t X

It might be expected, and spectroscopic data suggest, that the efficiency of the halogens in promoting recombination of N atoms follows the order 12 > BrZ > Clz. A kinetic analysis has been made of the vacuum ultraviolet emissions observed during the reaction of active nitrogen with IX compounds (X -:: I, Br, CI, or CN). The results indicate that, in certain cases at least, the

[152] M . J . Welch, Chem. Commun. 1968, 1354. [153] J . T. Herron and R. E. Huie, J. physic. Chem. 72, 2235 (1968). [154] L. I . Avramenko and V. M . Krasnen’kov, Izvest. Akad. Nauk SSSR, Ser. Chim. 1966, 417; Bull. Acad. Sci. USSR, Div. chem. Sci. (English translation) 1966, 1394. [I551 G. Liuti, C. Kunz, and S. Dondes, J. Amer. chem. SOC. 89, 5542 (1967). 11561 M . R . Grigor and L . F. Phillips, 11th Intern. Combustion Sympos., Berkeley, 1966, p. 1171, Combustion Institute, Pitts- burgh 1967. [157] C. G. Freeman and L. F. Phillips, J. physic. Chem. 68, 362 (1 964). [158] L . F. Phillips, Canad. J . Chem. 43, 369 (1965). I1591 D. I . Walron, M . J . McEwan and L. F. Phillips, Canad. J. Chem. 43, 3095 (1965). [160] K . S . Raxworthy and L. F. Phillips, Canad. J . Chem. 42, 2928 (1964).

N; produced in reaction (31) was Nz(3A,)[571. An analysis of energy requirements suggested that this state has an energy of 185 i 3.5 kcal mole-1. This value is considerably greater than that (171 kcal mole-1) de- rived from a direct spectroscopic measurement of 3Au --f B 3ng transition [1611.

From a study of infrared emission, Phiflips has shown that the intensity of the first positive bands increases markedly with addition of trace amounts of 12[*621.

This effect has also been observed when other reactants, such as 02, CH4, and CzH4, are added to an active nitrogen streamrsll.

A mass spectrometric study of the reaction of active nitrogen with C120 indicated a rapid reaction of N atomsr163J. For long reaction times and [C120] = 5 [Nl the ratio of C120 consumed to N consumed was 0.93 -2 0.10. A rate constant at room temperature, k32 9.1 x 10-12 cm3 molec-1 s-1, was estimated for the postulated primary attack by N atoms:

The final products contained N2, 0 2 , and Clz, but not NO. Similarly, a spectroscopic study of the reaction of active nitrogen with (2102 gave n o evidence for the reaction 11641

N + C102 + NO + CIO (33)

The reactions of active nitrogen with the halogen acids HI, HBr, and HCI clearly show the importance of bond energy in determining the course of a reaction. Both HI and HBr react quite readily, probably according

N + H X +. N H i X (34)

to [165-1681

to give free halogen and the ammonium salts, NH41 and NH4Br, respectively. Addition of Br2 or Hz during the HBr reaction causes a decrease in the rate of HBr destruction.

However, for HCI, reaction (34) would be highly endo- thermic and direct attack by N atoms is not likely [Zsl.

Only slight decomposition of HCI to produce H2 and Cl2 is observed. It is possible that this is due to energy transfer from excited N2, or, alternatively, to the catalyzed recombination of N atoms

N (35) H C I + N +. [HCI N] + Nz + H + - C I

[161] H. L. Wu and W. Benesch, Physic. Rev. 172, 31 (1968).

[162] L . F. Phillips, Canad. J. Chem. 46, 1450 (1968). [163] C. C. Freeman and L. F. Phillips, J. physic. Chem. 72, 3028 (1968). [164] €3. P . Broida, H. I . Schiff, and T. M . Sugden, Nature (Lon- don) 185, 759 (7960). [165] E . J . B . Willey and E . K . Rideal, J. chem. SOC. (London) 1927, 669. [166] E. R. V. Milton and H. B. Dunford, J. chem. Physics 34, 51 (1961). 11671 H. B. Dunford and B . E. Melanson, Canad. J . Chem. 37, 641 (1959). [168] R . H . Ewart and W . H. Rodebush, J. Amer. chem. SOC. 56, 97 (1934).

Angew. Chem. internat. Edit. Val. 9 (1970) / No. 3 189

The addition of small amounts of HCI to reactions of active nitrogen with hydrocarbons, e.g., CH4 or CzH6, has been found to increase markedly the HCN production from such reactions[32.1691. There is no net consumption of HCI. The probable mechanism is

C I + R H --f R + H C I (36)

followed by a rapid reaction

4.2.12. Hydrogen C y a n i d e a n d Cyanogen

Both HCN and (CN)2 are too strongly bonded to be attacked directly by N atoms. Thus, any apparent reaction is probably the result of a reaction with excited Nz molecules. Only slight decomposition of HCN by active nitrogen appears to occur, to produce H2 and (CN)zl14*,1701. Although it has been reported that the addition of HCN to active nitrogen does not significantly decrease the N atom concentration [1421, it has been found that upstream addition of HCN greatly decreases the ability of active nitrogen to react with CH3CI 11711.

The decomposition of (CN)z by active nitrogen produced large quantities of a black polymer on the walls of the reaction vessel 11721. A substance with rnje = 76, corresponding to C2(CN)2, was also formed [1421. Ad- dition of H2 or CH4 to the reaction mixture was found to yield HCN [1721.

The (CN)2 reaction is probably initiated by N2(A 3c:) [I431 and might involve C atoms in its overall mechanism [1701:

(CN)z+NZ + N z + 2 C N

C N + N + N z + C

C + ( C N h + C * N + C N

C 2 N + N + 2 C N

The reaction is inhibited by certain additives 11731, the relative efficiencies of which (02 > CH4 > N20 > CO > COz > SF6) might reflect their competitive reactions with either C N or C, e.g.,

C H 4 + C N + C H 3 C N + H

C H 4 + C N + H C N + C H 3

C + 0 2 + C O + O (43)

The addition of 0 atoms favors the reaction and in- tensifies CN emission.

[169] D . R . Safrany, P. Harteck, and R. R . Reeves j r . , J. chem. Physics 41, 1161 (1964). [170] D . R . Safrany and W. Jaster, J. physic. Chem. 72, 3305 (1968). [171] H . Brody and C . A . Winkler, unpublished results; H . Brody, M. Sc. Thesis, McGill University, Montreal 1955. [172] C . Haggart and C. A . Winkler, Canad. J . Chem. 38, 329 (1 960). I1731 D . R . Safrany and W. Jaster, J. physic. Chem. 72, 3318 (1968).

4.2.13. Me ta l s a n d Metal l ic C o m p o u n d s

The effects of adding metals to an active nitrogen stream in the region of the Lewis-Rayleigh afterglow have been studied in considerable detail 1171. Nitride formation has been observed with some metals, e.g., Hg, K, or Ca 1174,1751. However, emission from excited metal atoms is a much more general result. From studies of these emission, early workers made estimates of theenergy content of active nitrogen. Active nitrogen has also been used for many years as a low energy source for excitation of line and band spectra of many molecules1531761. The production of Ba+ when Ba is added to active nitrogen at 1125 OK suggests that NI is responsible for this reaction r176aI.

Reactions of the more volatile metal halides with active nitrogen are usually accompanied by emissions from excited metal atoms 1171, probably as a result of a collision with an electronically excited nitrogen molecule, perhaps in the (5cL) state 1176,1771.

The enhancement of the first negative system of Nz, and Na line emission, have been observed during the reaction of active nitrogen with anhydrous N a ~ S 0 4 11781

The enhancement of the first negative system of Nz and Na line emission, have been observed during the reaction of active nitrogen with anhydrous Na2SO4 [1781.

These observations suggest simultaneous production of a neutral Na atom and a N; ion.

The reactions of active nitrogen with metal carbonyls result in a rapid, stepwise degradation of the carbonyls by the reaction r49J

M(C0)n + N + M(C0)n-I t NCO (44)

These reactions are accompanied by intense metal atom emission.

The reactions of active nitrogen with organometallic compounds have received only limited attention. In the temperature range 130 to 330 OC, a rapid, temperature- independent production of HCN was observed during the reaction of active nitrogen with Hg(CzH5)z [1793.

It was suggested that the HCN resulted from a reaction of N atoms with C2H5 radicals, the presence of which was indicated by the production of small amounts of saturated hydrocarbons.

Both HCN and NH3 (more HCN than NH3) were obtained as products of the reaction of active nitrogen with methyl silanes [180,1811. Maximum HCN production increased with increase in temperature. Similarly,

[174] J . Okubo and H . Hamada, Philos. Mag. 171 8, 375 (1928). [175] A . E. Ruark, P. D . Foote, P . Rutnick, and R . L . Chenault, J . opt. SOC. America Rev. sci.Instruments 14,17 (1927). [176] L. F. Phillips, Canad. J. Chem. 41, 732 (1963). [176a] R . J . Oldham and H . P. Broida, J. chem. Physics 51, 2764 (1969). 11771 L. F. Phillips, Canad. J. Chem. 41, 2060 (1963). 11781 E. A . ANen and E. Leisman, J. chem. Physics48,3335 (1968). [179] D . A . Armstrong and C. A . Winkler, Canad. J. Chem. 34, 885 (1956). [180] H . A . Dewhurst and G. D . Cooper, J. Amer. chem. SOC. 82, 4220 (1960). [181] H . A . Dewhurst, J. physic. Chem. 63, 1976 (1959).

190 Angew. Chem. internat. Edit. 1 Vol. 9 (1970) NO. 3

the main product of the reaction of active nitrogen with [(CH3)2Si0]7 was also HCN [1*21.

Spectroscopic studies of the reactions of active nitrogen with certain organometallic compounds suggest that transfer of energy from an electronically excited nitrogen molecule causes dissociation of the organometallic compound, and excitation of the products L183-1841. The observed emissions from the excited metal atoms and from vibrationally excited CN suggest that the species concerned might be Nz(5S;).

4.3. Organic Reactants

The reactions of active nitrogen with organic compounds have been reviewed in some detail a number of times 111,15-171. In general, the qualitative features of these reactions are fairly well understood, but rele- vant kinetic data are limited and the mechanisms that have been proposed are still dubious.

4.3.1. A lkenes

The reactions of active nitrogen with alkenes have been studied in considerable detail. They are of partic- ular interest as a basis for much of our present knowledge of the reactions of active nitrogen with organic compounds. Until quite recently, the olefins, especially ethylene, were used as a chemical titrant for N atoms and have, therefore, been of special interest.

When alkenes are allowed to react with active nitrogen, HCN is rapidly formed as a major product. The amounts of alkene reacted and HCN produced increase linearly at first, then reach a constant (“plateau”) value as the flow of alkene is increased [23,185, 1861 For alkenes, this “plateau” value is generally independent of temperature. Similar behavior is observed for the active nitrogen reactions with many organic compounds. The attainment of such a temperature-independent “plateau” for HCN production from the olefins prompted the suggestion that it represented a measure of the N atom concentration in the system. Although HCN generally constitutes more than 95 % of the N-containing products from reactions of active nitrogen with alkenes, appreciable quantities of CH3CN and ( C N ) 2 are also formed C187-1901. When

[182] J . L . Weininger, J. Amer. chem. SOC. 83, 3388 (1961). 11831 H. P . Broida and K . E. Shuler, J. chem. Physics 27, 933 (1957). [184] R . E . March and H. I . Schiff, Canad. J. Chem. 45, 1891 (1967). [l85] J . H . Greenblatt and C. A . Winkler, Canad. J. Res. B 2 7 , 721 (1949). [1861 J . T. Herron, J . L. Franklin, and P . Bradt, Canad. J. Chem. 37, 579 (1959). 11871 G. Paraskevopoulosand C. A . Winkler, J. physic. Chem. 71, 947 (1967).

11881 Y. Shinozaki, R . Shaw, and N . N . Lichtin, J. Amer. chem. SOC. 86, 341 (1964). [1891 P . T . Hinde, Y. Titani, and N. N . Lichtin, J . Amer. chem. SOC. 89, 1411 (1967). [I901 Y. Titaniand N . N . Lichtin, J. physic. Chem. 72, 526 (1968).

~ ~

[N] > [alkene], NH3 production may also be signifi- cantc321. Small quantities of C H 4 , C2H6, C z H 2 , and possibly a C 3 hydrocarbon, are also found as products of the reaction of active nitrogen with C2H4‘23 ,1X71 .

The reaction with propene produces significant quantities of C 2 H 4 as well as smaller amounts of C 2 H 6 and C3H8 [187,188,190,1911. Experiments with labeled C 3 H 6

showed that some of the C 3 H 6 recovered is resynthesi- zed from hydrocarbon fragments produced during the reaction with active nitrogen [1*91.

In the reactions of active nitrogen with the butenes, the same “plateau” value for HCN was observed, but whereas butane is the main hydrocarbon product of the reaction with isobutene, both 1-butene and cis-2- butene produce mainly C2 and C3 hydrocarbons [*111921.

The reactions of active nitrogen with alkenes are mod- erately fast (k -: 10-14 cm3 molec-1 s -1 at room temperature) and have low activation energies ( E < 2 kcal

Rate constants for the reaction of active nitrogen with C 2 H 4 have recently been calculated from photometric measurements of the intensities of the first positive and second positive systems of the nitrogen afterglow 11951.

The value found from the second positive emission was about six times that estimated from the first positive emission. Although the significance of these results was not discussed the difference might be due to different rates of reaction of N(4S) and N(2D) with ethylene, as reported earlier 11961.

A “unified” mechanism of the type

mole-1) [187,193,1941.

was first proposed to explain the main features of these reactions [ I l l . More recently, Herron has suggested that the discrepancy between the N atom concentration measured by the N O “titration”, and that inferred from the C 2 H 4 reaction, may be explained by a mechanism that involves H atoms as an essential compo- nent [331, e.g.,

H f C z H 4 + C2Hs (49)

N + C 2 H 5 + N H + C z H 4 (50)

[191] G. S . Trick and C. A . Winkler, Canad. J. Chem. 30, 915 (1 95 2). [*I Recent results indicate that the “plateau” HCN yields from the reactions of active nitrogen with isobutene and 2-methyl-2- butene are considerably less than those from reactions with the other alkenes [341. [1921 H . Gesser, C. Luner, and C. A . Winkler, Canad. J. Chem. 31, 346 (1953). [1931 K . D. Foster, P . Kebarle, and H . B. Dunford, Canad. J . Chem. 44, 2691 (1966). 11941 J . 7’. Herron and R . E. Huie, J . physic. Chem. 72, 2538 (1968). [195] S. Miyazaki and S. Takahashi, Bull. chem. SOC. Japan 41, 1456 (1968). [196] J . Dubrin, C . MacKay, and R . Wolfgang, J. chem. Physics 44, 2208 (1966).

Angew. Chein. internat. Edit. Vol. 9 (1970) No. 3 191

H + C z H s 3 2CH3

N + N H + N z C H

(51)

(25)

N + C H 3 + H C N + 2 H (52)

A loss of N atoms might be expected from reaction (50) followed by reaction (25). Sufruny and Jusfer studied the effect of adding H atoms from a second discharge to the active nitrogen-ethylene reactionr321. They found that the maximum HCN produced under these conditions is in essential agreement with the NO “titration” made in the presence of the added H atoms, which probably catalyze N atom recombination (see below). This would be in qualitative agreement with Herron’s mechanism since the addition of H atoms would favor reaction (51) and thus increase HCN production. They also found that, assuming CH4 to be produced from CH3 radicals and C2H6 from CzH5 radicals, HCN was produced mainly from the C2H6 precursor. They suggested that this is due to a rapid “cracking” reaction

N + C I H ~ --z HCN 4- CH, - i - H ( 5 3 )

which would be faster than reaction (51). This does not imply that HCN production from the reaction of N atoms with CH3 radicals is slow. These reactions were studied under conditions of an excess of hydrocarbon, which would favor reaction (53). There is experimental evidence that the corresponding reaction with CH3 radicals is also fastr1971. This is supported by the rapid production of HCN from the HCl- catalyzed reaction of active nitrogen with CH4, which must result from reaction of N atoms with CH3 radicals (32). Sufruny and Juster suggested that, under ordinary conditions, the disappearance of N atoms could proceed via NH radicals according to

(54)

N + C H x > HzCN-- H ( 5 5 )

(56)

(57)

N+C2H5 + HzCNJ-CH?

N 4 HzCN +- N H f H C N

H j HzCN + HCN - Hz

When [N] > [HI, the formation of NH by reaction(56) would lead to loss of N atoms by reaction (25), while addition of H atoms, so that [N] x [HI, would sup- press formation of Nz by increasing the importance of reaction (57).

The reactions of active nitrogen with higher alkenes are assumed to be reduced essentially to reactions of N atoms with either CH3 or CzH5 radicals by the intervention of “cracking” reactions.

Several features of the reactions of active nitrogen with alkenes cannot be explained adequately by this mechanism. Perhaps the most striking discrepancy lies in the formation of NH3 as a product when ICzH41 < [N]. If the loss of N atoms were caused entirely by reaction (25), the loss of NH, with formation of N2, should become more pronounced when N atoms are in excess. Furthermore, spin considerations suggest that reaction (24) should be at least four times as important as reaction (25)

Reaction (25) should not be dismissed as of no significance. It has been shown that, when H atoms from a second discharge are added to active nitrogen, the concentration of N atoms (NO “titration”) is decreased by about 17 % 132,1931. Since no known reaction of NO with H atoms could possibly compete with its very fast reaction with N atoms, it seems likely that the NO “titration” remains valid under these conditions, hence that H atoms may catalyze N atom recombination. These mechanisms are also not adequate to explain the observed pressure dependence of t h e NO/HCN ratio [299 1397 1991.

This might, in part, be explained by a reaction of the type N H + N H 3 N 2 + 2 H (26)

N + H + M -+ NH + M (58) The effect of adding H atoms could then be explained if the reaction

N H + H + H z + N (24)

could regenerate N atoms a t a rate appreciably faster than the rate of NZ formation by reaction (26). Sufruny has suggested that the reactions of H, 0, and N atoms proceed by similar mechanisms, and that the difference in the final products obtained is simply a result of the different valences of the atoms[1981. He reinterpreted his earlier results on this basis, and concluded that HzCN must be an important reaction intermediate in the reactions of active nitrogen with hydrocarbons. The possibility that this species might be involved in the mechanism of the reaction with ethylene had been suggested earlier [231. Important steps of the overall mechanism might then be

[197] D. A . Armstrong and C. A . Winkler, Canad. J. Chem. 33, 1649 (1955). [198] D . R . Safrany, personal communication.

It has been reported that HCN can be produced when mixtures of C2H4, or other hydrocarbons, with N2 are allowed to react in a shock tube [ZOO]. The temperature was too low (1400-6000 OK) to produce a significant number of N atoms. It was postulated, therefore, that HCN is produced by the reaction of a vibrationally excited nitrogen molecule with hydrocarbon radicals. The measured activation energies for the process involved are in the range 23 to 54 kcal mole-1, much higher than those for corresponding N(4S) reactions.

HCN has also been obtained from the radiolysis, with 6OCo y-rays, of a gas phase mixture of nitrogen and CzH4 [2011. It is interesting to note that HCN production is insignificant when fruns-2-butene is used instead

[199] L. I . Avramenko and V . M . Krasnen’kov, Izvest. Akad. Nauk SSSR, Otdel. Chim. Nauk 1963, 1196. [2001 V. V . Rao, D . Mackay, and 0. Trass, Canad. J. Chem. Engng. 45, 61 (1967). 12011 T. Oku, R . Kato, S. Sato, and S . Shidu, Bull. chem. SOC. Japan 41, 2192 (1968).

192 Angew. Chem. iniernat. Edit. Vol. 9 (I9701 No. 3

of C2H4. Furthermore, the presence of butene inhibits production of HCN from C2H4 in such systems. It has also been shown that HCN is not necessarily the major product when alkene-liquid nitrogen solutions are irradiated with y-rays[20a,20bl.

tive nitrogen, and “plateau” values for HCN production are observed even at room temperaturel30,2071.

The rate constants for the reactions of active nitrogen with alkanes are lower than those for the reactions with alkenes (k E 10-15 cm3 molec-1 s-1 at room temperature), and activation energies are higher 122,2031.

4.3.2. A lkanes 4.3.3. Alkynes

The reactions of active nitrogen with the saturated hydrocarbons are similar in many respects to those with alkenes. However, the initiation reactions are probably different, especially for the lower members of the series, such as CH4, C2H6, and cyclopropane.

At room temperature, the production of HCN from the reactions with the lower alkanes is appreciably decreased whenNH3or SO2is added to the active nitrogen stream between the discharge and the point of hydrocarbon addition [140,2021. It has also been found that the Arrhenius plots for the reactions of active nitrogen with certain alkanes have a definite change of sloper221. These observations indicate that the alkane reactions are initiated by excited N2 molecule reactions such as

N ; + R H -f N ~ + R + H (59)

To conserve spin, the N; would have to be in either a singlet or a triplet state. The alkyl radical formed in such an initial step should react rapidly with N atoms.

As with the alkenes, the main product of the reactions of active nitrogen with alkanes is HCN. Other N-containing products that have been identified are NH3, (CN)2, and CH3CN [30,196,198,202,2031. These are generally accompanied by various hydrocarbons C171.

At room temperature, the reactions with CH4 or C2H6 showed an “induction” effect in the yield of HCN with hydrocarbon flow rate, which was overcome, however, by the addition of H atoms. Temperatures above, 200 “C were required to obtain “plateau” values for HCN production from C2H6 and C3H8 130,32,2041. The “plateau” value was then identical with that obtained from the reactions with alkenes [301. It should, perhaps, be noted that the rate of Hatom attack on CH4, or C2H6, is quite slow at temperatures below about 200 “C 1205 I 2061. Thus, the behavior of the active nitrogen reactions may be based on the change in rates of H atom attack on the alkanes with increase in temperature.

The presence of secondary C atoms in the butanes or higher alkanes increases the rate of reaction with ac-

[202] A . N . Wright and C. A . Winkler, Canad. J . Chem. 40,1291 (1962). [203] E. R . Zabolotny and H . Gesser, J. physic. Chem. 66, 854 (1 962). [204] P . A . Gartaganis and C. A. Winkler, Canad. J. Chem. 34, 1457 (1956). [205] E. W. R . Sreacie: Atomic and Free Radical Reactions. Reinhold Publishing Corp., New York 1954.

12061 R. W . Walker, J . chem. SOC. (London) A, 1968, 2391.

Recent studies indicate that the reactions of active nitrogen with alkynes should be divided into two groups: (a) the reaction with acetylene; (b) the reactions with other alkynes.

Unlike the reactions with most other hydrocarbons, the reaction of active nitrogen with C2H2 produces considerable amounts of (CN)2 as well as the usual product, HCN [208,2091. The reaction is also accompanied by deposition of a brown polymer (C = 65%, N = 27%, H = 7%) on the walls of the reaction vessel [170,2083. Other N-containing products obtained are NH3 and cyanoacetylene (C2HCN) [186,2031.

It has been reported that the initial attack of C2H2 by N atoms is a slow process (for [C2H2]/[N]O.O5, k -5 x 10-15 cm3 molec-1 s-1 at 300 OK) t170,194,211,211aI. It was found, however, that the rate of reaction increased rapidly with increase in [CzHz] (0.05 < [C~HZ] / IN] < 1). When [C2Hz]/[N] > 1, a comparatively slow reaction is observed ‘2llal.

The mechanism of this reaction is probably quite com- plex 11861. The formation of CzHCN, identified mass- spectrometrically, indicated that displacement reactions might be important, e.g.,

Safrany and luster have suggested that the overall behavior of the reaction is greatly influenced by the presence of (CN)2 as a product[1701. Although the production of HCN is increased by about 50% by the upstream addition of NH312101 it has little effect on the rate of the slow reaction ([C2Hz]/[N] > 1) but decreases the rate of the fast reaction[211al. In their study of the kinetics of this reaction, Kistiakowsky and co-workers [211al suggest that the slow reaction ([CzH2]/[N] > 1) proceeds by a mechanism similar to the “unified” mechanism of Evans, Freeman, and Winkier. They also suggest that Nz(A3 c:) may initiate the fast reaction.

[207] R . A . Back and C. A . Winkler, Canad. J. Chem. 32, 718

[208] A . Schavo and C. A . Winkler, Canad. J. Chem. 37, 655 (1959). [209] J . Versteeg and C. A . Winkler, Canad. J. Chem. 31, 129 (1953). [210] D . R . Safrany, R . R . Reeves, and P . Harteck, J. Amer. chem. SOC. 86, 3160 (1964). [211] L. I . Avramenko and V. M . Krasnen’kov, Izvest. Akad. Nauk SSSR, Otdel. Chim. Nauk 1964, 882; Bull. Acad. Sci. USSR, Div. chem. Sci. (English translation) 1964, 770. [211a] C. A . Arrington j r . , 0 . 0. Bernadini, and G. B. Kistia- kowsky, Proc. Roy. SOC. (London), Ser. A 310, 161 (1969).

(1954).

Angew. Chem. internat. Edit. Vol. 9 (1970) / No. 3 193

The reaction of active nitrogen with propyne and 2-butyne resemble the analogous reactions with alkenes. Both compounds produce significant quantities of (CN)2 (about 50% of total HCN), but relatively little polymer. The Arrhenius parameters for the reactions of active nitrogen with alkynes other than acetylene are comparable with those for the corresponding reactions with alkenes 11941.

The reactions of active nitrogen with isomeric allene and propyne were found to be remarkably similar [2121.

Both reactions produce significant quantities of C2H2. Addition of H atoms to these reactions increased the yields of both HCN and C2H2, and it was suggested that production of CzH2 in the active nitrogen systems results from H-atom reactions, e.g.,

4.3.4. A r o m a t i c H y d r o c a r b o n s

The reactions of active nitrogen with aromatic compounds have not been studied in detail. The production of tars and solids, which deposit on the walls of the reaction vessel, seem to be characteristic of these reactions [181,213-2151. Small yields of phenyl isocya- nide, benzonitrile, and pyridine (but no aniline) were obtained from the reaction of active nitrogen with C6H6 1181,2151.

The emission spectrum recorded during the reaction of active nitrogen with solid benzene shows the vibrational structure of the phosphorescence spectrum of C6H6 shifted to higher wavelengths [2161. This was attributed to reaction with N2(A 3z3.

4.3.5. O t h e r O r g a n i c C o m p o u n d s

The reactions of active nitrogen with several alkyl halides have been studied in considerable detail. The perfluorinated alkenes C2F4, C3F.5, and perfluoro-2- butene yield a polymer (C = 26 %, F = 65 %, N = 9 %) as the major N-containing product 12171. Fluorocarbons are also obtained, together with small quantities of FCN. It was suggested that the yield of FCN might be reduced by its reaction with the parent alkene to form a polymer, e.g. ,

[212] D . R . Safrany and W. Jaster, J. physic. Chem. 72, 3323 (1 968). [213] L . B. Howard and G. E. Hillbert, J. Amer. chem. SOC. 60, 1918 (1938). [214] P . M. AronGvich and B. M . Mikhailov, Izvest. Akad. Nauk SSSR, Otdel. chim. Nauk 1956, 544. [215] P . M . Aronovich, N . K. Belsky, and B. M . Mikhailov, Izvest. Akad. Nauk SSSR, Otdel. Chim. Nauk 1956, 696. [216] 0. Dessaux and P . Goudmand, Bull. SOC. chim. France 1968, 2011. [217] M . Madhavan and W. E. Jones, Canad. J. Chem. 46, 3483 (1 968).

The main features of these reactions were explained by a mechanism similar to the “unified” mechanism for the reaction of active nitrogen with ethylene, proposed earlier by Evans, Freeman, and Winkler 1111.

The kinetics of the reaction of active nitrogen with perfluoro-2-butene were studied by the diffusion flame technique 12181. The-rate constant was estimated to be 7 x 10-14 cm3 molec-1 s-1 at 313 “K, which does not agree with the value suggested by a mass-spectrometric study, i.e., k 4 6 x 10-1~cm~molec-~ s-1 at 500 “K [1941.

The reactions of active nitrogen with the chlorinated methanes, except CC14, produce HCI and HCN as the main products (27,2191. These reactions are unusual in that the maximum HCN yield seems to be temperature- dependent. It is possible that the HCI produced acts as a “catalyst” in a manner similar to that observed when HCI is added to the reactions of active nitrogen with certain hydrocarbons [32,1691.

The reaction of active nitrogen with CC4 is accompanied by an intense reaction flame and produces CNCl and Cl2[219]. The rate constant has been estimated as k -= 2 x 10-12 exp(-2.1 kcal/RT) cm3 molec-1 s-1 from an analysis of the product distributions [2201.

The reactions of active nitrogen with CH30H and C ~ H S O H produced HCN and H2O as the main products [221,2221. The activation energies for these reactions were estimated to be 3.1 and 3.4 kcal mole-1, and the steric factors to be 1.7 x 10-3 and 3.6 x 10-3,

respectively.

The reaction of active nitrogen with CH3CHO showed no evidence for hydrogen abstraction by N atoms [2231.

The main products of the reaction are HCN, H2, and CO, with small amounts of glyoxal and substances with m/e = 43 and m/e = 86. The reaction with CH3CDO does not seem to produce any DCN, and the H2 : HD : D2 ratios are 1 : 0.88 : 0.33. The proposed mechanism was

(64)

( 6 5 )

N + CH3CHO + HCN + Hz + CHO

CHO + CH3CHO + CO + H2 + R

To account for the m/e = 43 peak, it was suggested that R might be CH2CHO. The rate constant for reaction (65) was estimated to be k65 = 2.0 x 10-14 cm3 molec-1 s-1 at 296°K. A qualitative study of the corresponding reaction with CzHsCHO suggested that it probably reacts by a similar mechanism 12231.

[218] M . Madhavan and W . E. Jones, J. physic. Chem. 72, 1812 (1968). 12191 S . E. Sobering and C. A . Winkler, Canad. J. Chem. 36, 1223 (1958). [2201 V. N . Balavunov and K . P . Bychkova, Kinetika i Kataliz 9 , 173 (1968); see Kinetics and Catalysis (English translation) 9, 140 (1968). [221] M . J . Sole and P . A . Gartaganis, Canad. J. Chem. 41, 1097 (1963). 12223 P . A . Gartaganis, Canad. J. Chem. 43, 935 (1965). 12231 R . M. Lambert, M . I . Christie, R . C. Goldsworthy, and J. W . Linnett, Proc. Roy. SOC. (London), Ser. A 302, 167 (1968).

194 Angew. Chem. internat. Edit. 1 Vol. 9 (1970) 1 No. 3

The reactions of active nitrogen with CH3NH2 and CH3CN typically produce HCN as the main product [28,181,2241. For both reactions, the “plateau” value for HCN production apparently increases with temperature.

5. CN Emission

The reactions of active nitrogen with C-containing compounds are generally accompanied by a characteristic CN emission. Both the red (A 211 + X 2c) and violet (B 2 5 -f X 2 3 ) bands of CN have been observed in the flames that accompany such reactionsr701, and they have been studied in considerable detail in an attempt to elucidate the mechanism by which these electronically excited states of CN may be populated.

It was noted earlier that (CN)2 production from most of the active nitrogen-hydrocarbon reactions is small and, as Strutt pointed out, the intensity of the reaction flames need give no indication of the extent to which the main reactions occur to yield HCN as the final product 15231. This observation has recently been confirmed by Avramenko and Krasnen’kov, who showed that the rates of (CN)2 or HCN formation are not related to the intensity of the emission spectra of CN, CH, and Cz from the reactions of active nitrogen with CH4, C2H2, C2H6, CH30H, and CCl4L2251.

The mechanism for the population of electronically excited CN radicals in active nitrogen reactions is com- plex. Intensity measurements of the CN emissions, from reactions with cyanogen derivatives or halogenated hydrocarbons, showed three distinct vibrational energy population distributions in both the A ZII and the B 2 3 states [2261. Three distinct mechanisms were proposed to account for these distributions. The first population distribution was observed mainly during reactions with cyanogen derivatives, YCN, at pressures < 0.1 torr and was attributed to either

N + N + YCN + CN*(A zn or B 2 c ) + Nz(X 1ci)+ Y (66)

or

N; + YCN +- C N ” + N2(X ‘Xi) + Y (67)

A second distinct population distribution was observed with halogenated hydrocarbons at higher pressures and fairly high reactant concentration. Excitation of CN was attributed to

N + C X + C N * + X (68)

[224] G. R . Freeman and C. A. Winkler, J. physic. Chem. 59,780 (1 955). 12251 L. I. Avramenko and V. M . Krasnen’kov, Izvest. Akad. Nauk SSSR, Ser. Chim. 1966,1911; Bull. Acad. Sci. USSR, Div. chem. Ser. (English translation) 1966, 1849. [2261 T. Iwai, M . I. Savadatti, and H. P. Broida, J. chern. Physics 47, 3861 (1967).

A third population distribution was observed in the reactions of cyanogen derivatives and halogenated hydrocarbons under “blue” flame conditions, i.e., low reactant concentrations, when the first positive system of active nitrogen is still visible and emission results from excitation of previously formed CN according to

or

Absolute intensity measurements and the overall kinetics suggest that the N,’ species of reaction (70) is Nz(A 3x3, or possibly Nz(X 12;) in a high vibrational level (with energy > 149 kcal mole-1) [2271.

This mechanism has been confirmed by results of opti- cal absorption measurements of ground state CN in active nitrogen flames [2281.

6. The N2(A3Z:) State

In the past, reactions of active nitrogen which could not be adequately explained in terms of N atom reactions have often been attributed to reaction with Nz(A 3c:) molecules. Because decay to the ground state Nz(X 12;) by light emission is spin forbidden, N2(A 3x:) molecules are relatively long-lived, and afford a particularly attractive choice as a second reactant [12a, 171. Several workers have recently obtained significant concentrations of the Nz(A 3c:) species in systems which contain a relatively small number of N atoms [53,54,1431.

As mentioned previously [Section 3, eq. ( S ) ] , perhaps the most important reaction of Nz(A 3x3 molecules is the rapid reaction with N(4S) atoms to produce vibrationally excited ground state molecules [531. The rate of this reaction may be very sensitive to the vibrational energy of the A state molecules [228al.

Energy pooling by

N2(A 3C:) + N2(A 3XJ + N2(X ‘Xi) + Nz(C 3nU) (71)

has also been observed [228b1. The initial production of H atoms in reactions of active nitrogen with organic compounds might well be due to energy transfer from N2(A 3x1:). This species, with energy of 142-146 kcal/ mole, may also serve as a triplet sensitizerrl431. With reactants having low-lying triplet levels, such as (CN)2, SOz, or CS2, reaction of N2(A 3c:) generally leads to emission from the reactant due to a rapid triplet-triplet transition. Other reactants, such as N20

[227] J . C. Boden and B. A . Thrush, Proc. Roy. SOC. (London), Ser. A 305, 93 (1968). [228] T . Iwai, D . W. Pratt, and H. P. Broida, J. chem. Physics 49, 919 (1968). [228a] M . P. Weinreb and G. G. Mannelfa, J. chem. Physics 50, 3129 (1969). [228b] D . H. Stedman and D . W. Setser, J. chem. Physics SO 2256 (1969).

Angew. Chem. internaf. Edit. J Vol. 9 (1970) /No. 3 195

or 0 2 , are partially dissociated by N 2 ( A 3 x 3 mole-

Excitation of NO by N?(A3C:) has been shown to occur efficiently by

cules 1104,1431

N2(A 3c:) t NO(X 211) + Nz(X 1s;) + NO(A 2s-) (72)

Excitation of NO results in approximately one-half of the de-exciting collisions of Nz(A 33:) with NO 12291.

It has been suggested that the mercury 2537 A emission might be used as a test for N 2 ( A 3 3 ; ) [56,230-232J.

The importance of Nz(A3X:) molecules in active nitrogen is, however, somewhat limited by their relatively low concentration, which is estimated to be m 10-3 [N] 12041.

Received: March 24, 1969 [A 744 IE] German version: Angew. Chem. 82. 187 (1970)

[229] R. A. Young and G. A. S t . John, J . chem. Physics 48, 898 (1968). [230] A. Granzow, M. 2. Hoffman, N . N . Lichtin, and S. K. Wason, J. physic. Chem. 72, 3741 (1968).

[2311 A . Granzow, M . 2. Hoffman, N . N . Lichtin, and S. K. Wason, J . physic. Chem. 72, 1402 (1968). (2321 R. A. Young and G. A. St. John, J. chem. Physics 48,2572 (1968); C. H. Dugan, Canad. J . Chem. 47, 2314 (1969).

Magnetic Resonance of Paramagnetic Complexes

By Heimo J. Keller and KarI E. Schwarzhans[*J

The line width of the E S R and N M R signals of paramagnetic transition metal complexes is determined mainly by the electron spin-lattice relaxation time q-. Values of re greater than 10-9 lead to ESR spectra that are readily resolved, while values smaller than 10-11 give N M R spectra having small line widths. Since fast relaxation processes are effective in nearly all transition metal complexes with several unpaired electrons and in all complexes having an orbitally degenerate ground state, the N M R method has a wider scope. The sign and magnitude of the electron-nucleus coupling can be determined with great sensitivity from the N M R spectra, whereas only the magnitude of this interaction can be determined from the ESR spectra. Free spin densities can be found very accurately from the N M R shifts, and the method can therefore be advantageously applied to kinetic measurements, e.g. on short-lived contact complexes.

1. Introduction

The use of both the electron spin resonance (ESR) and the nuclear magnetic resonance (NMR) effects can lead to the solution of a number of chemical problems in connection with paramagnetic transition metal complexes. Several reviews of both methods have been published in recent years, and results for transition metal complexes have also been discussed 11-81, but their combined application to paramagnetic complexes

[*I Dr. H. J. Keller and Dr. K . E. Schwarzhans ~-

Anorganisch-chemisches taboratorium der Technischen Hochschule 8 Miinchen 2, Arcisstrasse 21 (Germany)

[ l ] NMR, ESR: A. Carringfon and A. D. McLachlan: Intro- duction to Magnetic Resonance. Harper & Row, New York 1967. [2] NMR: D. R. Eaton and W. D. Phillips, Advances magnetic Resonance 1 , 103 (1965). [3] NMR: M . L. Maddox, S. L. Sfafford, and H. D. Kaesz, Advances organometallic Chem. 3, 1 (1966). [4] NMR: E. DeBoer and H. van Willigen, Progr. nuciear magnetic Resonance Spectroscopy 2, 111 (1967). [5] N M R : A. Kowalsky and M . Cohn, Annu. Rev. Biochem. 33, 481 (1964). [6] NMR: D. R. Eaton, A . D. Josey, and W. D. PhiNips, DIS- CUSS. Faraday. SOC. 34, 77 (1962).

has been largely ignored. A question of prime interest to chemists is when to use NMR measurements in practice, and when ESR.

2. Theoretical Principles

2.1. Resonance Conditions

The problems will be considered on the basis of the simplest possible system, i.e. hydrogen atoms, assuming at first that these atoms are not influenced by their neighbors, so that it is possible to discuss the resonance properties of a single hydrogen atom. It is well known that the hydrogen atom consists of one proton and one electron. Each of these elementary particles has a characteristic angular momentum (spin), which, according to the quantum theory, can only assume certain specific values. The angular momentum J , which differs in magnitude for the two particles in question, is therefore, figuratively speaking,

[7] ESR: B. R. McGarvey, Transition Metal Chem. 3, 89 (1 966). [8] ESR: W . Low: Paramagnetic Resonance in Solids. Acade- mic Press, New York 1960.

196 Angew. Chem. internat. Edit. 1 Vol. 9 (1970) 1 NO. 3

the chemical behavior of active nitrogen

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