the infrared and raman spectra of dicyanodiacetylene, nccccccn

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Page 1: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

Spectrochimica Acta, 1967, Vol. 23A, pp. 1415 to 1429. Pergamon Press Ltd. Printed in Northern Ireland

The infrared and Raman spectra of dicyanodiacetylene, N&-C~C-C~C-C~N*~

FOIL A. MILLER and DONALD H. LEMMON Department of Chemistry, University of Pittsburgh and Mellon

Institute, Pittsburgh, Pa. 15213

(Received 21 July 1966)

AbStrsct-Dicyanodiacetylene, N=C&C--C=C-C%N, was prepared by improved pro- cedures which are given in detail. Its infrared spectrum was measured from 35 to 4000 cm-l in the vapor and solution phases. Raman spectra with polarizations were obtained for solutions. D,, symmetry is fully satisfactory, and all fundamentals have been assigned. All other symmetries present serious problems. Thus the experimental evidence clearly supports the expected linear symmetrical structure. Dicyanodiacetylene is therefore believed to be the longest coniirmed linear molecule now known.

INTRODUCTION

THIS paper reports the first study of the vibrational spectrum of dicyanodi- acetylene, N&3-C=C-CzC-C-N, which will hereafter be termed DCDA. (The Chemical Abstracts name is hexadiyne dinitrile.)

This compound is of interest to us for several reasons. (1) It is now, as a result of this work, the longest linear molecule whose linearity has direct experimental support. $ (2) Our laboratory has had a continuing interest in linear molecules [l-4], acetylenic compounds [l, 2, S-71, and cyano compounds [l, 2, S-10], and DCDA combines all three characteristics. In addition it is a natural extension of our earlier work on dicyanoacetylene [1, 21 and diacetylene [a]. (3) The compound is unusual in having four conjugated triple bonds. The vibrational spectrum of such a system has not previously been studied.

* This work was supported by the National Science Foundation under grant GP-5050. t From a thesis to be submitted by D. H. LEMMON in partial fulfillment of the requirements

for the degree of Doctor of Philosophy at the University of Pittsburgh. $ Triacetylene also haa eight atoms, and is almost certainly linear, but its symmetry has

not been proven. We are studying it at present.

[l] F. A. MILLER and R. B. HANNAN, JR., J. China. Phya. 21, 110 (1953). [2] F. A. MILLER, R. B. HANNAN, JR., and L. R. COUSINS, J. Chem. Phya. 23,2127 (1955). [3] F. A. MILLER and W. G. F~~~~~~,Spectrochint. Acta 20, 253 (1964). [4] F.A.MILLER,D.H.LEMMoN~~~R.E.WITK~WSJII, Spectrochim. Acta 21, 1709 (1965). [5] J. H. WOTIZ and F. A. MILLER, J. Am. Chmn. Sot. 71, 3441 (1949). [6] F.A. MILLER~~~R.P.BA~, J. Chmn. Phya. 22, 1544 (1954). [7] C.V. BERNEY, L. R. COUSINS and F. A.MILLER, Spectrochim. Acta 19, 2019 (1963). [8] F. A. MILLER and W. K. BAER, Spectrochim. Acta 19, 73 (1963). [9] F. A. MILLER, 0. SALA, et al. Spectrochim. Acta 20, 1233 (1964).

[lo] F.A. MILLER, S.G.FRANKISS and 0. SALA, Spectrochim. Acta 21, 775 (1965).

1415

Page 2: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

1416 F. A. MILLER and D. H. LEMMON

The synthesis of DCDA was first reported by MOREAU and BONGRAND in 1920 [ll]. BROCKMAN [12] improved the preparation, established the identity of the compound more certainly by preparing a derivative, and reported some X-ray diffraction lines from a single crystal rotation pattern. SAGQIOMO [13] also de- scribed the synthesis, measured the vapor pressure curve, and gave a qualitative infrared spectrum in Ccl, solution obtained with NaCl optics. (His spectrum contains three bands at 8.0,9.2, and 9.8 p which seem to be due to silicone grease; another silicone band at 12.5,~ is not seen because of solvent interference.) Finally, a study of the mass spectrum has been reported [14].

PROPERTIES AND PREPARATION

DCDA is a white crystalline solid which melts at 64.5-65’. It has a vapor pressure of ~15 torr at room temperature [13], and sublimes readily and is easily handled by vacuum transfer techniques. It has an irritating odor, and is a strong lachrymator. DCDA darkens slowly in vucuo at room temperature, but is quite stable at -75’. Although soluble in the common organic solvents, it decomposes in or complexes with many of them. For example solutions in CSz, Ccl,, CHCl,, furan, and hexane slowly become amber colored even in vacua; in cyclohexane the change is fast. Solutions in toluene turn blue slowly, but those in benzene turn blue almost instantly.

Our synthesis followed the general method described in [ll-131, but several changes were made which either inoreased the yields or simplified the operation. Because the preparation has many pitfalls, we shall describe our procedure in detail. The reaction scheme was:

H-C&C-COOH - H-C=C-COOCH, liq. H-CEC-CONH, ‘+

H___CsC____CEN ‘“’ ‘la 2- Cu__C=C-CGN =, Fe (‘x)$

NEC-C=C--C=C-&N. Details for each step follow.

1. HC-CCOOCH,

Twenty-five grams of cold (0”) propiolic acid (Aldrich Chemical Co.) was added dropwise Firith stirring to a cold (O”) solution of 4.1 ml of cont. H,SO, in 55 ml of absolute CH,OH, the temperature being kept below 10’ during addition. The reaction mixture was stored in the dark at room temperature for four days. (This seems to be the optimum time; & one day decreases the yield.) The system was then stirred while 100 ml of saturated NaHCO, solution was added slowly; solid NaHCO, was then added until the water layer remained basic. The organic layer was separated, dried overnight over CaSO,, filtered, and vacuum distilled to separate the ester. Boiling point range ~64-72” at 180 torr. Yield, 13.5 g

[Ill C. MOREAU and C. J. BONQRAND, Ann. Chim. (Parie) 14,47 (1920). [12] F. J. BROCKMAN, Caw J. Chem. 83, 507 (1955). [13] A. J. SAGGIOMO, J. Org. Chem. 22, 1171 (1957). [14] V. H. DIBELER, R. M. REESE and J. L. FRAN-IN, J. Am Chews. Sot. 83, 1813 (1961).

Page 3: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

The infrared and Raman speotra of dicyanodiacetylene 1417

(46%) of colorless liquid. Attempts to separate the ester directly from the reaction mixture of propiolio acid and methyl alcohol by vacuum distillation resulted in extensive polymerisation.

2. HC-CCONH,

The original preparations [ll, 131 used aqueous ammonia and resulted in poor yields and an impure product. We found that adding the ester dropwise to an excess of liquid ammonia gives ~100 per cent yield in sufficient purity to use directly in the next step. The excess NH,, and the CH,OH formed in the reaction, were pumped off. Melting point of the pure white crystalline amide: 60-61’.

3. H-C’EC-C=N

Fifteen grams of amide, 60 g of calcined white sand for heat transfer and 40 g of P,O, were mixed in a three-neck round-bottom flask. A thermometer was inserted into the reaction mixture via one neck. A stopcock was fitted to another neck to facilitate opening the system after the reaction was completed. The third neck was connected to a trap (equipped with stopcocks) through which the system was thoroughly evacuated. The system was then closed and the trap was kept in liquid nitrogen while the reaction temperature was gradually raised to 226” over a two hour period. Pure cyanoacetylene was collected in the trap. Yield 9.0 g (81%). This colorless liquid should be stored under vacuum in dry ice; it quickly turns blue at room temperature.

4. Cu-C=C-C%N

Twenty grams of Cu,Cl, was stirred under nitrogen with 60 ml of 14 per cent aque- ous ammonia for 20 min. Then 1800 ml of H,O was added, and stirring was continued for 30 min more. The slurry was filtered, and the deep-blue filtrate was placed under N,; 5 ml (4.1 g) of cyanoacetylene was added dropwise to the filtrate with stirring, and the stirring was continued for 10 min more. The resulting suspension was filtered and the yellow precipitate of Cu-C-C-C=N was washed with H,O and transferred to a beaker. It is important to keep the precipitate wet became it is ex$osive when dry. (The final filtration again gives a deep blue filtrate but it is too dilute in the cuprous complex ion to be re-used.)

6. NAJ-CEC-CA&CEN

This step was done in a cold room at 5’C, and all reagents used were at this temperature. To the product from the previous step was added 25 ml of Ccl, and 25 ml of H,O, and the resulting slurry was stirred rapidly. Forty ml of saturated K,Fe(CN), solution was added dropwise, and stirring continued for 10 min more. (The timing is critical; a longer time apparently results in extensive oxidation of the product.) The slurry was then centrifuged, the CCI, layer (bottom) withdrawn via a pipette and filtered through paper to remove traces of water and sludge. The product was recovered from the solvent by repeated bulb-to-bulb vacuum transfer. Yield: -_9 g. Additional extraction of the sludge with Ccl, gave negligible product. We found that Ccl,, which was used in [12], gave better

Page 4: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

1418 F. A. MILLER and D. H. LEMMON

recovery in our vacuum line method than benzene which was used in [ll]. We also tried CHCl, because its higher vapor pressure would be helpful, but its density makes separation from the sludge difficult.

Raman SPECTROSCOPE PROCEDURES AND RESULTS

The spectra were obtained with a Cary Model 81 Raman spectrophotometer using Hg 4358 A excitation and 10 cm-l slits. The samples were ~10 percent by weight solutions in carbon tetrachloride, hexane, furan and toluene in 7 mm dia. Raman tubes. Qualitative polarizations were obtained by the usual two-meas- urement method using cylinders of Polaroid concentric with the sample.

The results for carbon tetrachloride and hexane solutions, which exhibited all the observed bands, are given in Table 1. Frequencies should be accurate to ~2 cm-l. The strongest Raman band, 2235 cm-l, was also observed by Hg 4047 and Hg 4339 A excitation, giving the apparent lines at 468 and 2135 cm-l. (The 2235 line excited by Hg 4347 A would come at 2178 cm-l, where it would be masked by the 2183 band. The latter is much too intense to be due to Hg 4347 A excita- tion.)

Infrared

Vapor spectra were measured from 35 to 4000 cm-l with Beckman IR-9 and IR-11 grating spectrophotometers. Path lengths were 2.8 and 8.2 m, and the resolution was l-2 cm-l everywhere. Measurements on Ccl, and CS, solutions covered the 400-4000 cm-1 region only. The results are given in Table 1 and Fig. 1. Frequencies should be accurate to Al cm-l unless marked as broad or shoulder.

ASSIGNMENTS

D,, symmetry was assumed initially. The fundamental vibrations, selection rules and our assignments are given in Table 2. HERZBERG’S conventions [15] are used throughout. For comparison, the fundamental frequencies of dicyanocetylene (DCA) are also included [2].

A. Raman-active fundamentals

The three polarized Raman lines at 2235, 2183 and 1287.5 cm-l are surely yl, vg and va respectively, but there is no polarized line for vq. The 170 cm-l band is reasonable for vlO. Since 468 is really v1 excited by Hg 4047 A, the only candidates left for vq, vs and vg are 455, SO1 and 571 cm- l. All three appear to be depolarized, so it is a bit difficult to decide which one is vq. Fortunately the infrared band at 563 cm-l provides some help. It is a doublet, and therefore a parallel band. It can be explained as 501 + 61.5 = 562.5. There is no doubt that 61.5 is the lowest nu fundamental. Because a mu fundamental can give a parallel binary sum tone only by combining with a rr, fundamental, this requires that 501 be rrg.

[15] G. HERZBERG, Infrared and Raman Spectra of Polyatomic Molecules, Van Nostrand (1960).

Page 5: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

Table 1. Dicyanodiacetylene: Raman and infrared spectra (cm-‘)

ccl,

Raman, solution Infrared, solution Infrared, vapor Rel. peak

Hexme Intensity P&n. ccl, cs, cm-’ Intell.

170

* *

501

671

1287.5

(2136.6)

2183

2235

2286.5

2670.6 2697.6

UW$ t

2,b dpl

455 6

(468) -5

600.6 60

669

1286.5

2181

2233

2567.6

2

8

4

46

1000

2

9 2

dp ? v9

dp

dp

490 488.6

666 654

721 717.6

943

*

2095 2091.6

2160 *

2232 2228.5 2264 2260

2296.5 *

t 61.5 vs

189 m, b 276 ““S 281 TV, sh 289 w, sh

303 “YW 311

I

vvw 319 “YW 332 336.5 w I 439.6 445.5 In I

490.6 “8

606 VW

644.t 650

714 720

944

1011

s 490.6 + [156] = 646.6

w, b

VW, b

2097

2160

2221

2266

m

m,b

m,b VW S

2290.5 m 2296 I

V18 VlO 3 x 61.5 = 1841

VIP 276 + a’( - v& 276 + 2v, - 2v&

Impurity?

490.5 - [166] = 334.6

501 - 61.6 = 439.6

Hg 4047 A - 2236

VI1 VB ?

501 + 61.6 = 562.5

(

466 + 490.5 = 945.6 (II) 7 501+ 443 = 944(l)

466 + 663 = 1018

VI VI Hg 4339 - 2236.5 3 x 717 = 2151 (II) ?

VI vg for W?

Vl % ?

2235 + 61.6 = 2296.5 (1) ?

2 x 1287.6 = 2575 2264+ 443= 2707

w, m, s = weak. medium, strong; v = very; b = broad; sh = shoulder; p. dp = polarized, depolarized. * Solvent interferes. t Solutions not exmnined below 400 cm-1 in the infrared. t [166] Estimated v&e for vapor. See “Assignments”. 0 v, = 61.5 or 170 cn+.

2400 2ooc

2400 ZOOC

Fig. 1. Infrared spectrum of dicyanodiacetylene vapor. Press. ~15 ton-. A = 8.2 m. B = 2.8 m path. Note the changes in the frequency scale.

1419

Page 6: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

1420 F. A. MILLER and D. H. LEMI~XON

Table 2. Fundamental vibrations and assigmnents for dicyanodiacetylene and dicytmoaoetylene

D% Species

Assignmenta Aotivity No. Schematio description DCDA DCA [2]

%3 + R (P), - 1

2

3

4

cl&+ -, i.r. ([I) 6

6

7

=I! R (dp), - 8

9

10

rla -, ix. (JJ 11

12

13

CzN str tic str c-c str cc str CEN str c4 str c-c str Bend

Bend

Bend

Bend

Bend

Bend

2236 -2290

2183 2li9

1287.6 692

571 -

2266 2241

2097 -

717 1154

601 604

466 263

170[156] -

490.5 472

276 107

61.5 -

[166] Estimated vapor value. See “Assignments.” DCDA: Dicyanodiacetylene. DCA: Dicyanoaaetylene.

It follows that vp, a C-C stretch, is either 455 or 571 cm-l. The former seems too low, so we assign 571 to vq. This leaves 501 for vs and 455 for vs.

B. Infrared-active fundamentals

1. Band contours. In the vapor phase the parallel bands should be doublets with a missing Q branch. From the bond distances for dicyanoacetylene [ 161, we estimate the moment of inertia of DCDA to be 890 a.m.u.-A*. This gives a computed spacing between the maxima of the P and R branches of 5.6 cm-l [17]. Six doublet bands are observed, with the actual spacings varying from 4.5 to 6.0 cm-l. The appearance of so many well-defined doublets is some slight evidence for linearity. (It would provide conclusive proof if one could show that there were really a zero gap at the center [18]; our resolution was nowhere near good enough to do this.)

Because of the large moment of inertia, the perpendicular bands of DCDA are not expected to exhibit any structure. This distinction is very useful.

2. Species T, (1 bands). The bands at 61.5, 276 and 490.5 are good choices for the bending modes v13, v12 and vrl respectively because of their great intensities and their contours.

3. Species IS,+ ( 11 bands). Oddly, the parallel bands and the triple bond stretch- ing fundamentals provide the hardest problems in the infrared assignments. Usually, these are the easiest to assign. There are two o,+ triple bond stretching

[la] R. B. HANNAN and R. L. COLLIN, Acta Cry&. 6, 350 (1953). [17] Ref. [15], p. 391. [18] Ref. [15], p. 384.

Page 7: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

The infrared and Raman spectra of dicyanodiacetylene 1421

fundamentals, Ye and Ye, and three reasonable candidates: 2293 (m, doublet), 2266 (a) and 2097 (m). It seems certain that 2266 must be one of the fundamentals because of its intensity, but which is the other? The doublet contour suggests 2293, but the frequency value favors 2097.

It is well established that the stretching frequency of a conjugated C%N group is usually in the range 2240-2215 cm-l, whereas conjugated C=C stretches are typically below 2125 cm-l, and thus are about 100 cm-l lower [19].* This indicates that the choice of 2293 and 2266 would make the frequencies unusually high. The alternative, 2266 and 2097, is much more reasonable, and we take this pair for Ye and Ye. Neither of them has the expected doublet contour, and we suggest that the central minimum is filled in by hot bands involving the 61.5 vibration. Why only the high-frequency stretches have their contours filled in this way we do not know, but it seems to happen in many oases. C&O, provides a good example [3], and for CF,CrCH and CF,C-CD [7] and H,Si&CH and D,Si-C-CH [20, 211 the individual hot bands can be clearly seen.t These values for Ye and Y,, agree well with the corresponding fundamentals in H-C=C-C%N, where they are 2271 and 2078 cm-l [22]. (Incidentally in an infrared spectrum of H-C=C-C=N which we obtained, both of these bands were doublets as expected.) One last problem remains: we cannot account for the rejected 2293 band. The sum 2235 + 61.5 = 2296.5 fits numerically, but would give a per- pendicular band, whereas a parallel contour is observed.

This leaves only Y,, the antisymmetric C-C stretch, to be assigned. There are three strong parallel bands which are likely candidates; 717, 647 and 563 cm-l. The latter two have satisfactory explanations as binary combinations, whereas none could be found for 717. We therefore take 717 for v,. Further support for this is found in the fact that 563 and 647 are not really useful in explaining left- over bands, whereas 717 gives an acceptable explanation of 2150 as 3 x 717 = 2151 (vapor). In solution both the 717 and 2150 bands increase in frequency, which is unusual. We calculate 3 x 721 = 2162, and observe 2160. This in- creases our confidence in this explanation.

C. Remaining bands

Explanations for many of the remaining bands are included in Table 1. All are compatible with the selection rules except the band contours for 944 and 2150 cm-l. Only binary combinations were searched systematically, although a few ternary ones that we stumbled onto are included.

* Bellamy gives the C%C frequency of DCA as 2267 from [l], but this was revised in [2] to 2119 cm-l.

t There is still some question aa to whether these features are really hot bands or have some other, as yet unknown, cause [20]. [19] L. J. BELLAMY, The InfraredSpectra of Complex Molecules, pp. 263,59, John Wiley (2nd Ed.)

(1958). [ZO] R. B. REEVES, R. E. WILDE and D. W. ROBINSON, J. Ch.cm. P&Y. 46,126 (1964). [21] E. A. V. EBSWORTH, S. G. FRANKISS and W. J. JONES, J. Mol. Spectvy 13, 9 (1964). [22] V. A. JOB and G. W. KING, Can. J. Ch.em. 41, 3132 (1963).

Page 8: The infrared and Raman spectra of dicyanodiacetylene, NCCCCCCN

1422 F. A. MILLER and D. H. LEMON

Postulated [166] value for vl,,. An interesting case involves the infrared doublets at 334 and 647 cm-l. It was noted that these are 490.5 f [156]. The latter frequency was not observed, but is believed to be the vapor phase value for vi,, which is at 170 cm-l in solution. This is supported by the fact that the 647 cm-1 vapor band becomes 656 in CCI, solution; it is nicely explained by the solution values 490 + 170 = 660. It is now well established that low frequencies are higher in condensed states than in the vapor, and the proposed 14 cm-l shift from 156 to 170 cm-l is not unusual [23]. It follows that [156] should be used in place of 170 whenever a vapor phase frequency is needed. This eliminates some possible binary combinations, such as:

(1) Explaining 443 as 276 + 170 = 446.

(2) Explaining 505 as 334 + 170 = 504. If 334 is the difference tone as assigned, it would also invalidate this.

(3) Explaining 2097 as 2266 - 170 = 2096. Also arguing against this is the fact that the corresponding sum tone was not observed, and that we have taken 2097 for vg.

Unfortunately there are still a few bands for which we do not have satisfactory explanations. The most serious problem in the infrared is with 2293 cm-l, and in addition we are not content with the explanations for 189 and the 303-311-319 trio. The only difficulty in the Raman spectrum is with 2286. We note, too, that the postulated Raman overtone at 2570 is as intense as its funda- mental at 1287. Another puzzle for which we have no answer is why the infrared band at 563 cm-l is strong in the vapor but is completely missing in solution.

Symmetry Discussion

The observed spectrum fits D,, symmetry very well, whereas the use of any other point group causes serious difficulties. It is particularly clear that there are no real infrared-Raman coincidences, so the molecule almost certainly has a center of symmetry. This narrows other possibilities to a trans-bent model (C,,). The spectroscopic problems with the latter will not be discussed beyond mentioning (a) that it permits seven polarized and two depolarized Raman fundamentals, and (b) that the assignments become very difficult. The interpretation on the basis of C,, has to be forced to an unreasonable degree, whereas it follows easily for D,,. It is therefore our view that the data confirm the expected linear symmetrical structure.

Mixing of the vibrations

It is interesting to note that the vibrations which have been schematically described in Table 2 as C-N stretches or C-C stretches seem to be just that to a rather good degree. Their frequencies come close to the ranges found for simpler molecules [ 191, so that there is no obvious evidence of the C-N and C-C stretches

[23] W. G. FATELY, I. MATSUBAFLA and R. E. WITKOWSKI, Spectrochim. Acta 20, 1461 (1964).

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The infrared and Raznan spectra of dicyanodiscetylene 1423

mixing as might have been expected. This is particularly clear for y1 and Q, where there was no problem with the assignments.

The two C-C stretches in o g+ show just the opposite behavior. va is un- expectedly high, and vq strikingly low, indicating that they have undergone a strong first order interaction. Their r.m.s. value is 995 cm-l, which is a reasonable value for a C-C stretch.

Comparison of the bending frequencies in DCDA and C,O,

In earlier work [4] we emphasized that the lowest bending mode in carbon suboxide is abnormally low (63 cm-l) and abnormally weak. Comparison with DCDA provides further illustration. The lowest bending mode in DCDA is at 61.5 cm-l, which is almost identical with the value for C,O,, even though DCDA has three more atoms and three nominally single bonds. The DCDA band is also very strong, in marked contrast to the one for C,O,.