The and ~~~o~et P(NCO), and POWCO),

AI&r&-The infrared spect.rum from 120 to 5000 cm-r and the Kaman spectrum wit.h polariza- tions were obtained for phosphorus (III) isocyanate and phosphoryl (V) isocyanate. They ~~~nonstr&~~ that both moI~ctIIes have at least a Cs.axis of s~rnrn~~~. For P(lr;CO)s, try (~1~~~ trigonal s~r~~c~~~~) may be ~o~~itel~ ~I~~~inat~~I. Cs* sy~~rnetr~ (plan structure) is aIso ~Iirni~l~~~~, but with less ~ort~~~y. G’s, s~~~~rnot,ry seems most probable for both molocliles. This does not establish wIl~t,hor Lha I*-N===C-0 roups are linear or bent. ~~I~ldarn~~tal frequencies are assigned for both subst,anccs.

In the ultravialet. spectrum P(NCO), has no band above 2300 -4, but PO(WO), has a weak band at 2660 A in W&Y, solution (cmbr zz 21 1. mole-r cm-r).

Tms wax+ wiaa motiva~d by a recent s~,~d~ of the vibrational ~~ctr~~rn of Sj(~~O)~ by Miller and Paulson [l]. They found that this compound is t~etr~h~ral, which means that each Si-N&=0 group is linear. The skeleton of H,SiNCS is also thought to be linear 121. On the other hand there is good ~vjden~ that HNCO [3J, CW,NCO 133, Hh’CS [4f and CH,NCS [5] have bent skeletons with a bond angle ntt the nitrogen atom of 125 to 160”. This difference can be rationalized by postulating thaL in t*he first two com~unds there is iderable double bond’ te the silicon. ~~surnabl~ this ~~~ol~~s the non-bonds air of eIcctrons on the ogen atom and an empty 3&orbital on silicon. It csn be represented sohem~tic&lly by

* Prom a thesis to be submitted by W. K. BAEK in partiai fuIfiIIment of the requirements for thr degree of Doctor of Philosophy at the University of Pittsburgh.

+ Thie work was supported by the United States Atomic Energy Commission under Co~tr~et AT(30.1). 1093.

1312 F. A. MILLER and IV. K. BAER

Recause hydrogen and carbon have no suitable d-orbital, this double bonding does not occur with them and the skeletons of the last four compounds are bent.

If this explanation is correct, it also ought to apply when phosphorus replaces silicon. Phosphorus too has empty 3d-orbitals, and the fact that its electronegativity is greater than that of silicon should favor the added bonding. Hence the P---X= C===C group should be linear. To try to test this prediction, we have obtained the infrared and Raman spectra of P(~(~O)~ and P~(~CO)~. These have not been re- ported before, and there has been no structural determination for either compound by any means.

There are five reasonable structures for P(NCO), which have some symmetry. They are-shown in Fig. 1. Each of these five models has a twofold symmetry axis. If an oxygen atom is added on this axis to give PO(NCO),, the symmetries reduce to the following.

~yn1metry of: ~rn~~~~ of P(NCO), portion P(KCO), ~O(~C~)~

-~ _“__ _..___~_~_ .~~_._._. --. .--- ---.- -_I_~~

a Plenar trigonal Q3h G’3, b Plantar pin-whocl c 3h c3

% d Trigonal pyramid Gse c3, 0 Nonplanar pin-wheel cs =%

Although it is also possible that P(pI’CO), may have no symmetry, it will be shown below that the spectrum contradicts this.

The infrared and Raman spectra of P(NCO), are surprisingly simple, and at first glance seem most compat~ible with the Da, or Csh models. There is, however a serious flaw in this evidence. The symmetry of PO(KCO), cannot be higher than C,, as shown above. For C,, and G, all the spectroscopically-active fundamentals are altowed in both the infrared and Raman effect. A much richer spectrum is therefore

expected upon conversion from a Da, to n C,, structure, or from C, to C,. This is not

observed ; the spectrum of PO(NCO), is qualitatively not greatly different from that of P(NCO), except for the addition of two I’==0 vibrations. It is therefore misleading to attach too much significance to missing bands. Certainly many which are formally active in PO(~CO)~ are not observed; the same may also be true for P(NCO)a. We have made strenuous efforts to observe more fundamentals for both compounds by using prolonged Raman exposures and thicker infrared samples, but with no success.

The results for the two molecules will now be discussed in some detail, starting with PO(~C~)~.

~~(~CO)~

Properties and preparation

Phosphoryl isocyanate is a liquid at room temperature, melting at 5” and boiling at 193°C. Although it is reported to be colorless, our sample was pale yellow in a thickness of several cm. The ~om~und reacts with moistun? on exposure to the atmosphere and must therefore be handled in a dry box. The liquid is fairly stable at room temperature but it slowly discolors when heated.

The infrared, Raman and ultraviolet spectra of P(NCO), and PO(SCO), 1313

The sample was prepared by the method of ANDERSON fcif. 300 g of A@CO, 127 ml of PUCl,, and 225 ml of benzene were refluxed for 44 hr with cou~t~nt stirring, The

solids were removed by filt~tion and the liquid portion uw ~r~~tjona~d at reduced pressure to remove benzene and ur~re~ted POCl,. The crude product WM further

(b) C3h

(c 1 c,, (d) C,, (e) C, Fig. 1. Possihla ~tr~ctu~~ for P(KCO),

(a) D,. Planar tri$Q~~. (b) Can. Planar pin-wheo1. (c) C3”. Trigonal pyramid. Linear PSCO groups.

(d) C,“. Trigonal pyramid. Each PSCO group bent at the K, but. remaining in a plane of symmetry.

(0) C,. Sonplaner pin-wh64.

purified by ~veral additional distillations at reduced pressure. Approximately 4 ml of purified sample was obtained. Several attempts to de~rmi~e its purity by chro- rn~~~phy were ~~~suece~ful. Two column packing8 were t:ried-Apiozon L on firebrick and Apiezon L on “Fluoropak 80’“~using several column temperatures for each. Only broad, seymmetrical peaks indicating decompoeition were observed.

A’lpectroscopic methods

Three instruments were used to obtain the infrared spectrum. The 120-400 cm-l

region was covered with a small sin~lo-bairn grating Bpe~tro~~et~r described pre- viously [7 J. For the 3~0-50(~0 em-’ rangy, a Beckman IR-4 $~ctr~photomet~r (do~~~l~ beam, double ~lon~)~h~omator) with C&r and NaCl optics was ufied. Frequ~nci~$ for all the bands in the 11004000 ~rn‘-~ range were measured with a Perkin-Elmer Nod&l 12 spectrometer (single beam, double pass) using &her NaCl or CaF, optics

[S] H. H. Asnr,rrso~, J. Am. Chem. Sec. 64, 1757 (1942). [7] F. A. MILI.EH, G L CARLYON and 1%‘. B. CVHITE, Sperfrochh acta 15, 709 (19.59).

1314 F. A. MILLER and W. K. BAER

where appropriate. Infrared frequencies are believed to be accurate to f 1 cm-l from 120 to 1000 cm-l, to &2 cm-l from 1000 to 2000 cm-l and to &4 cm-l at 3000 cm-i.

Cells were filled in a dry box. The infrared spectrum of the pure liquid is shown in Fig. 2 and the observed frequencies are listed in Table 1.

% 00 400 600 600 1000 1200 1400 1600 1800 2200 2600 3000 3400 3800 4200 4600

Frequency, cm“

Fig. 2. Infrared spectrum of PO(NCO),. a = 0.132 mm; b = 0.058 mm; c = capillary film; d =

film, > c; e = 0.050 mm; f = solution in Ccl,. Raman bands are indicated at the top of the figure.

The Raman spectrum was obtained with a photographic grating instrument described earlier [8]. A “Toronto” type mercury arc was the source, and the sample volume was about 4 ml. For Hg 4358-A excitation, exposures ranged from 40 to 90 min with Eastman 103a-0 plates; for Hg 5461-A excitation, they were 4 and 8 hr on 103a-F plates. Qualitative depolarizations were determined by the method of CRAWFORD and HORWITZ [9], using 4358-A excitation. The observed frequencies are given in Table 1; they are believed to be accurate to -&3 cm-l for sharp bands.

Discussion of results

It must first be established whether the compound is an isocyanate or a cyanate: PO(NCO), or PO(OCN),. ANDERSON [S] did not offer convincing evidence for preferring the isocyanate structure. Fortunately group frequencies provide strong support for it, as summarized in Table 2. For the cyanate (-0-CrN), no funda- mental frequencies are expected between 1200 and 2000 cm-l. For the isocyanate (-N&LO), the pseudo symmetric stretch is expected near 1400 cm-l. For both PO(NCO), and P(NCO),, two strong bands are found at 1420-1455 cm-l. They are good evidence for the isocyanate structure. These results are similar to those for

Si(NCO), and Ge(NCO), [l]. (The 1282 cm- l band in PO(NCO), is the P=O stretching frequency).

The only reasonable symmetries for PO(NCO), are: (1) C,,, derived from structures a, c, or d of Fig. 1;

(2) C,, derived from structures b or e of Fig. 1; and (3) C, (no symmetry).

[s] F. A. MILLER and R. G. INSKEEP, J. Chem. Phya. 18, 1519 (1950); F. A. MILLER, L. R. COUSINS and R. B. HANNAN, Ibid. 23, 2127 (1955).

[9] B. L. CRAWFORD, JR. and W. HORWITZ, J. Chevn. Phys. 15, 268 (1947).

The infrared, Raman and ultraviolet spectra of P(NCO), and PO(NCO),

Table 1. Raman and infrared spectra of PO(NCO),

1315

Raman Infrared Assignment

A; Int. P Y Int. (cm-l) (cm-‘)

301 385 425 513 577 603

704 6 P

1282 6 P

1453 10 P

4 dp 4 dp 4 dp 2 P 9 P 1 dp

301

429

600 vs

620 V8

761 vs. b 984T m

1202t W

1283 8

1429 8

-2217* w, sh 2265* vs 2320* W

2552 VW

2664 VW

2750 VW

-2846 VW, ah

-2963 VW, sh 3016 m

-3071 VW, sh -3549 VW, sh

3679 m 4429 vw

m

S

e fund. e fund. e fund. al fund. a, fund. e fund. e fund. a, fund. e fund. Imp.? 385 + 600 = 985 Imp.? 2 x 600 = 1200 al fund. e fund. a, fund. 1453 + 761 = 2214 e fund. a, fund. 2 x 1283 = 2566 2265 + 301 = 2566 1283 + 761 + 620 = 2664 1429 + 2 x 620 = 2669 2320 + 429 = 2749 2 x 1429 = 2858 2265 + 577 = 2842 2265 + 704 = 2969 2320 + 704 = 3024 2265 + 761 = 3026 2320 + 761 = 3081 2265 + 1283 = 3548 2265 + 1429 = 3694 (9) 2265 + 1453 + 704 = 4422 (9)

w = weak m = medium s = strong v = very sh = shoulder b = broad - = approximately

* Determined from a CCI, solution. The 3016 cm-l and 3679 cm-l frequencies, determined for the pure liquid, were observed at 3019 cm-l and 3670 cm-l in a CCI, solution. We conclude there is no large shift on going from the pure liquid to a CCI, solution.

t Intensity is variable. Probably an impurity.

The symmetry classification of the vibrations for C,, is summarized in Table 3. (Herzberg’s conventions [lo] are used throughout this paper). If the symmetry is reduced from C,, to C,, species a, and a2 coalesce, causing the two inactine a2 vibrations to become allowed. It will be virtually impossible to distinguish between C’s and C,, from the vibrational spectrum. We shall adopt C,, because, as will be seen shortly,

[lo] G. HERZBERG, Infrared and Raman Spectra of Polyatomic Molecules. Van Nostrand, New York (1945).

1316 F. A. MILLER and W. K. BAER

Table 2. Group frequencies for cyan&es and isocyanates (cm-l)

Predicted -0-C-N -N=C=O

Observed PO(NCO), P(NC0 )3

-2250 ~2250 2320 2300 2265 2230

< 1200 -1400 1453 1430 1429 1421

Table 3. Fundamental vibrations and frequencies for PO(NCO), and P(NCO),.

(C,, symmetry)

Species Activity No. Schematic description*

a1 R(p), I.R. 1 -N=C=O pseudo antisym. stretch 2 -N=C=O pseudo sym. stretch 3 P=O stretch 4 P-N sym. stretch 5 Deformation A 6 Deformation B 7 PN, sym. deformation

a2 -, - 8 Deformation A 9 Deformation B

e R(dp), I.R. 10 -N=C=O pseudo antisym. stretch 11 -N=C=O pseudo sym. stretch 12 P-N degen. stretch 13 Deformation A 14 Deformation A 15 P=O wag 16 Deformation B 17 Deformation B

18 PN, degen. deformation

Assignment PO(NCO), P(NCO),

2320 2300 1453 1430 1283 -

704 699 577 588 513 ? ? 362 ? ? ? ?

2265 2230 1429 1421

761 681 620 603 600 577 429 -

385 388 301 316 ? 291

t t * Deformation A is a bending P-N=C=O

4 4

t t Deformation B is a bending P-X=C=O

J- 1

The inframd, Raman and ultraviolet spectra of P(SCO), and l’O(KCO), 1317

there are not even enough observed bands for it. The case for C, is far worse. It allows twenty-been f~ndamen~ls to be both Raman and infrared active, which is so different from obs~~ation that we shall not consider C, further.

For CsV, seven Raman active f~~r~darn~ntals may bc polarized. We have observed five polarized lines, and know that there must be a sixth one belonging to an -NC0 stretch near 2300 cm-r. C,, allows sixteen of the eighteen fundamental frequencies to be active in both the R.aman and infrared spectra, but only four coincidences have been observed: 361-301, 425-42’3, 603-600 and 1282-1283 cm-‘. C, symmetry allows eighteen funda~~entala to be both infrared and Raman active, and permits nine of these to be Raman polarized. Thus C, does not fit the obviations well, but C, is somewhat worse. We conclude that ~O(~GO9~ is probably C,,, but that for some unknown reason several of the fundamentals which are formally allowed in both the infrared and Raman spectra actually appear only in one or the other of them. This happens to a certain extent in most molecules, but seldom to the degree found for PO (NGO),.

Our assignments for the fundamentals are given in Tables 1 and 3. There is no need to discuss them in d&ail, but a few comments may be appropriate. The sche- matic descriptions are often only app~ximatio~, of coursesc, but they are conve~ent labels. They may not always be ~sociated with the proper frequency; it is quite possible that their order should be changed in some cases.

The two fundamentals in the 2300 cm-r region will be mentioned first. Surely the 2265 cm-i infrared band belongs to one of them because of its intensity. The other could be either 2320 or 2217 cm-l. We favor the former for two reasons:

(1) The 2320 cm-1 value corresponds better with bands found for Si(NGO), (2347 cm-l, p), Ge(~CO)*(2304 cm-l), and P(NGO)~(2300 cm-“, p).

(2) The 2320 cm-r band cannot be explained as a binary combination, whereas 2217 cm-r can. Also 2320 cm-r is useful in accounting for the 2750 and 3071 cm-r hands. We assign 2320 cm-l to species a, and 2265 cm-i to species e by analogy with P(NGO),. In this compound, as will be seen in Table 4, the lower of the two fundamentals, 2230 cm-l, is depolarized in the Raman effect and very strong in the infrared. The higher, 2300 cm-l is Raman polarized and weak in the infrared.

This leaves five polarized Raman lines for t*he six rem&~ing c%i fundamen~ls. The missing one is believed to be t#he lowest frequency in the species, the PNa defor- mation. The P==O stretch is at 1283 cm--l, in good agreement with POGl,( 1295 cm-r).

Tn species e there are fundamentals at 620 and 600 cm-i which have both been attributed to deformations. These frequencies may seem too close for vibrations in the same species and of the same t,ype, but the two deformations occur in mutually perpendicular planes and so will not interact with one another.

The bands at 984 and 1202 cm-i became weaker upon purification but never di~p~ar~(~ completely. Alignment for them, and for the remaining bands, will be found in the t,ables. There are two allowed fundamentals for which no frequencies were observed, even though a vigorous search was made. The spectrum is remarkable in two respects : in having relatively few Raman-infrared coincidences, and in having no combination tones below 2000 cm-*. The bands are either intense or are cnm- pletely missing.

1318 F. A. MILLER and 5%‘. K. I~AER

If the symmetry were really C,, two more allowed fundamentals would be missing, both of which would give polarized Raman lines. C,, provides the better fit with observation, but unfortunately C, cannot be rigorously eliminated.

Propertim and preparation

Phosphorus isocyanate is a colorless substance which melts at --2’ and boils at 169°C. It reacts readily with moisture, but can be handled easily in a dry atmosphere. We found, in ag~ment with others [ 1 I], that it exhibits some peculiar phase changes. The sample slowly solidifies on standing. Presumably the sample is polymerizing, since the solid will revert to the original liquid on warming.

Frequency, cm-’

Fig. 3. Infrared spectrum of P(NCO),.

a = O-060 mm: b = capillary film; c -A 0+050 mm; d = solution in WI,. l&man bands arc indicated at

the top af the figure.

The sampIe was prepared by the method of FORBES and ANDERSON [I I]. 25 ml of freshly distilled PC& was slowly added to 100 g of finely ground AgNCO in 130 ml of warm benzene with continuous stirring. After 30 min of refluxing, the solution was filtered and the ~maining PCl, and benzene were distilled off at atIno~~heric pressure. Several distillations at reduced pressure were made to purify the crude product, Approximately 10 ml of pure P(NCO), was obtained. The sample was gas-chromato- graphed through a column of Apiezon L on “Fluoropak 80”. With freshly distilled samples, a single peak was observed. Samples a few days old gave a shoulder on the main peak which grew as the P(NCO), aged. All spectroscopic me~urements were therefore made on freshly diatilled samples.

The equipment for ob~ining the infrared ~~ctrurn was the same as for PO(~CO~~. The spectrum is shown in Fig. 3, and the observed frequencies are given in Table 4.

Some unusual temperature-dependent changes in intensity and frequency were noted in the 300-400 cm-* region of the infrared spectrum which introduce some doubt as ~"-~ [Il] CI. S. FOKBES and f-l. H. ANDSBSON, J. Am. G&m. SW. 82, ‘761 (1940); I-x. H. ANDEBS~N.

r&x 7% 193 (1950).

The infrared, Raman and ultraviolet spect~ra of X’(NCO), and PO(NC’O),

Tab10 4. Raman and infrared spcctm af P(NCO),

1319

Raman

AF (Cm-“) XnL

- _” ._.- . --- _ .._ “_ ^_._ -..

291 4

362 6

N 381 sh

577 5b

588 9

675 rt: 10 sh

699 6 asym.

1178 0

1430

2230 I dp 2300 I P

I(9

Infrared

G

P (cm-~“) Int. ~~~ignnlent __._ _ __._“._ -I----

dP 316

P h 365 dp? 388 dp 577

p 603

--. 681

1’

111

w, sh

m vs

VI3

vs, b

I__ m-1179

1202

1282

1421

p 2008

VW, sh

W

W

V8

VW

2190* 2239* 2293+ 2520 2587

W

VB

w

VW

\‘W

-2812 VW, sh 2927 m 3623 m

-3693 \‘w, sh 4334 VW

e fund. e fund. a1 fund. e fund. e fund. a, fund. e fund. e fund. a, fund.

i 603 -i- 577 = 1180 2 x 588 = 1176

2 x 603 = 1206

i 681 + 603 = 1284 699 -I- 588 = 1287

e fund. a, fund.

! 1421 -1. 588 = 2009

1430 i- 577 = 2007 1421 i- 2 x 388 = 2197 e fund. a, fund. 2230 i- 291 = 2521 2230 + 362 = 2592 2300 -7 291 = 2591 2230 -L 588 = 2818 2230 i 699 = 2929 2230 i- 1421 = 3651 (?) 2300 + 1421 = 3721 (P) Many ternaries

w = wertk m = medium 8 - strong v 2 very sh = shoulder b 7 broad

* Measured in CCI, solution. The 2927 cm-.’ and 3623 cm-* frequencies duterminod for the pure liquid were observed at 2928 cm”’ and 3631 cm- 1 in a (XI, solution. \Te conclude there is no large shift on going from the pure liquid to a CCI, solution.

to the v~lidjty of the results there. Knowing t,hat there is a Raman fine at 362 cm-r, we wanted to get a better mea~uremcnt of a weak shoulder observed near 365 cm-1

in the infrared to see if it really is coincident with the Raman frequency. WC had hoped that by going to the solid, the bands might sharpen and the shoulder be resolved. The vapor was condensed as a solid deposit on a cold window in a conven- tional cold cell at 20°K. A new band appeared at 350 cm-’ ; the 388 cm-1 and ~36.5 cm-r bands completely disappeared, but the 316 cm-’ band shifted only to 318 cm-r and seemed to be intensified. A similar effect was observed when a 0.025 mm liquid sample was frozen by cooling to approximately -75°C. &Again, the 358 and 365 em-1 bands di~ap~ar~~~, a new band at, 350 cm-l appeared, and the band near 318 cm-’ increased several fold in jn~~~~it~. However when the aample was put

3

1320 F. A. MILLER a,nd W. K. BAER

in CH,Cl, solution and cooled to -75”C, the 388 cm-r band did not disappear, although the other changes occurred. They could be reversed by bringing the solution back to room temperature. We believe that these variations are due to the peculiar phase change, which we suspect to be a polymerization. Naturally one wonders whether any of the room temperature frequencies reported in Table 4 are the result of the “polymerization”. To test this the infrared spectrum of a warmed sample was obtained; it was identical with the room temperature one.

Table 5. Expectations for various models of P(NCO),

(See Fig. 1 for the structures)

C$ 8 a [R(p), I.R.] + 8 e [R(dp), I.R.].

c : CT;;:

6 aJR(p), I.R.1 + 2 a,[-, -_I + 8 e [R(dp), I.R.].

5 a’ [R(p), -1 + 3 a” [-_, I.R.] + 6 e’ [R(dp), I.R.] + 2e” [R(dp), -_I

Dab: 3%’ [R(P), -1 + 2%’ [-, -1 + 3 +” [-, I.R.] + 6 e’ [R(dp), I.R. ] + 2 e” [R(dp), -_I.

R = Raman active I.R. = infrared active - = forbidden

c3 c 37) c 3h D3h Obsv’d

No. of Raman-active fundamentals 16 14 13 11 10* No. of infrared-active fundamentals 16 14 9 9 s-9* No. of Raman-infrared coincidences 16 14 6 6 2-6* No. of polarized Raman lines 8 6 5 3 _

No. of polarized Raman lines also 8 6 0 0 1-i infrared active

* 1178 cm-r not included. It cannot be a fundamental because of its numerical value.

Raman results, also summarized in Table 4, were first obtained photographically with Hg 4047,4358 and 5461 A excitation. The sample was kept warm (>5O”C) in order to minimize the “polymerization” which produced particulate matter that enhanced the background scattering. Sample volume was 4 ml and exposure times were comparable to those for PO(NCO),.

The Raman spectrum was also obtained with a Cary-81 photoelectric spectro- photometer (4358-A excitation) using a 5-ml sample. Several new features were noted that were not detected photographically. A band near 381 cm-l (depolarized?) was observed as a shoulder on the 362 cm-l frequency. Also, a definite asymmetry was observed on the low-frequency side of the 699 cm-l band, indicating the possi- bility of another band in this region. Attempts to resolve the band using a narrow spectral slit width were unsuccessful. The polarizations of all Raman lines except 675 and 1178 cm-l were obtained; these two were too weak for measurement.

It will be recalled that our combined error in measuring “good” bands in both the infrared and Raman spectra is thought to be 45 cm-l at most. Consequently we believe that 1430 and 1421 cm-l are due to different vibrations. On the other hand 2230 (Raman) is paired with 2239 (infrared), and 2300 witch 2293 cm-l. In these two cases the infrared frequencies were obtained from Ccl, solutions because the bands were too intense even as capillary films. Since a comparison of two other bands of

The infrared, Raman and ultraviolet spectra of P(NCO), and PO(NCO), 1321

P(NCO), in pure liquid and in Ccl, solution showed shifts of 1 and 8 cm-l, we believe the above pairs may really be due to the same vibrations, and the frequency differences arise from the change of state.

Finally, there is no doubt that 577 and 588 cm-l are really two separate lines. The latter is strong and sharp, and overlays the broad 577 cm-l band. It is also strongly polarized, and the proper polarization exposure largely removes it and leaves the 577 cm-l line underneath.

Discussion of results The spectrum of P(NCO), is surprisingly simple. It is therefore a temptation to

conclude that the molecule has a high symmetry until one realizes that its spectrum is rather similar to that of PO(NCO),, whose symmetry cannot be higher than C,,. Clearly, caution is necessary. Equally clear, however, is that C, need not be con- sidered.

It has already been demonstrated that the molecule is an isocyanate and not a cyanate (Table 2). Fig. 1 shows the five structures which will be considered. The classification of the fundamentals and their selection rules for each structure are summarized in Table 5, together with our observations. Superficially, agreement seems best for D,, or C,,. DSh can be eliminated, however, because it has only three totally symmetric fundamentals, whereas five polarized Raman lines were observed. One can be sure that none of these five are combination tones because combinations are relatively weak in the Raman effect, whereas four of the five polarized lines are strong. For the fifth one, at 2300 cm-l, no binary combination is possible. Conse- quently the D,, model (planar trigonal structure) can be eliminated with assurance. Note that the argument is based on features seen in the spectrum, not on features that are missing.

Cab symmetry (the planar pin-wheel structure) can also be eliminated on the basis of two arguments. Under this symmetry the normal vibrations of P(NCO), divide as follows among the species. (Selection rules are added for later use.)

5a’[R(p), -_I + 3a”[--, I.R.] + Be’[R(dp), I.R.] + Be”[R(dp), -_I.

First, this symmetry requires that no polarized Raman frequency should appear also in the infrared spectrum (except by accidental coincidence or by the breakdown of selection rules in a condensed state). The 362 cm-l band does appear weakly in the infrared, and there may also be a coincidence at 2300 cm-l. The second argu- ment concerns the infrared bands at 1179, 1202 and 1282. These have been ex- plained as the following binary combinations:

Combination C,, symmetry ____-

1179: 603 + 577 = 1180 a” x e’ = E”

2 x 588 = 1176 a’ x a’ = A’ 1202: 2 x 603 = 1206 a” x a” = A’

1282: 603 + 681 = 1284 au x e’ = E”

588 + 699 = 1287 a’xa’=A’

Infrared selection rule

___ Forbidden for C,, Forbidden for C,, Forbidden for C,, Forbidden for C,, Forbidden for C,,

Let us now consider whether these would be allowed by C,, symmetry. The 603 cm-l band appears only in the infrared ; it is therefore probable that it would be an a”

1323 F. A. MILLER and \\*. K. BAER

fundamental. The 588 and 699 cm-l bands are Raman active and polarized, so they would surely be a’ fundamentals. Finally, 577 and 681 cm-1 appear in both spectra, so they would be attributed to e’. These symmetries, and their direct products, have been indicated for the above combinations. In every case the combination is forbid- den in the infrared. The inference is that C,, selection rules do not apply. These two arguments are perhaps not convincing individually, but taken together they have convinced us that C,, symmetry is improbable.

The next symmetry to be considered is C,, (t,he trigonal pyramid structures). This was the favored symmetry for PO(NCO),. Since the only change on going from P(NCO), to PO(KCO), is to add a l’=O stretch and a doubly degenerate P=O bending mode, the assignments for the two molecules should be rat.her similar. That t,hey can easily be made so is shown in Table 3. There is no need to discuss the choices in detail. Both 316 and 388 cm-l have been used, although they are suspect because of their sensitivity to temperature changes. Because the 388 cm-’ band disappeared on cooling, it might be the difference tone 681 - 291 = 390 cm-r. However the corresponding sum tone at about 681 + 291 = 972 cm-i should thenalso be observed. Since it was not, this possibility is rejected.

The argument against C, symmetry is exactly the same as for PO(NCO),. The two a2 frequencies merge with the a, species and become spectroscopically active. There is already a shortage of observed frequencies for C3”, and the situation would become somewhat worse for C,, In other words there is no need to postulate C3, and doing so would introduce additional difficulties. We conclude that C,, symmetry best. fits the observations.

The vibrational spectra of PO(NCO), and P(SCO), indicate that both of these molecules have at least a three-fold axis of symmetry. This is shown by the finding of only two -N-C=0 stretching frequencies near 1450 cm-l in each molecule. Also the spect,ra should be much richer in bands if there were no degeneracy. Cer- t,ainly the results are incompatible with C, symmetry.

For PO(XCO), the data fit C,, symmetry best; C, is less satisfactory. For P(NCO),, D,, can be eliminated rather definitely, and C,, also but with less assurance. C,, is acceptable; C, again is less satisfactory. We conclude that C,, is the most probable symmetry for both molecules? although the evidence is not. as convincing as we had originally hoped it would be.

Assignments for the two compounds an the basis of C,, symmetry are quite similar. For each there are many vibrations which are formally allowed in both the Raman and infrared spectra which actually a.ppear in only one or the other of them. This seems to be t,he pattern wit,h isocyanates. MILL~~R and CARLSOK [l] found with Si(SCO), that four of the sixf, vibrations appeared only in the infrared spectrum in spite of being also Raman allowed. This leads to the unhappy conclusion that it may be unusually misleading to deduce structures from selection rules for isocyanates. Certainly it will be much safer t,o argue from observed spectral features than from t,he absences of bands.

Finally it should be noted that our original question of whether the P-N==C=O groups are linear or bent is still not set,tled. They can be either and still give C,, symmetry (Figs. lc and Id).

The infrared, Raman and ultraviolet spectra of P(XCO), and PO(NCO), 1323

ULTRAVIOLET SPECTRA

The ultraviolet spectra of P(NCO), and PO(NCO), were obtained as methylene chloride solutions on a Gary model-14 spectrophotometer. The only absorption shown by P(NCO), is due to a band whose maximum is below the cut-off point of the solvent (~2300 A). For PO(NCO),, a band was observed at 2660 ,k (urnax ‘v 211. mole-l cm-l). Although it is possible that this band is due to an impurity, attempts at further purification of the sample gave no change in the intensity of the band. We therefore believe that it is due to PO(NCO),.