Spectrochimica Acta, vol. 26A, pp. 1677 to 1588. Pergamon Press 1969. Printed in Northern Ireland

The infrared and Raman spectra of (CF8),C==C=0, (cF,),C=N=N and (CF,),C=NH*t

FOIL A. MILLER and F. E. KMAT Department of Chemistry, University of Pittsburghs

and

Mellon Institute, Carnegie-Mellon University, Pittsburgh Pennsylvania 15213

(Received 21 October 1968)

Abstract-The infrared and Raman spectra of (CF.J&=C=O, (CFs),C=N=N, and (CF.&&= NH are reported and assigned. C,, symmetry is satisfactory for the first two substances. The torsions could not be located. Our data, when compared with that for hexafluoroacetone, indicate that there is no need to assume C, symmetry for the latter as concluded by Berney; C,, is equally satisfactory.

INTRODUCTION

PREVIOUS work in our laboratory has dealt with the vibrational analysis of molecules containing trifluoromethyl groups, and with the evaluation of the barriers to internal rotation for CF, rotors [l-4]. We initially undertook a study of the low frequency infrared spectra of bis (trifluoromethyl) ketene, (CF,),C=C=O, bis (trifluoromethyl) diazomethane, (CF,),C=N=N, and hexafluoroacetone imine, (CF,),C=NH, in order to investigate the barriers to internal rotation of the two CF, rotors in these molecules, thus extending earlier work on molecules having two CH, rotors attached to the same atom [5]. Unfortunately the torsional vibrations could not be observed. We give here the hitherto unreported infrared and Raman spectra of these molecules, since they continue our interest in the vibrational analysis of compounds containing CF, groups or containing several multiple bonds. The vibrational assignments are made on the basis of a C,, representation for the (CF,),C!=X=Y molecules, which proves to be satisfactory. The correlation of the observed vibrational frequencies of these three molecules with those of hexafluoroacetone, (CF,),C=O, clearly requires an alternative assignment for one of the vibrational modes of (CF,),C=O. This change removes the main reason for BERNEY’S conclusion that the conformation of hexafluoro- acetone is C, [4].

* From a thesis to be submitted by F. E. Kiviat in partial fulfillment of the requirements for the degree of Doctor of Philosophy at the University of Pittsburgh.

-78;5? IS work was supported by the National Science Foundation under Grants GP-5050 and

$ To which correspondence should be addressed.

[l] R. L. REDINGTON, J. Mol. Spectvy 9, 469 (1962). [2] C. V. BERNEY, L. R. COUSINS and F. A. MILLER, Spectrochim. Acta 19, 2019 (1963). [3] C. V. BERNEY, Spectrochim. Acta 20, 1437 (1964). [4] C. V. BERNEY, Spectrochim. Acta 21, 1809 (1965). [5] W. G. FATELJZY and F. A. MILLER, Spectrochim. Acta 18, 977 (1962).

1577

1578 FOIL A. MZLER a& F. E. KIVIAT

(CF,),C=C=O is a colorIess liquid which boiIs at 5’C. It was first prepared by ENGLAND and KRXSPAN [6], and our sample was a gift from Dr. C. G. Krespan of E. I. du Pont de Nemours and Co. The sample contained a small amount of POF,. Attempts to purify it by vacuum line techniques produced no change in the infrared spectrum and it was therefore used as received. Contrary to a previous report [?J, we found that (CF,),C==C=O is stable in the gaseous state. A sample was kept in a standard 10 cm gas cell with KBr windows at a pressure of 40 torr for periods of up to eight hours without any change in the infrared spect~m.

(CF,),C=N=N, a yellow liquid which boils at 13°C was prepared according to the method of GALE et aE. [8]. The sample was purified by means of trap-to-trap distillation on a vacuum line, followed by passing the vapor through a dephleg- mator at -78°C [9]. The (CF,),O=N=N condensed in the receiver, while the major impurity, (CF~)~C=NH, remained a solid on the walls of the dephlegmator. Gas ~hromato~aphy at 0°C with Chromasorb P wet with silicone oil showed impurities amounting to less than 0.7%.

(CF,),C=NH, a colorless liquid which boils at 16”C, was prepared according to the method of MIDDLETON and KRESPA~ [IO]. The sample was purified by means of several trap-to-trap transfers on a vacuum line, Gas-liquid chromatography at room temperature and at 0°C with Chromasorb P wet with silicone oil showed impurities amounting to less than O.7o/o.

Infrared spectra of the vapors were obtained from 33 to 4000 cm-l with Beckman IR-9 and IR-11 spectrophotometers. A Beckman 10 m variable path gas cell with polyethylene and pol~ropylene windows was used from 33 to 450 cm-l, and a 10 cm gas cell with KBr windows was used from 400 to 4000 cm-r. Sample pressures were varied from 0.5 to 250 torr.

The Raman spectra of liquid (CF&C=NH and (CF,),C==C=O were measured with a Cary Model 81 Raman spectrometer using Hg 4358 A excitation. The samples were contained in 7 mm o. d. tubes cooled to -35°C by a technique described by MILLER and MATSUUARA [I 11. Qualitative depolarizations were obtained by the usual two-exposure method using appropriate cylinders of Polaroid wrapped concentrically about the double walled glass cylinder in which the sample tube was placed. The Raman spectrum of liquid (CF,),C=N=N was also measured with a Cary 81 Raman spectrometer, but using 6328 A radiation

[6] D. C. ENGLAND and C. G. KRESPAN, J. Am. C&m. Sot. 88,5582 (1966). [7] SW. NADZHLUUTDINOV, N. A. STOVOKHOTOVA and V. A. KARGIN, Russ. J. Phys. Chew. 40,

479 (1966). [$I D.M.GALE, W.J.&%IDDLETON andC.G.~~~~~~,J.Am.t7hem.Soc.87,657 (1965). [9] E. KIRELL, ~~n~~~o~ of Laboratiw ~s~~~t~t~o~ (Edited by E. C. LUMB), p. 260. Elsevier

(1963). [lo] ~.J.~DD~~o~ and C.G.KRESPAN,J. Org. Chem.30,1398 (1965). [ll] F. A. MILLER and I. MATSUBARA, Spectrochim. Acta 22, 173 (1966).

IR and Raman spectra of (CF,),C=C=O, (CF,),C=N=N and (CF,),C=NH 1579

from a He-Ne laser and the standard Cary co-axial laser excitation optics. The sample was contained in a sealed capillary tube.

Frequencies are believed to be accurate to f 1 cm-l in the infrared and to f2 cm-l in the Raman spectra unless the bands are very weak and/or broad.

3. Results

The observed infrared spectra are presented in Figs. 1 and 2. The frequencies and their assignments to vibrational modes are presented in Tables l-3. Table 4 summarizes the vibrational assignments. -

$00 ( , , , ( , , , ( , , , , , , , , , , ,

A: 60

CF 40

1' /c-c=0

=F,

A-tOTon B-3 Tar

Path Length tOcr

0 J l I I I I I

2800 2400 taoo (600 1400 1200 1000 800 600 (

n !. WC

I 1 I

‘rrl 10 Torr

‘ath Length 62m -

10 Ton Path Length 6.2171

I I I , 200 0

Fig. 1. Infrared spectra of (CF,),C=N=N and (CF,),C=C=O.

1. Point group

STRUCTURAL MODELS

There is no doubt that the >C==C=O and >=N=N groups are linear. This

is supported by numerous structural investigations of H,C=C=O, (CH,),C=C=O, and H,C=N=N [12]. The main question concerns the conformation of the CF,

F2] Tables of Interatomio Distances and Con&wation in Moleculee and Ions. The Chemioal Society (1958); Supplement (1965).

5

FOIL A. &fILLER and F. E. %X’IAT

A-3OTorr B- 3Tar 135 Tcfr

FatI Lnp,h mlh LM.#l tocm mm

Fig. 2. Infrared spectrum of (CF,),C=NH.

groups. If they are freely rotating, the effeotive vibrational selection rules will be those for C,,. If the internsl rots&ion is strongly hindered, the CF, groups could be oriented to give either Cao, C,, C,, or C, symmetry. It will be nesrly impossible to distinguish between these four cases from the vibrational spectrum. We therefore start by assuming the highest possible symmetry, G,,. One of the Czu conform&ions is pictured in Fig. 3.

In (CF,),C=NH, the C==NH group is undoubtedly angular at the nitrogen atom. The highest symmetry the molecule can have is 0,. However, it is con- venient to classify the (CF,),C==NH vibrations under C,, symmetry for ease of comparison with the two (CF,)&==X=Y molecules. The a, and b, species of C,, become the a’ species of C,, and the a8 and b, species of C,, become the a” species of c,.

2. Band contours

The principal moments of inertia given in Table 5 were c&ulated from the following assumed parameters : r(C-F) = 1.33 8, r(C--C) = X.51 8, r(C==C) = 1.31 A, r(C=N) = 1.32 d, r(C=O) = 1.16 A, r(N=N) = 1.12 8, r(N-H) = 1.09 8, /‘(C--&=-c) = 120“, ,/(F-C-F) = 109*28’, L(F-C-C) = 109”28’, and ,/(C=N-H) = 117*.

The moments were used to calculate the Badger-Zumwalt parameters for an asymmetric mole&e [13]. These parameters were then used to csJcul&e P-R separations for type A band contours, and Q-Q seprtrations for type B band contours. The results are inoluded in Table 5. Figure 4 illustrates the most ideal example of each type. Unfortunately not many of the observed bands can be classified with confidence in this way. The C type is not useful for assigning frequencies because it is featureless for these molecules [l3].

The moments of inertia are independent of the orientation of the CF, rotors about their C-C axes, Con~quently the C’,,, C,, C,, and G, co~orm~tions of (CF&C%=X=Y will have the same band shapes, and the contours will be of no aid in deducing the correct conformation.

[13] R. M. BADGER snd L. R. ZTJWVALT, 6. Chews. Phys. 6, 711 (1938).

- Table 1. Observed infrared and Raman spectra of (CF,),C=C=O

IR (g4 Ramen (liq. -3fYC) Band Rel. Rel. peak

am-’ tspe inten. cm-l inten. Polzn. Assignment

136 171 292

334 448

(473) (483) 640 650 555 679 693 668 728 763

(873) 91%

991

1029 1036 1081 109% 1135

1194 1282 1308 1313 1343 1417 1475 1493 1540 1685 1612 1636 1676 1718 1760 1787 1857 1882 1955 2012 2067 2110 2140 2194

2300 2323 2384 2410 2600 262% 2618 2642 2681 2762 2788 2830 2969 3386

A w w w

m

m

(II) w (I) w

m m

m w

w

m

A m B m

(II) x.2 VW

8

w

w

w

w

m

vs m 8 s

v-s s w

TV

w

w

VW

w

w

w w

w

w

m

w YW

m

w

w vs

w w

w

w

VW

VW

w w

VW

VW

w

w

VW

w

w

138 12

314 12 336 27 448 70

541 33

697 670

766

9 7

100

1167

1199

18

5

1420 39

2197 1%

3604

w, m, s = weak, medium, strong; v = very; P = polarized.

1581

Skefetal vibn. wzl CF, rook CF, rock CF, rock vIt

P CCC scissors Ye P CF, deform. v,

POF, POF,

P CF, deform. ye C=C=O bend (Ye,?) CF, deform. v18

P c-c sym. str. Yg POF,

728 + 314 = 1042 I417 - 334 = 1083 763 + 334 = 1097 1308 - 171 = 1137 CF, str. Ye,,

\

i

CF, strs.

P ’ C==C=O pseudo sym. str. v, 1308 + 171 = 1479 763 Jr 728 = 1491 991 + 560 =t 1541 1417 + 171= 1688 P 1343 + 292 = 1636 1343 + 314 = 1676 991+ 728 = 1719 1417 + 334 = 1761 1343 + 448 = 1791 2194 - 334 = 1860 1157 + 728 = 1886 1194 + 763 = 1967 1417 + 593 = 2010 1343 + 728 = 2071 1343 + 763 = 210% 1417 + 728 = 2146

P C =C =O pseudo ant&m. Sk. Y1 1313 + 991 = 2304 2194 + 136 = 2329 1194 x 2= 2388 1343 + 763 + 314 = 2420 1308f 1194= 2602 2194 + 334 = 2628 1313 + 1308 = 2621 2194 + 448 = 2642 1343 x 2 = 268% 1417 f 1343 = 2760 2194 + 693 = 2787 1417 x 2 = 2834 2194 + 763 = 2957 2194 + 1194 = 3388 2194 + 1417 = 3611

1582 FOIL A. MILLER and F. E. KWIAT

Table 2. Observed infrared and Raman spectra of (CF,),C=N=N

cm-’

IR (gd Ramen (liq., 25%) Band Rel. Rol. peak

tYpe inten. cm-’ inten. P&n. Assignment

103 147

326 460 496 624 545 662 595 642 720 740 764

~830 564 997

1030

1183 1210 1324 1361

1350 1440 1456 1631 1559 1652 1693 1711 1755 1507 1565 1966 1951 2040 2136

2200 2230 2296 2369 2412 2676 2660 2710 2731 2550 3327 3605

103 14 156 16 314 7 333 21 P 449 45 P 445 <1 625 <l

555 <l 695 10 643 <l

766 100 P

C =N =N bend CF, deform. Ye C=N=N bend (Y,,?) CF, deform. vIB 695 + 147 = 745 C-c sym. str. Yg 524 + 314 = 5351 720 + 147 = 567 C-C antisym. str. VI6 P

1061 <l 720 + 333 = 1063 1167 4 CF, star. vIo

1205 3

1360 4

1375 6 P

2136 4 P

Skeletal vibn. vIs Skeletal vibn. vpl CF, rock vIs (CF,),C scissor vg CF, deform. y, CF, deform. 2CF, deforms.

+

1 6 CF, strs.

1 C=N=N pseudo sym. str. vg 720 X 2= 1440 997 + 496 = 1492 ? 997 _t 595 = 1696 1167 + 495 = 1662 1350 + 314 = 1694 997 + 720 = 1717 1210 + 662 = 1762 1361 + 460 = 1511 1324 + 645 = 1572 1361 + 595 = 1969 1350 + 595 = 1975 1324 + 720 = 2044 C=N=N pseudo antisym. str. y1 1210 + 997 = 2207 2136+ 103= 2239 ? 1153 x 2 = 2366 1210 x 2 = 2420 2136 + 460 = 2656 1324 X 2 = 2645 1350 + 1324 = 27041 2136 + 595 = 2734 2136 + 720 = 2556 2136 + 1153 = 3319? 2136 + 1350 = 3516

w, m, s = weak, medium, strong; Y = very; P = polarized.

IR and Raman spectra of (CF,)&==C=O, (CF,),C=N=N and (CFJ,C=NH 1683

Table 3. Observed infrared and Raman spectra of (CF,),C=NH

cm-’

IR kw) Band R.31.

we inten. cm-1

Reman (liq. -36°C) Rel. peak

inten. P&n. Assignment

171 197

w

w

293 333 379 481 601 637 640 638 706 739

w

w m

B B

m m m

B m A 8

w

776 A w 927 B * 986 w

1023 w 1067 w 1076 w

1133 m

1196 1212 1261 1389

1407 1476 1621 1662 1636 1678 1702 1867 1986 2038 2104 2173 2332 2366 2424 2446 2611 2669 2627 2774 2888 2963 3089

3311 m 3368 w

169 202 282

334 381 481 606 637

638

3 28 10

26 17 26

7 7

18

177 927

100 6

1186

1260

4

7

1686 7 1706 31

3249 18 3304 26

CF, rook CF, rock vp CF, rock vI1 CF, rook

P ccc scissor Yg c ==N wag (Y,,?)

P CF, deform. Y? 2CF, deforms.

+ c==N wag CF, deform. vS CF, deform. v18 N-H out-of-plane wag

(v,,?) P C-c sym. str. VS

C-C antisym. str. VI6 ? 739 + 282 = 1021 ? ? 638 + 601 = 1139

CF, str. vto \

I SCF, strs.

706 X 2 = 1410 776 + 706 = 1481 1196 + 333 = 1628 N-H in-plane wag (vl, T) 1261 + 379 = 1640 1389 + 293 = 1682

P C =N str. vt 1389 + 481 = 1870 1212 + 776 = 1988 1662 + 481 = 2043 ? 1702 + 481 = 2183 1702 + 638 = 2340 ? 1212 x 2= 2424 1261 + 1196 = 2466 1261 X 2 = 2622 1 1702 + 927 = 2629 1662+ 1212= 2774 1702 + 1196 = 2897 1702 + 1261 = 2963 1702 $ 1389 = 3091 N-H Btr., i&~Ol.

P N-H str., intramol. vI N-H str., free

w, m, s = weak, medium, strong: v = very; P = polarized.

1584 FOIL A. MILLER and F. E. KIVIAT

Table 4. Approximate description of the fundamentd vibrations of (CF,),C=X=Y moleoulas (Cz, symmetry)

Species and Schematio \ \ \ ectivity NO. desoription c=c=o C=N=N C=NH*

/ / /

a1

R(P), IR GYP0 B)

%

R(dp.)-

bl B(dp.1, IR (tma A)

b,

R(dp.), IR (type C)

:, 3 4 6 6 7 8 9

10 11 12 13 14 16 16 17 18 19 20 21 22 23 24 25 26 27

C =X=Y paeudo antisp. str, 2194 2136 3311 C=X=Y pseudo ~yxn. str. 1417 1380 1702 CF, etr., sym. CF, str., originally degen. > + t t

C-c sym. str. 763 764 776 CF, deform., originally degen. 693 598 638 CF, deform., sym. 448 450 481 (CF,),C scissors 334 326 333 CF, rock ? ? 197 CF, str., origin&y degen. 1167 1167 1185 CF, deform., origin&y degen. N.O. N.O. N.O. CF, rock 314 314 282 CF, torsion N.O. CF, str.. sym. CF, str., origin&y degen. > f “‘:* “;“’ C-C *ntisym. str. 991 997 927 C=X=Y in-plane bend 668? 6421 16621 CF, deform., originally degen. 728 720 705 CF, deform., sym. CF, rock ! !

195 ik7

;

Sk&t& tibn. CF, str., originally degen. C =X =Y out-of-plane bend : :

: 7391

CF, deform., originally degen. $ CF, rock r r

Skeletal vibn. N:O. lb3 3.791 CF, torsion N.O. N.O. N.O.

* True syrnmetq is 0,. The dasariptive ns;mes are not &ways appropriate. + Region ~1200-1400 cm-.‘. f~;o~o~~O-J-~_om-?

. .;

In (CF,),C===NH the H &tom, which is situ&d off the C--N axis, will produce o&y a slight change in the orientation of the inertial s,xes relative to those for the (CF,),C=X=Y molecules. Consequently the band contours for vibrational modes involving the CF, groups and C-C bonds should be similar for all three molecules.

VIBRATIONAL ASSIG-NMENTS

The (CF,),C=X=Y molecules have 27 fundamental modes of vibration. Assuming C,, symmetry, the reduoed representation is 9a, + 4a, + 8b, + 6b,. The modes and our assignments are summarized in Table 4. It is impossible to assign all of the observed frequencies unequivocally, a-nd some of them are at- tributed only to a type of vibration rather than to a specific mode.

1. 4000-750 cm-l

In this region one expects the C==X=Y stretches, the C-F stretches, and the C-C stretohes. The correlation of vibrationtcl frequencies in the three molecules is not quite straightforward for two reasons. First, gaseous POE’, is present in the (CF,),C=C=O sample, and has absorptions at 1418 and at 994 cm-l which may mask bands of (CF,),C=C=O. Secondly, the C=N--H group modes do not correlate with those of C==C=O and C=N=N and must be considered separately.

IR and Raman speotra of (CF&C=C==O, (CE‘,)&=N=N and (CF&QNH 1686

Fig. 3. A C,, conformation of (CFs)&=X=Y. The A and B inertial &xes am shown; the C axis is perpendicular to the plane of the paper.

1.1 C=X=Y stretches. The pseudo antisymmetrio stretoh y1 of c==C=O and C=N=N is clearly the polarized Ra;msn line at 2197 and 2136 cm-l respectively. The C-F stretches of CF, groups, which are in the range 1160-1400 cm-l, are very weak in R&man spectra [2-41. Consequently the strong polarized R&man line at 1420 cm-l for (CF,),C=C=O is a good candidate for the C=C==O pseudo symmetric stretch. However it is necessary to consider the presence of POF,.

Table 5. Principal moments of inertia, Pfb separations of type A bands, and Q-Q separations of type B bands of (CF&C=C=O, (CF,),C=N=N, and (CF,),c=NH

(CF,),C=C=O (CF,),C=N=N (CF,),C=NH

Principal moments of inertia (a.m.u.-Aa)

IA 1, &?

366 479 667 364 479 647 222 479 522

Type A bmd PR separation

(cm-1)

10 10 10

Type 3 band Q-Q separation

(cm-1)

2 2 3

The observed’ infrared band at 1417 cm-l in the speotrum of (CF,),C=C==O is completely structureless, whereas the 1418 cm-1 infrared band of POF, has a definite P&R structure [14]. Also, for POF, the 1418 cm-l bend is leas intense than the 473 cm--l one, whereas for our sample the reverse is true. We therefore believe that (CF,),C=C==O has a band at 1417 cm-r, and we assign it to the C=C=O pseudo-symmetric stretch Q. (Some of the intensity may be due to POE’,.) Further evidence is that the Raman speotrum of liquid POF, has a line at 1395 cm-l (corresponding to the infrared band st 1418 cm-r of the vapor), whereas the R&man spectrum of liquid (CF,),C==C!==O has a line only at 1420 cm-l.

The C=N=N pseudo symmetric stretch is assigned to the polarized Ramsn line at line at 1378 em-1 for (CF,),C=N=N, although this line is of approximately the same intensity as the C-F stretches.

In (CF,)$===NH, the t&N stretch is undoubtedly the polarized Raman line at 1705 cm-l, and the N-H stretch is the polarized line at 3304 cm-l. Oddly, the NH

[14] H. SELIC and H. H. CLASSEN, J. Chem. P&p. 44, 1404 (1966).

1586 FOIL A. MILLER and F. E. KIVIAT

stretch scarcely changes on going to the vapor, for it is at 3311 cm-l in the infrared spectrum. This suggests that many of the molecules are hydrogen bonded in the vapor, probably by an intramolecular bond to a fluorine atom. In the vapor there is a weaker band at 3368 cm-i which we cannot explain as a sum tone. It may be due to some free N-H groups. In the Raman spectrum there is a second band at 3249 cm-l which we also cannot explain as a sum, and which may be inter- molecularly bonded N-H.

100

90

60

70

‘0 ; 60 E = ; 50

5 E 40 a

30

20

10

0

\

A

I I I

I I I 775 765 755

Wovenumber tn cm-1

Fig. 4. A; Example of & type A infrared band contour (720 cm-’ of (CF&C=N--- N). B; Example of 8 type B band contour (763 cm-l of (CF,),C=C=O).

The N-H wags are difficult to locate. (Deuteration should be helpful, but it was not done because this molecule was incidental to our main interest.) The in-plane wag is tentatively taken as 1662 cm-l and the out-of-plane one as 739 cm-l.

1.2 C-F &et&es. There will be six of these if the three-fold symmetry of the CF, groups is sufficiently altered. One will be an a2 mode and only Raman active. It can be identified for each of the molecules. For the other five stretches it is impossible to make specific assignments. (For two of the molecules there are only four available frequencies.) See Tables 1-3.

1.3 C-C stretches. (CF,),C=C=O has a strong infrared band at 991 cm-l. However the strongest infrared absorption of gaseous POF, occurs at 994 cm-l,

IR and Raman spectra of (CF,),C=C=O, (CF,),C=R’=N and (CF,),C=NH 1687

and its contour is similar to that of the 991 cm-l band in the spectrum of (CF,),C= C=O [la]. In the spectrum of (CF,),C=N=N there is a corresponding strong infrared band at 997 cm-l, and this similarity leads us to attribute both of these frequencies to the C-C antisymmetric stretch Q. These choices are not certain, although they do agree with the assignment of this mode to an infrared band at 972 cm-l in hexafluoroacetone [4].

The C-C symmetric stretch Ye should give a polarized Raman line. We assign it to bands at 766 cm-l in both (CF,),C=C=O and (CF,),C=N=N. They are the strongest Raman lines in the two spectra. In hexafluoroacetone the corresponding frequency is 774 cm-l [4].

(CF,),C=NH presents a problem because its strongest Raman line (777 cm-l, polarized), which we also assign to the C-C symmetric stretch Q, has a correspond- ing infrared band at 776 cm-l with an A type contour, whereas a B type contour is expected. A similar contradiction exists for the infrared band at 927 cm-l which is assigned expected, but a B difficulties.

2. 760-250 cm-l

as the C-C antisymmetric stretch yi6. An A type contour is type one is observed. We do not know how to circumvent these

In this region come the CF, rocks, the CF, deformations, the (CF,),C scissors, and the C=X=Y bends.

2.1 CF, rocks. Both (CF,),C==C=O and (CF,),C=N=N have a Raman line at 314 cm-l without any corresponding infrared band. These lines are assigned to the a2 CF, rock vi2. For (CF,),C=NH a Raman line occurs at 282 cm-l and an infrared one at 293 cm-l. It is a general rule that below ~350 cm-l gas phase frequencies are lower than the corresponding liquid phase ones [15]. Since the converse holds here, they are attributed to separate vibrational modes. The 282 cm-l Raman line of (CF,),C=NH is therefore assigned to the a2 CF, rock via. The infrared frequencies occurring at 292 cm-l in (CF,),C==C=O and at 293 cm-l in (CF,),C=NH are assigned to CF, rocks of undetermined species, but not a,.

2.2 (CF,),C sciseors. This vibrational mode belongs to species a,. The only polarized Raman frequency that is common to all three molecules in the region below 400 cm-l, where the scissoring mode is expected, occurs at 335 cm-l for (CF,),C=C=O, at 333 cm-l for (CF,),C=N=N, and at 334 cm-l for (CF,),C= NH. These are therefore chosen for the scissoring mode vg.

2.3. CF, deformations and C=X=Y bends. The region ~400-750 cm-l is expected to contain six CF, deformations and two C=X=Y bends. Of these, only two CF, deformations are permitted to occur as polarized Raman lines in the C,, representation.

The Raman spectrum of (CF,),C=C=O has two polarized lines, one at 448 cm-l and the other at 597 cm-l. These frequencies, as well as the corresponding infrared frequencies of (CF,),C=N=N at 450 cm-l and 698 cm-l, and of (CF,),C=NH at 481 cm-l and 638 cm-l, are assigned to the a, CF, deformations. The B type band contours of these bands in the infrared spectrum of (CF,),C=NH support this

[15] W. G. FATELEY, I. MATSUBARA and R. E. WITEOWSKI, Spectrochim. Acta 20, 1461 (1964).

1588 Forr, A. MILLER and F. E. fivwr

assignment. The infrared bands at 728 cm-l in the spectrum of (CF,),C=C=O, at 720 cm-l in the spectrum of (CF,),C=N=N, and at 705 cm-l in the spectrum of (CF,),C=NH have the A type band contour that is expected of a b, mode. Con- sequently, they are assigned to the b, CF, deformation yrs.

There is no band in the spectrum of (CF,),C=NH corresponding to the infrared bands at 668 cm-l for (CF,),C=C=O and at 643 cm-l for (CF,),C=N=N. These frequencies are therefore assigned as C=X=Y bends, although it is difficult to say whether in-plane or out-of-plane. (In the spectrum of gaseous dimethylketene an infrared band at 676 cm-l is attributed to the out-of-plane C=C=O bend [16].

3. Combination bands

Nearly all of the remaining bands can be satisfactorily explained as binary combinations as shown in Tables l-3. Whenever a difference tone is utilized, the corresponding sum tone is also observed.

4. Conclusion

Although C,, symmetry seems to be completely satisfactory for interpreting these spectra, the lower symmetries cannot be eliminated with certainty. The data do not allow the distinction to be made.

COMMENTS ON HEXAFLUOROACETONE

BERNEY concluded that (CFs),C=O has C, symmetry [4]. His chief argument had to do with the C=O wags. If the symmetry is CZU, both CL0 wags will be non-totally symmetric and will give depolarized Reman lines. If the symmetry is C,, one of the wags will be totally symmetric and will give a polarized Raman line. He assigned the polarized line at 318 cm- l to a wag, and concluded from the polarization that the symmetry is C,. Since we found strong polarized Raman lines at 335, 333, and 334 cm-i in our three compounds, it seems likely that they and 318 cm-l of (CF,),C=O are due to a mode which is nearly the same in all four molecules. This could not be the C=X wag because that would be very different in the four. We think the mode is the (CF,),C scissoring. This reassignment undercuts Berney’s main reason for preferring C, symmetry.

There was another reason which was based on the claimed polarized character of the 716 cm-l Raman band. This band was the weakest one reported, and our increased experience with the same equipment now makes us question whether it is in fact polarized. We regard this evidence as indecisive.

It is therefore our conclusion that one cannot distinguish between C, and C,, for hexafluoroacetone and that there is no reason for preferring C’,.

Acknowledgement-We me grateful to Dr. C. G. KRESPAN of E. I. du Pont de Nemours and Company for the sample of (CF,),C=C=O and for communicating to us details of the synthesis of (CF,),C=N=N not explicitly discussed in Ref. [8].

[16] W. H. FLETCDR and W. B. BARISH, Spectrochim. Acta 21, 1647 (1964).