Spectroohimica Acta, Vol. 28A, pp. 603 to 618 Pergamon Press 1972. Printed In Northern Ireland

Infrared and Raman spectra of cyclobutane and cyclobutane-ds*

FOIL A. MILLER and ROBERT J. CAPWELL? Department of Chemistry, Umverslty of Pittsburgh

Putsburgh, Pennsylvama 15213

and

R. C. LORD and DONALD G. REA$ Department of Chemistry, Massachusetts Institute of Technology

Cambridge, Massachusetts 02139

(Received 13 July 1971)

Abstract-New 1.r. and Raman measurements are reported for Gyclobutane and cyclobutane-de. They mclude 1.r. spectra on the gas at room temperature and on the sohd at about lOOoK. Raman spectra have been measured for the gas and liquid at room temperature and for the solid at 113’K.

These data allow the assignments (for D,,) to be put on a much firmer basis. Many of the tentative ones of LORD and NAKAQAWA have been conihmed and made more exact. Several of the fundamentals have been observed for the first time. All of the 21 spectroscopmally- active fundamentals for C,H, are well assigned except for two b, and one b, modes (we, rr,,, and

v15)*

A. INTRODUCTION

THE EVIDENCE in support of D,, symmetry for cyclobutane in its ground vibrational state is convincing. This conclusion stems from a wide variety of work : an electron diffraction study [l], assignment of the vibrational spectrum [2-4bJ, measurement of the pure rotational Raman spectrum [5], a normal coordinate calculation [6],

NMR studies [7, 81, and analysis of the pure ring puckering vibration in the Raman spectrum and of puckering frequencies deduced from mid i.r. combination tones [9-111.

* Based on theses submitted by Robert J. Capwell and D. G. Rea m partial fulflllment of the requirements for the degree of Doctor of Philosophy at the Umverslty of Pittsburgh and Massachusetts Institute of Technology.

7 Amencan Chemical Society-Petroleum Research Fund Fellow. $ Present address: Jet Propulsion Laboratory, California Institute of Technology, Pasadena,

California.

[l] 0. BMTIANSEN and P. N. SKANKE, quoted m A. ALMENNINGEN, 0. BASTIANSEN and P. N SKANEE, Acta Chem. Stand. 15, 711 (1961).

[2] G. W. RATHJENS, JR , N. K. FREEMAN, W D. GWINN and K. S PITZER, J Am. Chews. Sot. 75, 6634 (1953).

[3] D. G. REA, Ph.D. Thesis, Massachusetts Institute of Technology (1954). [4] D. A. Dows and N. RICH, J. Chem. Phya. 47, 333 (1967).

[4a] V. T. ALEKSANYAN, G. M. KUZ’YANTS, M. Yu. LUKINA, S. V. ZOTOVA and E I. VOSTOKOVA, J. Strmct. Chem. USSR 9, 123 (1968).

[4b] V. T. ALEKSANYAN, Opt. z Spektroskopya 29, 1075 (1970); Opt. Spectly 29, 574 (1970). [5] R C. LORD and B. P. STOICHEFF, Can. J. Phys. 49, 725 (1962). [6] R C. LORD and I NAKAQAWA, J. Chem. Phye. 29, 2951 (1963). [7] S. MEIBOOM and L. C. SNYDER, J. Am. Chmn. Sot. 99, 1038 (1967). [S] S. MEIBOOM and L. C. SNYDER, J. Chem. Phye. 52, 3857 (1970). [9] T. UEDA and T. S~NOUCE~I, J. Chem. Phys. 49, 470 (1968).

[lo] J. M. R. STONE and I. M. MILLS, Mol. Phys. 18, 631 (1970). [ll] F. A. M~LER and R. J. CAPWELL, Spectrochim. Acta 27A, 947 (1971).

1 603

604 FOIL A. MILLER ef. ah.

The most comprehensive vibrational study of cyclobutane prior to the present work is to be found in the thesis of REA [3], quoted by LORD and NAKA~AWA [6] REA obtained i.r. spectra of the gas and liquid, and Raman spectra of the liquid, for both C,Hs and C,Ds. However, the assignment of a large number of fundamentals, especially for C,D,, remain in doubt.

At the University of Pittsburgh the i.r. spectrum of the gas and the Raman spectrum of the liquid have been reinvestigated for both molecules. Also reported for the first time, and for both molecules, is the Reman spectrum of the gas (including depolarizations) and of the solid (at 113’K), and the complete i.r. spectrum of the solid at about lOOoK. These new data require some revision of the earlier assign- ments. Since REA’S thesis has often been quoted but never published, and since his sample of cyclobutane-d, was used for the recent measurements, this is presented as a joint publication from our two laboratories.

B. EXPERIMENTAL

1. Samples

Cyclobutane was purchased from Merck, Sharp and Dohme (Canada). The purity was determined by infrared and mass spectral analysis. Small amounts of CO, and other impurity(s)-probably hydrocarbons as suggested from mass peaks at 43, 42, and 29 m/e-were partially removed by vacuum transfer from a liquid-sohd CHsOH trap at -98°C to a liquid nitrogen one at -196°C. Although this procedure was not completely successful, the purity of C,H, is certainly greater than 99 %.

The cyclobutane-d, was prepared by Dr. D. G. REA [3]. The deuterium content of this material is about 97 %. A mass analysis of the sample gave the composition 83 o/o C,D,, 13 o/o C,D,H, and 3 o/o C,D,H, [5]. This rather high H content means that little confidence can be placed m the assignment of weak bands of the C,D, sample.

2. Infrared spectra

The i.r. spectra of C,H, and C,D, gas were measured from 200 to 4000 cm-l with a Beckman IR-12 spectrophotometer which was calibrated in the usual manner [12] All frequency measurements for C,H, vapor were obtained on samples contained in a 10 m variable path length cell fitted with KBr windows. For C,H, the spectral slit width was less than 1 cm-l over the entire region. For C,D, it was less than 2 cm-l. The frequencies are believed accurate to *0.5 cm-l for the sharp bands. The C,D, gas was examined in a 10 cm cell having CsI windows.

Both C,H, and C,D, were studied as solids in the usual type of low temperature i r cell [13]. The gas was slowly deposited onto a CsI window cooled by liquid nitrogen. The spectrum of the C,H, solid was obtained from 160 to 4000 cm-l. In the region between 160 and 300 cm-l, a Beckman IR-11 spectrophotometer was used. Solid C,D, was examined only from 400 to 4000 cm-l.

[12] IUPAC, Tables of Wavenumbers for the Calzbration of Infrared Spectrometers. Butterworths, Wmhmgton, D.C (1961).

[13] R C. LORD, R S MCDONALD and F. A. MILLER, J Opt Sot Am. 42, 149 (1952).

%T

%T

d

c-C4 H, GAS r 36bO I I LJI I

I 1 I I

10 3200 2600 2400 2000 1600 I

CM-'

605

I Infrared and Raman spectra of cyclobutane and cyclobutane-c&

I I I 800 600 ’ &ii- + IO

00

Fig. 1. Infrared spectrum of c-C,H, gas m 10 cm cell. (A) 180 torr. (B) 5 torr.

3. Raman spectra

Raman spectra were obtained with a Spex Ramalog instrument described elsewhere [14]. The instrument was calibrated with some of the strong atomic lines from the laser discharge. The excitation source was a Carson Laboratories Model 300 argon ion laser with an output power of between 0.9 and 1 .l W at the 4880 b line. For the recording of condensed state spectra the laser power was reduced to about 500 mW.

The gas cell was 8 cylindrical u v. type cell with a side arm. The flat windows were perpendicular to the laser beam, which was multipassed through the cell several times. The gas pressures for C,H, and C,D, were 730 and ca. 800 torr, respectively. The very strong totally symmetric modes were examined with a 1 cm-l spectral slit, and our reported Q branches are believed accurate to fl.0 cm-l.

Raman spectra were obtained for the liquid at room temperature and for the solid (low temperature phase) at 113’K. The low temperature sample holder has been described by MIILLER and HARNEY [15]. The accuracy of the sharp bands is thought to be fl.0 cm-l.

Quantitative polarizations were measured for both the gas and liquid

4. Resdts

The results for C,H, and C,D, are given in Tables 1 and 2, and in Figs l-4. The low frequency Raman spectrum of C,H, crystal undergoes a drastic change

[14] K.H. RHEE and F. A. I~b~~~,Spect~ochh Acta 27A, 1 (1971). [16] F. A. MILLER and B. M. HARNEY, Appl. Spectry 24, 291 (1970).

606 FOIL A. MILLER et&.

%T c-&D8 GAS

I

A t3 ’

%T ~,~~~ c-C,D, GAS

40000 1 3600 I 3200 I * 2600 2400 I_ 2000 1 I 1600 1600 I I I 1 I I

Cd

Fig. 2. Infrared spectrum of c-C,D, ges m 10 cm cell. (A) 200 torr. (B) 10 torr. The bands marked with * are due to lsotoplc lmpurltles. _ __,, __._ _~ -

CYCLOBUTANE GAS

CM-'

Fig. 3. Raman spectrum of c-C,H, gas. Pressure 730 torr. Spectral sht width 6 cm-l. Laser power l.lW at 4880 8. Time constant 1 sec. Scan speed 50 cm-l/mm.

Photon countmg. For re-run bands the gam WM decreased by l/10.

at about 142’K. Three lattice modes appear in the low temperature form at 245,

144 and 127 cm-l. The sudden appearance of the relatively strong 245 band is

particularly striking. The corresponding bands in ds are 195, 124 and 116 cm-’

RATHJENS and GWINN [16] report a phase transition for C,H, at about 145°K The

crystal structures of both forms are unknown [17].

[ 161 G. W.RATHJENS, JR.MI~W.D.GWINN, J. Am.Chem.Soc.75,5629 (1953). [17] G F. CARTER and D H. TEMPLETON, ActaCty~t.6,805 (1953).

Infrared and Raman spectra of cyolobutane and cyolobutane-ds 607

CYCLOBUTANE- dl GAS

$600 I I

1400

Cd

Fig. 4. Raman spectrum of c-C,D, gas. Pressure 800 torr. Spectral slit mdth 6 cm-l. Laser power 0.9 W at 4880 A. Tune constant 1 sec. Scan speed 50 cm-l/min. Photon countmg. For Fe-run bands at 1169 and CD, stretchmg region the gam was decreased by l/5 and for the 884 band the gam was decreased by l/10.

C. DISCUSSION OF RESULTS 1. ~~yrnmetry

The numerous transitions for the puckering mode which have been inferred from mid i.r. combination tones [9-111, and observed directly in the Raman spectrum of C,H, and C,D, vapor [lo, 111 show that the carbon ring is folded with an average dihedral angle of 35’. As a result of the relatively low barrier (518 & 5 cm-l for C,H, and 508 f 8 cm-l for C,D,), conversion from one stable conformation to the other is readily accomplished at room temperature. Strictly speaking, cyclobutane can therefore not be classified as a rigid molecule having D,, symmetry. When considering the inversion spectrum the levels must be assigned to species of the D,, group [IS]. However, the remaining normal vibrations can be approximately and usefully classified according to the D,, group.

Under D,, there are 6a, + 2a, + 3b, + 5b, + 7e fundamental vibrations. All but the a2 modes are Raman-allowed. Those of species b, and e are allowed in the i.r. The normal vibrations and our assignments are summarized in Table 3.

2. Band contours

a. Infrared. The b, modes give parallel i.r. bands. Their P-R separation was estimated by the method of SETH-PAUL and DIJKSTRA [19] to be 36 cm-l for C,H, and 30 cm-l for C,D,. Some data used in the calculation are given in Table 4. The contours of the species e i r. bands will vary depending upon the value of the Coriolis coupling constant zeta for each individual vibration.

b. Raman. In the gas phase the a, bands will have very strong, sharp Q branches. This is not conclusive proof for a totally symmetric band because degenerate modes can, by fortuitous accident, occasionally have a similar appearance. However the converse is true : a diffuse Raman line for a symmetric top molecule in the gas phase is strong indication of a non-totally symmetric vibration. This is at least as good a criterion as the depolarization ratio [20]. The contours of b, and b, modes should be similar-broad and diffuse, with a zero gap at the center. (The latter may be more or less filled if resolution is low.) The appearance of the degenerate modes may vary

[18] H C. LONGUET-HIGGINS, Mol. Phye. 6, 446 (1963). [19] W A. SETH-PAUL and G. DIJKSTRA, Spectroohim Acta 23A, 2861 (1967). [20] G HERZBERG, Infrared and Raman Spectra, p. 442. Van Nostrand, New York (1945).

FOIL A. MILLER et al.

9

k ui

a m

P-W-

Tebl

e 1

(um

t )

Ram

an

Infr

ared

Gas

(2

98’K

) L

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°K)

cm-l

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901

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926

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f

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Ta

ble

1

(co

nt.

)

Ram

an

Infr

are

d

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E (2

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K)

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uid

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+

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69=

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38

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g

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+

1004

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2977

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R

wit

h v

1 62

6 +

(2

X

1224

) =

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72

2987

+

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6 =

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65

2962

+ -j-

749

749

= =

3711

37

14

2962

+

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=

3863

P =

p

ola

rmed

, d

p =

d

epo

lnru

ed,

s =

st

ron

g,

m =

m

edw

n,

w =

w

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, v =

v

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, sh

=

Bh

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lder

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=

F

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son

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or

the

wsl

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11

Infrared and Raman spectra of cyclobutane and cyclobutane-d, 611

2 d

w d a

612 FOIL A. MILLER et al.

II II

E

:

: c

c

: 3 ’

3

3 d d

8% c-3 r* -.x3

V

3

I

Table

2 (

cont.

)

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an

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are

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Oes

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K)

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)

cm

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t.

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m.

cm

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ent

1

10

2044

2065

2

14

~2043

m.

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m

7 1169

+

884

=

2063

2

X

1042=

2084

2

X

1064~

2108

X

480)

+

1169

=

-2079

-2096

2109Q

21288

2131Q

1

2142

m,

nh

m,

sh,

b 8

8

m,b

2064

m

-2077

m,

b

2097

m

2118

2121 >

m

2130

m

?

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+

944

=

2113

V18

?

V.

-2173

2183 >

8,

b 2167

8

2188

m.

sh

2178

m

2

X

1084

=

2168

(2

x

6.5

61 A

1065

=

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’ ’

i 1169

+

884 +

167 =

2210

m

F

R.

mth

y1

(2

X

556)

+

1084

=

21967

~2227P

8,

b 22438

“B

2248Q

m

, sh

2254Q

m

. sh

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8

2219

8

2228

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2234

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Y,,,

and

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6

2242

m

2610

~2738

2796

29468

2969Q

N2969R

I

3190

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3266

w

w

w

m

2936

29

42

m

2948

I

’ ll

64+

1076=

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2

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1169=

2338

?

2178+

666=

27341

2243

+

666

=

2799

mpunty

TV

, vvb

2243

+

944

=

31137

-3280

m,

vvb

2234

+

1064

=

3298

2234

+

1066

=

3299

X

1086

=

2130

2062

2073

P

0.06

0

07

2043

3 s

h

1 ah

2sh

2098

2

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15

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)

2145

47

2161

10 s

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72

0 11

0 11

2140

69

2166

7

2166

4

33

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8 s

h

0 0

7

P

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5

2180.6

45

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6 s

h

-2224

2 s

h,

b

~2234

4sh

2247

42 b

2169

77

2178

9

W(P

)

0 08

2224

26

2238

60 b

P(T

) 2238

61 b

2

0 0

5

2326

2

0 09

-2

323

2b

2946

1

2944

1

2946

3

2963

2

2962

1

2962

2

3291 J

p

z pola

nee

d,

dp

=

dopola

nze

d,

B =

st

ron

g,

m

=

medm

m,

w

=

wea

k,

v

=

very

, sh

=

should

er,

b

=

bro

ad,

F

R.

=

Fen

m

Reso

nance

* F

or

the

awgn

men

t of

th

e

pure

ri

ng

puckem

S

frequenm

es

see

Ref.

[I

11

t Im

pullty

.

614 FOIL A. MILLER et al.

Table 3. Fundamental vlbratlons of cyclobutane and cyclobutane-d,

44, specms

Actlvlty NO Schematic desorlptmn Ass,gmnents

c-C,‘% (gas) c-C& (g=)

a1 R(P). -

0s -,-

b, Wdp). -

b, R(dp), 1 r (11)

W-b), I=. (J-j

1 CH, stretch, antlsym. 2974* 2210~ 2 CH, stretch, sym. 2906.t 21205 3 CH, sc1~sors 1468 67 1169 3 4 Rmg stretch 1004 5 884 2 6 CH, roclung 737** (~615) 6 Rmg puckermg 199 4 167 1

7 CH, twist 8 CH, wag

- -

- -

9 CH, wag 12347 1042** 10 CH, twist (1226) 926 11 Ring stretch 926 748

12 CH, stretch, antlsym. 2986 7 13 CH, stretch, sym 2945 14 CH, sc~sors 1463 9 16 Rmg deform&on 9991 16 CH, rock 626 6

2243 ? 1084 ? 480

17 CH, stretch, antlsym. 2965 18 CH, stretch, sym 2887 19 CH, swssom 1462 20 CH, weg 1260 21 CHI, twist 1224 22 Rmg stretoh 9014 23 CH* rock 749

2234 2129 1066 1054

944

( ) Calculated from Product Rule. * Inferred from resonanoe pau 2991-2962 t Inferred from rwonance pair 2931-2878. $ Inferred from resonance per 2247-2180 5 Inferred from resonance pen 2166-2073 7 Sohd state Raman frequency * * Llqud state Raman frequency

Table 4. Observed and calculated moments of m&la and contours of the parellel bands of c-C4H, and c-C4D,

c-C,DB

1. Assumed dlmenslons c-c C-H LHCH dihedral angle

2. Observed moments of mertla [5] 11 (amu-AZ)

3 Calculated moments of ma-ta 111 (amu-A2) 1, (amu-A2)

4. Calculated PR separation for 11 bands (cm-‘)

1.548 A 1 548 d 1.092 A 1.092 A llo” llo”

35O 35O

47.5126 65.8624

74 66 96.93 47.27 67 03 35 6 30.1

Infrared and Raman spectra of cylobutane and cyclobutane-d, 615

considerably, as in the infrared, depending on the value of the Coriolis coupling constant for the vibration.

Neither i.r. nor Raman contours were as useful in making the assignments as we had hoped.

3. Assignments

The assignments will now be discussed, using wavenumbers for the gas phase unless otherwise noted. C,H, and C,D, will often be referred to as d, and d, for brevity. Results from Teller-Redlich Product Rule calculations are given in Table 5.

Table 5. Teller-Redllch product rule for c-C,H, and c-C,D,

D2a species 7, calculated 7, observed

a1 4.000 -

a2 1.754 -

6, 2.000 -

62 3.736 -

e 6.340 6.337

a. Species al. These six frequencies are Raman polarized and i.r. forbidden. The CH, and CD, stretching regions are very complex, with both molecules exhibiting five medium to strong polarized bands. To explain this we must assume several Fermi resonance interactions. The totally symmetric CH, stretch us is inferred from the pair of bands at 2931.0 and 2877.8 cm-l. Considering the enormous intensity of the over- tone, 2vla, the unperturbed v2 level can confidently be placed at about 2905 cm-l.

The antisymmetric CH, stretching mode v1 and both v1 and vz for d, have been similarly assigned. However, the identification of the interacting combination tone is not as easily determined and the position of the unperturbed fundamental is less accurately known.

The va scissoring mode for d,, and da is at 1468.6 and 1169.3 cm-l respectively. For d, it was found only in the solid state spectrum, and it is considerably less intense than va for d,.

The ring stretching fundamental for d, is at 1004.5 cm-l and is assigned in d, to the 884.2 cm-l band. This agrees with previous work [2, 3, 61. The ring puckering (vg) is certainly 199.4 and 157.1 cm-l in d, and d, respectively. It has been discussed in detail elsewhere [ 111. This leaves vg, which is the only troublesome a, fundamental.

For d,, it is assigned to 737 cm-l from the liquid spectrum. The band appears to be polarized, although it is difficult to be certain because of serious over-lapping with the 749 band. We have no experimental value to suggest for vs in d,. Below 700 cm-l in the Raman spectrum of the liquid, using high gain, we find two very weak polarized bands at 671 and 366 cm-l. The one is too high and the other too low to comfortably be taken as vs. Possibly 671 and 366 cm-l could be the sum and difference tones vg f vs respectively. With the puckering fundamental vs firmly established at 157.1 cm-l in the gas phase, this would place vg at about 520 cm-l. However the Product Rule indicates a value of about 615 cm-l. Clearly the above explanation for the 671 and 366 cm-l bands is incorrect.

b. Species b,. These three bands are only Raman active. We attribute 926 in

616 FOIL A. MILLER et al.

d, and 748 in d, to vii. The 926 cm-l band is broad and has a distinct minimum at the center. This contour is consistent with that expected for an unresolved, non- totally symmetric and non-degenerate Raman band [20]. The high frequency component is the maximum of the O&x branch and the low frequency component is that of the sQx branch. This is the reverse of the usual situation because, in the energy expression for the sub-bands, the term involving the difference in rotational constants is negative for cyclobutane. For the 748 cm-l band of d, there is no corresponding doublet structure. This is not consistent with expectations, but since the band is only Raman-active we feel that it is a b, frequency.

The CD, wagging mode yg is given the liquid state value 1042 cm-l in d, It shifts to 1040 in the solid and does not have an infrared counterpart. There is no good candidate for vg in d,. For lack of a better alternative, we take the weak band at 1234 cm-l, which is observed only in the solid. For v10 the band of d, at 925 cm-l is a reasonable choice. Since nothing is available for vi,, in d,, a value is estimated from the Product Rule. It is 1225 cm-i, which agrees with the position calculated in Ref. [6]. If this is correct the band would be accidentally coincident with the e mode at 1224 cm-l. In our opinion the assignments of vg and vi,, for C,H, are the most dubious of all its spectroscopically-active modes.

c. Species b,. Fundamentals of this species should have parallel i r. contours with strong Q branches and P-R separations of 36 cm-l for C,H, and 30 cm-l for C,D,. Unfortunately, the contours were of little help because of considerable over- lapping of the fundamentals, and because of complex band structure resulting from upper stage bands involving the ring puckering mode.

The CH, antisymmetric stretch vi2 is readily assigned to 2986.7 and 2243 cm-l in d, and ds respectively. The symmetric stretch (via) is placed at 2945 cm-l in d,, but can not be located in d,. In d, this band is strong, and we have no doubt that it is a fundamental. STONE and MILLS [lo] also made this same assignment.

The CH, scissoring in d, is expected near 1450 cm-l. This region of the spectrum is very complex, but the data for the solid clarify matters. In the Raman spectrum of the solid there are three bands: 1439, 1444, and 1469 cm-l. The last does not appear in the i.r., so it has already been assigned to the a, mode vs. The other two are in the i r The one at 1444 cm-i is split in the i.r., which is a weak reason for assigning it to species e. This leaves 1438 cm-l for b,. They are all so close together that the actual choice makes little difference. On the basis of intensity the bands at 1453.9 in d, (gas) and 1084 in ds appear to go together and are assigned to the scissoring mode v14 The former band has a P-R separation in the i.r. of about 34 cm-r, and its Raman contour is suitable. STONE and MILLS [lo] provide other evidence that these bands belong to 13, modes.

LORD and NAK~GAWA (61 calculated the ring bend vi5 to be 998 cm-l. We have found in the Raman spectrum of the gas two weak bands on the low frequency side of 1004 cm-l. In the solid, only one of these remains. This one, 999 cm-l, is tentatively assigned to vls. The disappearing band is probably an upper stage band involving vg. It is odd that vis is not present in the i.r. No value is suggested for it in d,. This assignment has been questioned by KRAINOV et al. [21] on the basis

[Ql] E. P. KRAINOV, N. I. PROKOITEVA and L. M. SVERDLOV, Opt. z Spektroskoptya 16, 566 (1964); English translation: Opt. Spectry 16, 309 (1964)

Infrared and Raman spectra of cyclobutane and cyclobutane-ds 617

of their normal coordinate calculations. However, their treatment seems to offer no advantage over that of LORD and NAKA~AWA [S], and they give no new experi- mental evidence in support of it. On the other hand, the band 999 cm-l is of about the right intensity and frequency shift from the very intense ring breathing frequency yq at 1004.5 cm-l to result from the isotopic species C13C,12H,. We therefore do not regard the assignment as settled.

The remaining b, fundamental, the CH, rock (YJ, is readily found at 625.6 in d, and 480 cm-l in d, Multiple & branches are found on the R branches of both of them (and of 2243). STONE and MILLS have commented on the features found near 625 and 480 cm-l [lo].

d. Species e. We agree, for the most part, with the assignments made previously for the e modes of C,H, [2,3,6], except that yl, is put at 2965 cm-l. This is supported by the fact that in the i.r. this band splits in the crystal, as do four other e modes:

1452 (vr9), 1224 (Yap), 901 (Yap), and 749 (Ye.&. Dows and RICH [4] observed splitting of these modes at 21°K. Curiously, none of the corresponding Raman bands exhibited two components in the solid even when examined with a 1 cm-l slit. We have no explanation for this observation.

The 2887 cm-l band is taken as the CH, symmetric stretch Q,. The small P-R separation eliminates it as a b, vibration. The two CD, stretching modes are found at 2234 and 2129 cm-l.

The 1224 band of d, and the 944 one of d, have remarkably similar and unique contours in the Raman spectra of the gases, with especially deep minima at their centers. It is clear that they should be paired, and they are good candidates for yzl, the CH, twist.

The lowest e mode (Vet) is undoubtedly 749 in d, and 556 in d,, and the next lowest (Vet) 901 and 730 cm- l. The CH, scissoring (YJ has already been assigned to 1452 in d, gas (1443 in the solid). It is hard to locate in d, because of band overlap- ping, but we take 1065 for it. The CH, wag v2,, is thought to be the strong band in d, at 1260 cm-l in the i.r. only. Its counterpart in d, is taken as 1054. This region of the d, spectrum is also complex, but the solid shows three bands here that may be funda- mentals : 1034, 1049, and 1072. These have been assigned to r2,,, Q, and vi4 re- spectively.

4. Summary

A number of fundamentals of both d, and d, have been observed for the first time. Many of the tentative assignments of LORD and NAKACJAWA [6] have been confirmed. Some of the earlier ones for d, [3] have had to be interchanged. Gas phase frequencies are now known for most of the fundamentals.

Of the 23 distinct frequencies for C,H,, two are spectroscopically inactive and three others are uncertain (Ye, Q,, and Q,J. The other eighteen are now reasonably well established.

AcknowZedgement8-The work at the Umverslty of Pittsburgh was supported by Grant GP-9260 from the Natlonal Science Foundation, and the aoqmsltlon of the spectroscopic eqmpment was aided by NSF Instruments Grant GP-8287. The work at the Massachusetts Institute of Technology was supported by various NSF grants over a period of years, most recently by Grant GP-13473.

618 FOIL A. MILLER et al.

Note added mproof-Akksanyan et al. [4a] attribute the mfrared absorption of C,Hs m the region of 600 cm-1 to two different fundamental vlbratlons. This might worsen the agreement between the stattlstlcal and the calorunetrlc values of the entropy which was demonstrated by UEDA and SHIMANOUCHI [Q]. It is much more reasonable to explain the multiple Q branches between 638 and 684 cm-l as a single fundamental superimposed on various excited states of the pucker-

mg mode-1.e. as yls + 12~s - 1z~a. Many levels of vs are well populated [ll].