Vibrational spectra and normal coordinate analysis of ethyl cyanides

Xiang Zhu, Richard A. Farrer, Erez Gershgoren, Henry C. Kapteyn, and John T. Fourkas. The Journal of Physical Chemistry B 2004 108 (11), 3384-3386...
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Vibrational Spectra of Ethyl Cyanides

1129

Vibrational Spectra and Normal Coordinate Analysis of Ethyl Cyanides C. J. Wurrey,' Department of Chemistry, University of Missouri-Kansas City, Kansas City, Missouri 64 110

W. E. Bucy,* and J. R. Durig"' Department of Chemistry, University of South Carolina, Columbia, South Carolina 29208 (Received August 8, 1975) Publication costs assisted by the University of South Carolina

The infrared spectra of CH~CHZCN, CDsCHzCN, CH&D&N, CD~CDZCN, and CH3CH2l3CN have been observed in the vapor phase from 4000 to 200 cm-l. Both the vapor and solid phase spectra of the -do and -dg molecules were observed in the infrared region from 4000 to 150 cm-l. The Raman spectra of all five isotopes were observed in the vapor, liquid, andsolid phases from 4000 to 50 cm-l, with the exception of the solid phase of the 13CNisotope. An assignment of the 2 1 fundamental vibrations has been proposed, based on depolarization values, band contours, and group-frequency correlation. A normal coordinate calculation was also carried out in which 19 force constants were used to fit the observed frequencies to within 1.3%.

Introduction Much research interest over the past several years has been centered on the vibrational spectra and, in particular, the normal coordinate analysis of the ethyl halides. Using the frequencies determined by Miller and Kiviat3 for ethyl chloride and its deuterated derivatives, Deinpster and Zerbi4 performed a normal coordinate analysis and proposed reassignments for several modes. Similarly, ethyl bromide and its deuterated derivatives were investigated and a normal coordinate analysis was also carried More recently, Crowder6 and Durig et al.7 have studied the spectra of several ethyl iodides. Crowder used a 28-parameter force field to fit four isotopic species while Durig et al. used 19 force constants to fit three isotopic ethyl iodides. Since the cyanide functionality can be thought of as a pseudo halide it was thought to be of interest to investigate the spectra of ethyl cyanide and a number of its isotopically substituted derivatives in order to help confirm assignments in the other ethyl halides. Duncan and Janzs performed the first complete vibrational analysis of ethyl cyanide and performed a normal coordinate analysis for the skeletal modes only. Yamadera and Krimmg used a 29-parameter force field to fit the 21 observed frequencies of ethyl cyanide. Klaboe and GrundneslO also proposed an assignment for ethyl cyanide; however, like Yamadera and Krimm they assign the torsion to an accidentally depolarized Raman peak a t 226 cm-l, which is much too intense to be assigned to a methyl torsion. This also led to a misassignment of the a' in-plane CCN bending mode in both cases. Heretofore, no vibrational work has been done on any isotopically substituted ethyl cyanides. In order to help corroborate the assignments and make a normal coordinate analysis meaningful in the case of ethyl cyanide's low symmetry ( C s ) ,isotopic data are virtually essential. Isotopic derivatives of ethyl cyanide have, however, been investigated by microwave spectroscopyl1,l2where a Coriolis interaction between the torsion and the CCN bending mode was found to exist in CH3CH2CNl3 but not in C D ~ C D Z C N . ~ ~ Thus, in order to confirm the vibrational assignment of ethyl cyanide, especially in the low-frequency region where discrepancies exist, a vibrational study and normal coordinate

analysis of five isotopic species of ethyl cyanide was undertaken.

Experimental Section Ethyl-do cyanide was obtained from Columbia Organic Chemicals Co. and was purified by vapor phase chromatography. Merck Sharp and Dohme of Canada supplied the ethyl-d5 cyanide which was used without further purification. Stohler Isotope Chemicals Co. supplied the ethyl-l,l-dz iodide, the ethyl-2,2,2-d3 iodide, and the K13CN used in the preparation of the isotopically substituted ethyl cyanide species. The ethyl-d3 cyanide was prepared by refluxing a dimethyl sulfoxide solution of ethyl-d3 iodide with KCN for 8 h. Ethyl cyanide-13C(N)was prepared analogously using ethyldo iodide and K13CN. Fractions boiling a t 82 and 94 "C were collected separately and purified by vapor-phase chromatography using a 15%Carbowax 20 M on 60/80 Chromosorb W column heated to 105 "C. The higher boiling fraction contained almost pure ethyl cyanide while the lower boiling fraction contained a significant amount of ethyl iodide. In preparing the ethyl-& cyanide by this procedure, solvent exchange with the deuterated positions occurred, yielding an equal mixture of ethyl-do, -dl, and -dz cyanide. Therefore the preparation of ethyl-dz cyanide was done using MezSO-ds, supplied by CEA of France, as the solvent. The purification of the -dz compound was also done by vapor phase chromatography. The isotopic purity was checked by observing the microwave spectrum using a Hewlett-Packard Model 8460A MRR spectrometer in the frequency region 26.5-40.0 Ghz. The observed spectrum showed the sample to be a t least 95% isotopically pure. The pure samples were stored over activated molecular sieves to remove any final traces of water. The Raman spectra were recorded on a Cary Model 82 spectrophotometer equipped with a Coherent Radiation Model 53 G argon ion laser source using the 5145-A line for excitation. The instrument was calibrated with emission lines from a neon lamp over the spectral range 0-4000 cm-1. Generally 1.5 -2.5 W of power were used for gases and about 1W for solids and liquids. Raman spectra of the vapor were obtained using the Cary multipass accessory and a standard Cary multipass cell which was adapted with a PTFE greaseless stopcock and a side-arm reservoir. The windows of the cell and The Journal of Physical Chemistry, Voi. 80,No. 11, 1976

C. J. Wurrey, W. E. Bucy, and J. R. Durig

1130

the sample area were heated to about 40 "C to increase the vapor pressure of the ethyl cyanides. Liquid phase spectra were recorded with the samples sealed in a glass capillary. Spectra of the solid ethyl cyanides were obtained using a cold cell in which the sample holder is a solid brass plate at an angle of 15" from the normal. Samples were deposited directly onto the brass plate held at --190 "C and then annealed until the spectra showed no further change. Frequencies reported are expected to be accurate to f 2 cm-l. A Perkin-Elmer Model 621 grating spectrophotometer was used to record mid-infrared spectra from 4000 to 200 cm-l. The instrument was purged with dry nitrogen and calibrated as described in the literature.'5 The mid-infrared spectra of the solids were obtained by condensing the vapors onto a CsI window cooled with boiling liquid nitrogen and then annealed until the spectra showed no change. Infrared spectra of the vapor were recorded using a 20-cm cell equipped with CsI The Journal of Physical Chemistry, Vol. 80.No. 1 7 , 1976

lolo R 1022 9 2991 P

"i

2999

x

7

2988

2w5

2997 R 2961 0 2451 P

VI

2959

VI

P

2941

2450

3W7 3408

windows and a Perkin-Elmer 1-m cell equipped with CsBr windows. Frequencies reported are expected to be accurate to cm-'. The far-infrared spectra were recorded on a Beckman IR-11 spectrophotometer. The instrument was continuously purged with dry air and calibrated as described in the literature.16The low-temperature cell described previously17 was used for obtaining solid phase spectra of the -do and - d molecules. ~ The spectra were obtained by condensing the vapors onto a silicon substrate maintained at liquid-nitrogen temperatures and then annealing until no further changes appeared in the spectra. The gas-phase spectra were obtained using a Beckman variable path length cell equipped with polyethylene windows but only a path length of 8.2 m was used. Sample pressures of the -do and -ds compounds were the normal vapor pressure at 25 "C, -40 mm. Frequencies reported are expected to be accurate to f l cm-'. All observed frequencies are listed

1131

Vibrational Spectra of Ethyl Cyanides Table I V .

- a b l e Ill.Obterved and CdlCUlafld Y i b r a t l o n a l Frequencier f a r E t h y l CymIde-d3.'

"man

d"(Cm-11

Gar

Intensity

2959

w ih

2944

bd

9

VI

2874 R 2864 9 2855 P

s

2935

5

9 5

Gas

Int.

P"1cm-l

I

dp

Solid

2257 R 2250 9 2212 P

OeSCrlFtiOW

w

dp7

2975

2955

2979

v l j /a")

VI

p

2945

2P41

2943

Y,

(1') 991 CH.

2932

ih

.21

2935

/I

+ 799 = 2941

2853

I

p

2846

2948

2513

2253

m

.I9

2242

2244

2107 R 2099 9 2090 Q

m

1455 9 1452

I

m m m m

rh

.4l

2223

2222

2138

m

0

2137

2132

2095

m

p

2089

2089

h

2254

Int.

9

Solid

Int.

61s

ff:!

U i i l g n m e n t i bnd ApploXlmlte DeScPlFtlOnS

YdlWl

dp

Solld

LiqUld

1 7

2254

2257 2247

s

p

2245

Y

dp?

2234

2251

i

Calculstsd

d"(cn.11

2245

~ ~ ( 8 84% ' 1 GO,

224E.

u l / ( a " 1 90% GO, a n t > i y m v e t r i c itretch

2242

2253

~ ~ ( a 70% 11% ' l C-N COI as tnr et lt ci yh m t r i c s t r e t c h

2229

2217

Y1,l1"1

IynmPfllC r t r s t i h

2243

2253

a n t l i y m e t r l c sfvetch 121 c-11 s t r e t c h

10% C-CINI i t r e t c h

2255

88% COS I n t i i Y W e t l l c I t l e t C h b yllli"l 981 (0% a n t i r w t r i c s t r e t c h

vl(d')

d/ld'l 79X C-N l t r a t c h

10% C-GIN1 I t i e t C h l o x cos a n t i i y i m e t r i e s t r e t c h 2235

RlW" -

YlCrn~~I

24,

i

m

l00X CH3 m t i i y m e t r ! C s t r e t c h

Observed and Calculated V i b r a t i o n a l FIOIYEDCI~S fm E t h y l Cyanide-d5.'

d-

61s

ZZG6 22c2

I I

2110 R 2142 9 2135 Q

1455 R

Arrignmenli and ADprbXimdte

VdlYPI

Liquid

h

2254

Calculated

799 2137

vala'l

+VI

= 2246

98% CUI r y m e t r i c s t r e t c h

2 v l r i n F. R. w i t h

1443

YII

+

Y*

2232 91

Ih

2180R 2172 9 2158 Q

m

2157

::;!

1

2140

2141 F

m

2128

ih

2099 R 2089 p 2080 F

5

1174 R 1165 9 1155 P

m

m

1440 F

m m

1448

b

.66

1440

1134

1149

",(a')

1321 R 1313 9 1303F

m m n

I314

w

p

1310

1308

1340

vtIa'l63%CH~wag 23% CH, deformdtio" 15% C-GIN1 s t r e t c h

71% CXs d e f o l m i t i o n 22% CHI W W

VY

p

2154 2135

1103

911 COS i n t i l y m e t r l c s t r e t c h

+

1051 = 2164

rh

s

2138

2134

v I I i ' ] 89% Cui iymefrlc i t r e t c h

2155

v , I i ' l 918 COB rynmetric i t v e t c h

2117

9

P

2122

Y

p

2089

2088

I

Q

2081

2079

1150

1165

w

p

1151

1158

1179

1119

1121

YY

p

1120

1117

1118

", / a ' ) 21% 59% COS r y m e t 7 i c d e f o m t i 0 n 15% Go1 C-Clefhvll d e f m a ti itoi en t c h

1100

1103

w

p

1100

1095

1095

up

2vli

i n F. R. u l t h

i

m

m

= 1453

"17

2157

1112 R

rh

l a ' I 437% 2 X C-C(N1 C b wag i t i e t c n

l a ' ) 311 4 3 % Ccon - C ldeformatla" ~thyllitret~h 18% COS wag 10% c-c-c bend

T a ~ l eI:l. Observed and Calculated VibrdTiOndl Frequendes for E t h y l Cyanide-d3.' Table I Y .

w

Gus

:ntelilty

1258 Q

w

12359

W

1120 R

ih

1060 9

5

12

:k,w

Gas

Int.

dp

Solid

Liquid

1250

w(b1

dp

1253

1255

dp?

1211

1238

p

I108

1107

1113

w

Ram*" __

1251

86lCHs twist

"19(3''1

1112

"1

(a') 56%C-ClethyIlrtretch 54% CD, r y m e t r l r defOPmDtlin

w

dp

1051

1059

1045

ulild"l

w

.75

1027

1328

1051

v l l ( d ' l 8411 COi d n t l l y m e t r i c deformation

975 R 955 Y 958 P

m m

966

W

P

964

956

1000

930

w,rh

935

w

Q

943

938

943

Gar

Int.

1;:

7

95% CDI a n t i r y n m e t r i c deformation

VdlWl

Solid

Obi

Int.

dp

Solid

Liquid 1047

~ ~ 1 1 87X '1

1054

1052

1048

~,b(d"I

911

913

923

1062

1081 Q

s

9:;' $ 1

A r r i g n m ~ n l and i AppIoxlmlte oe3triptions

Calculattd

AvIcm-~l

YIC~~11

1030

m

Obieived m d Calculated Vibrational Frequencies for E t h y l Cyani@e-d5.'

d

1055

1065

vw

dp

91?

915

N

Q

13'1

bj

11% co3 fyMnetTic defomibtlon

,,,

31% C-C-C bend ld'115XCD~r~etri~dFfanatlon 16% C0a Pock 13% C-CINI stretch (jl'l

301 C-C/Nl s t r e t c h 21% c-c l e t h y l 1 , f t r e t c h

9

Y,I/B"I541 CHo

w

916

915

9

P

913

:!E

1 E

843

935

w

.71

w

838

8357

YW

.7I

w w

553

571

Y>,I."l :

;a p

;;

712 9

w

513 R

Y

498

Q

m

P

502

805

743

w

p

737

734

507

w

w

p

518

517

592

"11

18'1

69% COa rack 191 C-CINI r t i e t c h

7 575

yl(a8'j

490

Y L ~ I 451 ~ ' C-C-C )

c-c-c

bend

59% CDi t v i r t 1 3 C O i rock

l a ' ) 31% (0,

a35

801

a35

813

v i l l l ~ " l 5411 CDI rack 24X COO rock 15% GO, W i l t

5a5?

586

yIs/a'l

wag 21% CO, deformation 12% C-C/NI I t i e t c h l o x Cos l o c k 10% C-Clethyl l r t r e t c h

Pock

31% CDI rock 13X CHI t w i s t 799

23% CDI d e f o t r d t l a n

13% COS 23% C-C r( ye tmh ey lt]r l ri t rdeetfco hr n d t i o n 171

20% CD, lynmetric deformation 861

C O S a n t i i y m m e t r l r deformation

94% C D a a n t l r F e t P l c defolmlitlon

561 COi rock 30s CH, lock bend 311 C - C l e t h y l l s t r e t t h 16% C-C-N in-Dlane bend

a34 Q

582 573 581

R 9 Q

W

835

P

w

594 9 591

589

:i!

511

501

Y

53% 10s rock 23% C-CINI s t l e t c h

.77

590

594

591

~ , ~67% ( iCOB ' 'l olc k

p

510

510

483

vli/a'I

31% COB rock 18% CO, t W l i t

29% C-C-C 46% c-c ( e bend thyl] stretch .*% C-C-N in-.lane

bend

Table 111. Observed and CalCYloted V l b i a f i o n b l Fraquencier fa? E t h y l CyanidP-d3 a

Table I".

Gill

334

204

9

Observed ahd Calculated V i b r a t l a n a l Frequencies far E t h y l Cyanlde-di.'

lnfrared

rn

" ( d l

d"(Oi1

I

dp

Solid

LiWid

353

345

341

y l l ~ i l * ' j 84% C-C-N out-of-piane bend

, 7 ; ~ ~ ~ $ ~205 l

204

203

v I l l ~ ' ]81% C-C-N in-plane bend

163

YII(.''I

In:.

Solid

GLS

m

350

334

vw ,8011i '651gas! quldl

bd

208

193

x

Inf.

Calculated

Ariignmentr and Approximate Oerorlptlonl

WlWl

9% T o r r i o n

17%

c-c-c bend

15Zb 161? 8)

See Tabler 1 and 11 f o r abbreviations and mmnentr.

bl cl

Astuned equal t o -d5 t o r s i o n ; lee n b l e

1757

891 i o r l l o n 7% C-C-n out-of-plane bend

See t e x t f o r d i r c u r r i o n .

Iv.

in Tables I-IV (miniprint material, see paragraph a t end of text regarding miniprint material).

Results and Discussion Selection Rules and Band Contours. The assignments presented in this paper are based upon a structure with C, symmetry in agreement with the microwave results. Under C, symmetry the 21 fundamental vibrational modes are distributed as 13 a' 8 a", in which all 21 frequencies are both infrared and Raman active. The a' vibrations will give rise to polarized Raman bands, whereas the a" modes will be depolarized. For all of the CH3CH2X molecules, where X = a halogen or pseudo halogen moiety, the a'' modes are predicted to have C-type band contours in the infrared spectra since the largest principal inertial axis occurs out of the molecular symmetry

+

plane. The a' (in-plane) vibrations are predicted to have A, B, or A/B hybrid contours depending on the orientation of the oscillating dipole moment. Using rotational constants of 0.905 07,0.157 24, and 0.140 99 cm-l, the P-R separation for type A and B band contours was calculated to be -22 and -21 cm-l, respectively. The C-type contour was characterized by a very strong Q branch on a broad background absorption which made identification of many a" modes very straightforward. Many of the a' vibrations possessed well-defined PQR or P R structures with observed P-R separations in good agreement with the calculated values.

Vibrational Assignments Assignments of the vibrational frequencies were made using the data from the infrared and Raman spectra of the gas phase which are shown in Figures 1 and 2. Since many of the asThe Journal of Physical Chemistry, Vol. 80,No. 11, 1976

C. J. Wurrey, W. E. Bucy, and J. R. Durig

1132

I

I

2000 WAVENUMBER (CM-‘)

3000

I

1000

2oi

Figure 2. infrared spectra of gaseous ethyl cyanides. Spectra were recorded at room temperature with cells of 20-cm and 1-m pathlengths: (A) CDSCD~CN, (B) CD3CH2CN,(C) CHzCDpCN, (D) CH3CHzi3CN, and (E) CH3CH2CN.

WAVENUMBER (CM-’) Figure 1. Raman spectra of gaseous ethyl cyanides. Spectra were recorded at 40-50 OC with 5-cm-’ spectral band width: (A) CD&D&N, (B) CD3CHzCN,(C) CH3CD2CN,(D) CH3CH2I3CN,and (E) CHsCHpCN. signments are rather straightforward, only a summary of the more interesting or difficult points will be presented. Assignment of the normal modes for the 13C(N)molecule is almost identical with that of the -do assignment. C-H and C-D Stretching Regions. By combining both infrared and Raman data of the gas phase, it was possible to assign the five CH stretching vibrations in four isotopic species. The totally deuterated species added confidence to our assignment of the CH region by showing similar features in the CD region. Our assignment is consistent with the The Journal of Physical Chemistry, Vol. 80, No. 11, 1976

argument presented by Durig et al. for several isotopic ethyl iodides’ and the assignment by Gunthard et al. for several isotopic nitroethanes.18 In the -dz compound a strong A-type band centered at 3002 cm-l in the infrared spectrum with a weak Raman counterpart at 2999 cm-’ with a high depolarization ratio, 0.7, is assigned as the doubly degenerate CH3 antisymmetric stretch, v 1 and ~ 1 4 assuming , local C3” symmetry. The very strong Raman band at 2959 cm-l is assigned as the totally symmetric a’ CHs stretch, uz. There is an additional strong polarized band a t 2900 cm-‘ in the Raman spectrum with a B-type infrared counterpart centered a t 2903 cm-l. Fermi resonance between v 2 and the first overtone of the CH3 antisymmetric deformation, 2~16, always leads to a medium intensity band in both the infrared and Raman.lg This effect was observed in all of our spectra and we feel this assignment is correct. A strong polarized Raman band a t 2948 cm-l in the -d3 compound is assigned as v3, the CH2 symmetric stretch. A broad region of strong intensity in the infrared spectrum is observed at about 2940 cm-1 with the only noticeable feature being a C-type Q branch at 2936 cm-’. Table I11 shows the assignment of this region as resulting from a binary combi-

Vibrational Spectra of Ethyl Cyanides

1133

nation. A rather weak shoulder a t 2969 cm-l in the infrared with a Raman counterpart, which is depolarized, at 2965 cm-l is assigned as the CH2 antisymmetric stretch, ~ 1 5 . The CH stretching assignments of the ethyl-do cyanide and 13CN follow directly from the -4and -d3 assignments. The v i 5 band is obscured by the R branch of v2. However, in the solid phase a band of medium intensity is observed at 2973 and 2967 cm-l in the infrared and Raman spectra, respectively. The assignment of the CD region is complicated by presence of the CN stretch, v4. However the frequency of this mode is rather insensitive to deuteration and is assigned between 2250 and 2259 cm-l in all 12CNspecies. A shift to 2207 cm-l for the CN stretch is observed in the 13CN molecule. The partially deuterated species provide the key to many of the assignments of the -d5 compound. In the -d2 compound two medium bands appear in both the infrared and Raman spectra in the 2100-2200-~m-~region where v3, the CD2 symmetric stretch, should occur. Following the same arguments as presented by Durig et the higher frequency band a t 2184 cm-l may be attributed to 2 X 1085, in Fermi resonance with v3 which falls a t 2133 cm-l. A medium C-type Q branch is observed at 2237 cm-I in the infrared spectrum, which is therefore assigned as ~ 1 5the , CD2 antisymmetric stretch. In the -d3 spectra two medium intensity bands are observed in both the infrared and Raman spectra a t 2142 and 2099 cm-l. Fermi resonance between 2v1g and v2 is postulated to explain their similar intensity and the abnormally large anharmonicity of V16. v2 is assigned as the strongly polarized Raman band at 2138 cm-l whose infrared counterpart at 2142 cm-l has an A-type band contour. The assignment of v1 and ~ 1 in4 the totally deuterated and - d species ~ is complicated by the CN stretch. In the -d5 compound a Q branch shoulder appears on the very strong and broad CN stretch. The intensity of this band in both the -d3 and -d5 compounds in comparison to the weak bands in both the -do and -4compounds leads us to the assignment of and ~ 1 as 4 almost degenerate with v4 in both the -d3 and -d5 molecules. Therefore the antisymmetric CD3 stretches in the -d3 and -d5 molecules are assigned at 2254 cm-l. A broad weak band about 2235 cm-l in the Raman spectrum of gaseous ethyl-d3 cyanide is assigned to the sum 1446 799 cm-l. There is no equivalent band in the -do compound and there is no evidence of an infrared counterpart which was observed to be very strong in the -do and -dz molecules. The assignment of the other CD modes in ethyl-d5 cyanide follows directly ~ from the -dg and - d assignments. CH3 and CH2 Deformation Region. The methyl deformations are well-established group frequencies,20 which are demonstrated by our isotopic data. In the -dz molecule, a medium intensity A-type band occurred at 1470 cm-l in the infrared spectrum, and is assigned as V g , the CH3 a’ antisymmetric deformation. A sharp, strong C-type A branch at 1460 cm-l is readily assigned to V16, the a” antisymmetric CH3 deformation whose Raman counterpart a t 1459 cm-l is depolarized. A weak B-type band centered a t 1391 cm-l in the infrared spectrum is assigned to v7, the a’ totally symmetric methyl deformation. In the -d3 molecule a medium band in the Raman liquid spectrum at 1434 cm-l and a t 1448 cm-l in the Raman spectrum of the gas phase is assigned to ~ 5the , CH2 scissors. A B-type band centered a t 1446 cm-l was observed in the gas-phase infrared spectrum. This assignment is directly applicable to the -do and 13CNmolecules. I t should be noted that from the potential energy distribution among symmetry coordinates obtained from the normal coordinate analysis, V 6 is calculated at 1456 cm-l and v j a t 1475 cm-l.

+

However, the calculations give very good agreement with the observed frequencies for v5 and V 6 for the -d2 and -d3 molecules. Further discussion of this point will follow in the normal coordinate analysis. CHz Motions. The assignment of the a’ CH2 wag, v8, at 1313 and 1323 cm-l in the -d3 and -do, respectively, is unambiguous. A medium intensity A-type contour was observed in the infrared spectrum with weak Raman counterparts. The a” CH2 twist was observed at 1270 and 1258 cm-l in the -do and -d3 compounds, respectively. Depolarized Raman bands and sharp C-type band contours made this assignment straightforward. C-type Q branches at 894 and 915 cm-l in the -dp and -d5, respectively, are assigned as V17, the CD2 twist. The band at 915 cm-l is almost degenerate with a B-type infrared band whose Raman counterpart is polarized. A band with a sharp Q branch in the infrared spectrum of the -do molecule a t 784 crn-l, with a depolarized Raman counterpart in the liquid phase, is assigned as the a” CH2 rocking motion. This motion shifts to 665 and 594 cm-l in the -d2 and -d5 molecules, respectively. A C-type Q branch at 861 cm-l in the -d3 molecule has been assigned as the CH2 rock, apparently shifted from the -do molecule. However, this is in good agreement with the calculated value of 896 cm-l. C-C Stretching and CH3 Rocking Regions. The assignment of the spectral region from 800 to 1200 cm-l is best described as a mixing region. Group frequencies in this region are generally not well defined for the ethyl fragment. The assignment of this region was done by comparing similar band types and consistent intensities between infrared and Raman data for all the isotopes. In the -d2 and -d5 molecules medium A-type bands at 1183 and 1165 cm-l, respectively, were calculated to be mainly vg, the CD2 wagging motions. Similar B-Type bands at 1113 and 1103 cm-l in the infrared spectrum of the -d3 and -d5 species, respectively, were calculated to be mainly vg, the ethyl C-C stretch. Polarized Raman bands were observed to support this assignment. A strongly polarized band a t 1078 cm-l in the -do molecule with an B-type infrared counterpart was assigned to vg. A similar band a t 1085 cm-l was observed in the -d2 molecule. A polarized Raman band a t 1121 cm-’ in the -d5 molecule was calculated to be v7, the CD3 symmetric defofmation. This is in good agreement with the 1113-cm-l band of the -d3 molecule, which is equally mixed between the CD3 symmetric deformation and the carbon-carbon stretch of the ethyl group. The a’ antisymmetric CD3 deformation, V g , was observed as a very strong A-type band a t 1061 cm-l in the infrared spectrum of the -d5 molecule and was calculated to be almost a pure mode. It is interesting that a strong C-type Q branch at 1058 and 1060 cm-I in the -d5 and -d3 molecules, respectively, are almost degenerate with Vg. Local Csu symmetry can be invoked to assign these bands as V16, the CD3 a” antisymmetric deformation. A Raman band a t 1030 cm-l in the -d3 molecule had a depolarization ratio of about 0.73. This band was then assigned as V g . The a’ CHS rocking mode and C-C(N) stretch are highly mixed modes and their assignment to a particular frequency is highly ambiguous. Strong, polarized Raman bands occurring near 820 f 20 cm-l may be assigned as one of these fundamentals, most likely the C-C stretch. Infrared counterparts are very weak in the gas phase, but are of medium intensity in the solid phase. A weak A-type band in the infrared spectrum at 1008 cm-l in the -&molecule is assigned as the last a’ fundamental expected in this region. Corresponding bands are observed in the other isotopes a t 1000 f 40 cm-l which The Journal of Physical Chemistry, Vol. 60,No. 11, 1976

C.J. Wurrey, W. E. Bucy, and J. R. Durig

1134

TABLE V: Product Rule Check of Assignments and Principal Moments of Inertia (amu A2)2 Calcd

Obsd

A, %

CH3CH2’3CN/CH3CH2CN

a’ 0.9419 a” 0.9454

0.9404 0.9813

-0.16 3.80

CH&D2CN/CH3CH2CN

a’ 0.3736 a’’ 0.4094

0.3727 0.4266

-0.26 4.19

CD3CH2CN/CH3CH2CN

a’ 0.1974 a” 0.2994

0.2092 0.3055

6.00 2.04

CD&D2CN/CH&H2CN

a’ 0.0735 a” 0.1206

0.0736 0.1250

0.19 3.62

I,

Ib

18.626 18.642 23.451 22.469 27.632

107.209 107.761 109.838 120.227 122.468

CH3CH2CN CH3CH213CN CH3CD2CN CD3CH2CN CD3CD2CN a

IC

119.562 120.131 123.911 133.269 137.568

Calculated from the structure presented in ref 11which was also used in the normal coordinate analysis.

also have weak to medium intensity Raman bands. Further discussion of the remaining a’ fundamentals will follow in the discussions about the Teller-Redlich product rule analysis and the normal coordinate analysis. The CH3 a’’ rocking motion, v18, was observed as a medium intensity C-type Q branch a t 1000 cm-l in the infrared spectrum of the -do molecule. Similar medium intensity bands were observed at 712 and 834 cm-l in the infrared spectrum of the -d3 and -dg molecules. A very weak shoulder at 1033 cm-’ in the infrared spectrum of the -d2 gas was also assigned as v18. Bending Vibrations and Torsions. The C-C-C bends, ~ 1 2 , were assigned to peaks a t 520 f 20 cm-l in the various isotopes, in good agreement with group frequencies. Both v13 and v20, the C-C-N bending modes, are weak to medium bands observed a t 201 f 10 and 355 f 17 cm-l in all isotopes. Both bands have high degrees of depolarization in the Raman effect, a fact characteristic of C - C r N bending modes. However, consistent with the assignment of Durig et a1.21 of isopropyl cyanide, the band at 371 cm-l in the infrared spectrum of the -do molecule has a C-type Q branch, characteristic of the a” mode and the v i 3 band a t 211 cm-l has a B-type contour. The torsional mode v21 was not observed directly. Microwave splitting results from the -do and -dg molecules gave frequencies of -220 and -162 cm-l, r e ~ p e c t i v e l y .A~ pos~,~~ sible 2v21 band was observed for the -d3 compound in the Raman effect at 313 cm-l, which is consistent with the microwave results. The broad Raman line arising from the C-C-N out-of-plane bend obscures most of the 2v21 region of the other isotopes. Teller-Redlich Product Rule Calculations. An attempt to check the consistency of the isotopic assignments was made through the Teller-Redlich product rule. Generally, an agreement of 3-5% between calculated and observed ratios is sufficient to suggest the validity of the assignment. Our results are shown in Table V. The product rule was used as an aid in assigning the “missing” fundamentals in the a’ block of the -d2, -d3, and -dg molecules. A weak Raman band at 855 cm-l in the gas phase spectrum of the -d2 molecule and a medium band in the solid phase at 856 cm-l was consistent with the a’ block TellerRedlich product rule calculation. A weaker Raman band at The Journal of Physical Chemistry, Vol. 80, No. 11, 1976

904 cm-l was discounted because it placed the calculation over 5%. The assignment of fundamentals at 915 and 671 cm-l in the -dg compound also gave a consistent product rule calculation. The 915-cm-l B-type band was of medium intensity in the infrared spectrum and weak in the Raman effect. The large intensity of this band in the solid phase also made us feel it was a fundamental, not an overtone or combination band. The band at 671 cm-l was chosen over the band at 725 cm-l to be consistent with product rule calculations. The -d3 molecule presented the greatest difficulty in our assignment. The strong band in the Raman spectra of the gas at 799 cm-I was considered a fundamental. This is in direct contrast to the prediction of the normal coordinate analysis which calculates this band at about 700 cm-l. A band at 743 cm-l in the Raman spectrum of this gas was discounted as this fundamental due to lack of intensity. It should be noted that the 799-cm-l band was observed only in the Raman effect which was consistent with intensity of other C-C(N) stretching vibrations. However the lowest frequency mode should be mostly the CD3 rock. However the assignment of both the 799- and 743-cm-l bands as fundamentals made the product rule calculations inconsistent with the assignment of the other fundamentals which are believed to be correct. The “missing” fundamental was therefore assigned to the Raman peak at 938 cm-l. Normal Coordinate Analysis. As an aid in describing the molecular vibrations in a more quantitative manner, a normal coordinate analysis was undertaken. The calculations were carried out by the Wilson FG matrix method22with computer programs written by S c h a ~ h t s c h n e i d e r .The ~ ~ observed frequencies were given a weight of 1/X in the least-squares fitting procedure of the force constants, and no attempt was made to correct for frequency shifts due to energy level interactions, i.e., Fermi resonance. The internal coordinates for ethyl cyanide are defined in Figure 3. Symmetrization was accomplished using the symmetry coordinates listed in Table VI. The S14 and S15symmetry coordinates have been defined as zero-coordinate four-branch redundancies. To have symmetry coordinates which would be to a degree consistent with the previous l i t e r a t ~ r ewe , ~ use ~ ~ the following procedure to define Sg and Sl2. The Sg’ was written as ( l / d ) ( A a - A T ) which is orthogonal to s15. The S12‘ was then written as

Vibrational Spectra of Ethyl Cyanides

1135

TABLE VI: Symmetry Coordinates for Ethyl Cyanides A' CH3 CH3 CH2 CH3 CH3 CH2

Species Antisymmetric SI = 6-1/2(2Ar1- Ar2 - Ar3) stretch Symmetric S Z = 3-'I2(Arl + Ar2 + Ar3) stretch Symmetric S3 = 2-lI2(Adl Adz) stretch Antisymmetric S4 = 6-112(2Aa2 - Aal - A a 3 ) deformation S g = 6-1/2(Aa1 A a 2 + A a 3 - AB1 Symmetric - AD2 - Ap3) deformation Deformation S g = 24-ll2[(&+ 2)As - ( A-

+

+

2)Aa A&]

CH2 CH3 C-C C-C(N) C-N C-C-C

Wag In-plane rock Stretch (ethyl) Stretch Stretch Bend

Figure 3. Internal coordinates for ethyl cyanide.

+

( l / m ) ( 2 A & AT - A81 - A82 - Ay1 - Ay2) where the coefficients were determined by the Schmidt orthogonalization method to complete the orthogonal set of A' symmetry coordinates. However, this new set does not separate the contributions of SG'and S12' in the potential energy distribution. Therefore we tried the new combinations

r l

1.1

C-C-N In-plane bend Redundancy Redundancy A" CH3 CH2 CH3

which were found to be separated in the potential energy distribution. Table VI clearly shows S g to be mainly As, the CH2 deformation, and Sl2 as AT, the C-C-C bend. The calculated vibrational frequencies are listed in Tables I-IV along with the associated symmetrized potential energy distribution among the diagonal elements of the E" matrix. Using the relatively simple force field shown in Table VII, the average agreement between calculated and observed frequencies was found to be 1.27% or 11.4 cm-l. In general, the results of the calculation support the commonly accepted "group frequency" descriptions associated with the methyl and methylene groups for the -do and 13C(N)molecules. However, the CH3 deformation region should be explained in more detail. It was found that very small changes in the force field had an effect of reversing the assignment of u5 and V6. In an attempt to best fit the 105 observed frequencies, the entire force field was allowed to vary. It was this procedure that reversed the assignment of u5 and Ug. However it must be emphasized that small changes in the force field will reverse the assignment, with a corresponding small increase in the percent error. Since 3N - 6 = 21 for ethyl cyanide, we tried to restrict the number of force constants to less than 21. It would have been possible with the amount of isotopic data we had to include all possible interaction constants, many of which would have been very small. However, we tried to find those force constants whose significance was such that fewer than 21 were required to reasonably reproduce the observed frequencies. F Q ~F, Q ~F, R ~ FQ,, , F,s, F,,], Force constants FTp,Fa@,FRQ, Fog, FQS,FQS,and F,p were tried and found to have very little

CH2 CH2 CH3 CH3 C-C-N

- A71 - A 7 2 - A01 -

S7 = %(A71 + A 7 2 - A01 - A021 Sa = 6-1/2(2Ap~ - Ap2 - A&) Sg = AR Si0 =

AQ

Si1 = AP si2

+

= 24-'lz[(&2)A6 - (4 AT + A71 + A 7 2 A01 A&]

+

+

Si3 = Ap

+ + Apl + + si5 = 6-1/2(A71+ A 7 2 + A01 + AS2 + A6 + AT) = 0 si4 =

6-1/2(A~1 -I-A 0 2 A a 3 Ab2 Ap3) = 0

Species Antisymmetric Si6 = 2-1/2(Ar2 - Ar3) stretch Antisymmetric S I T = 2-II2(Adl - Adz) stretch Antisymmetric Sla = 2-ll2(Aal - Aa3) deformation Twist Si9 = '/z(-S~l Ay2 + A01 - AS,) Rock s20 = %(a71 - A72 + A01 - AOz) Out-of-plane ,521 = 2-1/2(Ap2 - Ab,) rock Torsion S22 = AT Out-of-plane s23 = Au bend

+

effect on the force field. It has been shown that the relationship FR, = - F R ~exists for tetrahedral angle^.^^,^^ Our value ~ for for F R of~ -0.25 is in good agreement with the F R values ethyl iodide7 of 0.20 and for ethyl chloride4 of 0.17 mdynI8,. Our value for FR.( of 0.56 is much higher than the value of 0.21 mdynlA in the ethyl chlorides and iodide^.^,^ However, this value is more consistent with the 0.328 mdyn/8, value used by Yamedera and KrimmegThe high value of this interaction constant with respect to other simiIar molecules (ethyl halides) might be attributed to a much different electronic structure. Experimental evidence is conclusive that the C-C (ethyl) bond in ethyl cyanide (1.54 A) is longer than in the ethyl halides.11J2 A shortening of the C-C(N) bond to 1.46 8, in ethyl cyanide has been attributed to partial double bond character. Our force constants HQand H R are consistent with approximately the 20% double bond character indicated." The lower methyl barrier to internal rotation in ethyl cyanide with respect to the ethyl halides is more evidence that ethyl cyanide has a very different electron distribution about the C-C bond than is observed in the halogens. Various attempts were made to raise the calculated frequency of the CD3 rock at 691 cm-l. However, none were The Journal of Physical Chemistry, Vol. 80, No. 11, 1976

C. J. Wurrey, W. E. Bucy, and J. R. Durig

1136

TABLE VII: Internal Force Constants for CHsCHzCN and Its Deuterated Derivativesa,b

Force constant Kr Kd KR KQ Kp H,

Hp Ha H., Hs

H, H, H, HTb F,, Fdd

F R ~ FR., Fss

Coordinates involved C-H, methyl C-H, methylene C-C,ethyl C-C(N) C-N H-C-H, methyl C-C-H, methyl H-C-H, methylene C-C-H, methylene (N)C-C-H, methylene C-C-C C-C-N in-plane C-C-N out-of-plane -CH3' C-H,C-H C-H,C-H C-C, H-C-H C-C, C-C-H H-C-H, C-C-H

Value 4.89 f 0.02 4.77 f 0..02 3.97 f 0.17 4.93 f 0.26 17.52 f 0.21 0.542 f 0.006 0.619 & 0.014 0.611 f 0.016 0.520 f 0.020 0.802 f 0.018 1.970 f 0.167 0.211 f 0.021 0.306 f 0,019 0.010 f 0.001 0.075 i 0.013 0.043 f 0.023 -0.253 f 0.036 0.565 i 0.052 0.128 f 0.016

a Stretching force constants in mdyn .&-I; bending constants in mdyn 8, radian-2; stretch-bend interaction constants in mdyn radian-l. See Figure 3. Torsional coordinate is defined as the sum of three trans torsions about the C-C bond.

successful. Concluding that the -d3 molecule was behaving anomalously, we did not use the -dg data in one calculation. As was expected, the fit improved to 0.91% or 8.9 cm-l. The overall force field, however, did not change significantly. Therefore, the reported force field is the result of the calculation from the data of all five isotopes. In conclusion, we feel that our results provide a more complete understanding of the vibrational spectrum of ethyl cyanide and its -dz, -d3, -d5, and 13C(N)derivatives. The relatively simple force field used in this paper accounts for most

The Journal of Physical Chemistry, Vol. 80, No. 11, 1976

of the fundamental vibrations, and also provides something more than a qualitative description of these motions. Acknowledgment. The authors gratefully acknowledge the financial support by the National Aeronautics and Space Administration by Grant No. NGL-41-002-003. Miniprint Material Available: Full-sized photocopies of Tables I-IV (12 pages). Ordering information is available on any current masthead page.

References and Notes (1) C. J. Wurrey received his Ph.D. from MIT under the direction of Professor Lord in 1973. Professor J. R. Durig received his Ph.D. from MIT under the direction of Professor Lord in 1962. (2) To be submitted in partial fulfillment of the Ph.D. requirements. (3) F. A. Miller and F. E. Kiviat, Spectrochim. Acta, Part A, 25, 1363 (1969). (4) A. B. Dempster and G. Zerbi, J. Mol. Spectrosc., 39, 1 (1971). (5) R. Gaufres and M. Bejand-Bianchi, Spectrochim. Acta, Part A, 27, 2249 (1971). (6) G. A. Crowder, J. Mol. Spectrosc., 48, 467 (1973). (7) J. R. Durig, J. W. Thompson, V. W. Thyagesan, and J. D. Witt. J. Mol. Struct,, 24, 41 (1975). (8) N. E. Duncan and G. J. Janz, J. Chem. Phys., 23,434 (1955). (9) R. Yamadera and S. Krimm, Specfrochim. Acta, Part A, 24, 1677 (1968). (IO) P. Klaboe and J. Grundnes, Spectrochim. Acta, Part A, 24, 1905 (1968). (11) G. Lerner and B. P. Dailey, J. Chem. Phys., 26, 678 (1957). (12) H. M. Heise, H. Lutz, and H. Dreizler, 2.Nafurforsch. A, 29, 1345 (1974). (13) V. W. Laurie, J. Chem. Phys., 31, 1500 (1959). (14) Y. S. Li and J. R. Durig, J. Mol. Spectrosc., 54, 296 (1975). (15) R. N. Jones and A. Nadeau, Spectrochim. Acta, 20, 1175 (1964). (16) R. T. Hail and J. M. Dowling, J. Chem. Phys., 47, 2459 (1967); 52, 1161 (1970). (17) F. G.Baglin, S. F. Bush, and J. R. Durig, J. Chem. Phys., 47, 2104 (1967). (18) P. Groner, R. Meyer, and Hs. H. Gunthard, Chsm. Phys., in press. (19) G. Herzberg, "Infrared and Raman Spectra of Polyatomic Molecules", Van Nostrand, New York, N.Y., 1945. (20) L. J. Beilamy, "The Infrared Spectra of Complex Molecules". Wiley, New York, N.Y., 1962. (21) J. R. Durig. C. M.Player, Jr., Y. S. Li, J. Bragin, and C. W. Hawley, J. Chem. Phys., 57, 4544 (1972). (22) E. B. Wilson, Jr., J. C. Decius, and P. C. Cross, "Molecular Vibrations", McGraw-Hill, New York, N.Y.. 1955. (23) J. H. Schachtschneider, "Vibrational Analysis of Polyatomic Molecules", V and VI, Technical Reports No. 231-64 and 57-65, respectively, Shell Development Co., Emeryville, Calif. (24) H. Hollenstein and Hs. H. Gunthard, Chem. Phys., 4, 368 (1974). (25) P. Groner and Hs. H. Gunthard, J. Mol. Spectrosc., in press.