A vibrational spectroscopic study on 1,1,2-tribromethane

Spectrochimica Acta,Vol. 37A, PP. 37 to 40 Pergamon F’re~sLtd., 1981. Rinted in Great Britain

A vibrationalspectroscopicstudy on 1,1,24ribromethane s. suzuiu Department of Chemistry and Applied Chemistry, University of Salford, Salford MS 4WT, U.K. and G. VERGOTEN Laboratoire de Physique Mathematiques, Faculte de Pharmacie, 59045-Lille-Cedex,

France

(Received 1 May 1980) Ah&act-On

the basis of new data on the Raman spectra of solid and gaseous 1,1,2-tribromoethane

a revised vibrational assignment is suggested.

INI’RODUCDON

RESULTS

Since the pioneering work by M~ZUSHMA and his co-workers on the conformational problems of 1,2dichloroethane [l], rotational isomers of chlorinated ethanes have been studied extensively by a great number of investigators. Among 12 possible chlorinated ethanes, 1,1,2-trichloroethane has been a subject of particular interest because of its intermediate structure between 1,2dichloroethane and 1,1,2,2-tetrachloroethane. Thus this molecule has been studied by various methods and fairly recently an especially detailed study was made by CHRISTIAN et al. [2]. However, relatively few work has been done on 1,1,2-tribromoethane. This molecule has been studied by dipole moment [3,4] NMR [4,5], and ultrasonic [6] measurements. 1.r. spectroscopic measurements on the energy difference between the C, and C, forms were first made by MALHERBE and BERNSTEIN in 1952 [7]. A Raman study was carried out in 1941 by KAHOVEC and WAGNER [8]. In 1970, this molecule was reinvestigated in detail by TORGFUMSEN and KLAEBOE [9]. They measured the i.r. spectra down to 200 cm-’ in the liquid and solid states, and also the Raman spectrum in the liquid state. We have measured, in addition, the Raman spectrum in the solid state and some i.r. and Raman frequencies in the gaseous state. Together with the new data, the assignment of the fundamental frequencies has been reviewed. Also, a normal coordinate analysis has been carried out. The results are presented and discussed below.

AND DISCUSSION

A. Tentative assignment of observed frequencies Our observed frequencies are listed in Table 1. The vapour pressure being so low, we could only detect positions of strong bands in the gaseous spectra. In some cases, it was possible to analyse the band shape. The results agree mostly with previous ones [8 91, when the comparison is possible, but a few previously reported bands were not detected in our spectra, although none of them had been assigned to a fundamental. There are two probable forms for this molecule, Cl and C,, and the C, form is less polar. It is believed that they both exist in the liquid state, but in the solid spectrum, the number of observed bands decreases and it has been concluded that only the Cl form is present in the solid state. The Raman spectrum in the solid state obtained by the present work leads to some modification of the previous assignment. The 206~cm’ band in the liquid Raman spectrum was previously assigned to a C, fundamental, since this band was not found in the solid i.r. spectrum. However, in the solid Raman spectrum this band was observed with strong intensity at 209 cm-‘, thus we assign this to a C, fundamental. Similarly the band at 148 cm-’ in the liquid Raman spectrum is assigned to a C, fundamental for the same reason. On the contrary, the band at 174 cm-’ in the liquid Raman spectrum should be assigned to a C, fundamental, since this band disappears in the solid spectrum. The assignment of 162cm-l band remains unchanged, i.e. a C, fundamental. The 112cm-’ band in the liquid Raman spectrum, of which presence was considered to be doubtful was observed in the present work. Further, the 91-cm-’ torsion band of .C, form was observed at 72 cm-’ in the gaseous spectrum. A puzzling problem pointed out by the previous investigators [9], i.e. the origin of 487 cm-l band in the liquid i.r. spectrum which disappears in the solid state, still remains unsolved. This was tentatively assigned to a combination band, 273+ 208, but now that the 208-cm-’ band is proved to be

EXPERIMENTAL The sample of 1,1,2-tribromoethane was kindly prepared by Dr J. M. BRUCE at the University of Manchester. 1.r. spectroscopic measurements were made with a Perkin Elmer 521 spectrometer and Raman measurements with a Cary 82 spectrometer equipped with an Argon laser as well as a Coderg T800 Raman spectrometer also with an Argon laser. The sample was sprayed onto a cooled surface under vacuum, when the i.r. spectrum in the solid state was recorded. In Raman measurements at low temperature, the sample was sealed in a capillary. The gas Raman spectrum was recorded with a Coderg TSOO spectrometer using its accessary gas-cell. 37

S. SUZUKIand G. VERGOTJZN

38

Table 1. Vibrational frequencies of 1.1.2-tribromoethane infrared gas

liq

3036

3022 8

2975

3002 s 2959 m

Rsmsll SO1

gss

3017 2979

2819 v 1430 sh? 1427 1270

1416 s

1440 VW 1415 8

1360 w

1370 VW

1272 m

1272 8

1272

1iq

SO1

al2

3000 I&p?

2998 m

OB dCl,

2959 m,p

alI1

s

2827 w

2956 m 2827 Y

2820 w

2817 w

1417 w,dp

1414 Y

G-I2

sciss(C1, Cs)

1272 m.p

1272 m

a2

wdCl)

1219 m

(1212)

1210 s

1219 m,p?

1222 m

1183 m

1145 1120

1148 s

1145 s

1119 m 1042 m

1148 Y

1124 m

1147 w.p 1120 VW

1042 m

1042 m,p

1043 w

1001"

CJ

(H bend(Cl)op

1214 w 1195 VW

1190

C*)

sq.

cH2 wdC*) 683 + 561 (Cl)

1245 m 1219 8

a str(C1 CJ

3021 w

1259 m 1219

assigmQSat

3020ii

999

1Colvw.p

872

879 w.p

770

770 VW

Ca bend(Cs)ip CH hend(Cl)ip cH2 twist(Cl) cc sWC1) cc str(C*)

894 w 880

CH2 rock(Cs)

(886) 879 s

882 s

815 VW

850 VW 815 w

770 VW

768 VW

883

610 l 208(C1) 610 + 160(Cl) 563 + 193(C1) 610 + 102(Cl)

755 VW 710 w 685

620 568

cH2 rockW1)

690 s

683 s

670 sh

667 sh

693

688 "8

688 vs

sh

672 sh

CBr str(C1)

610 vs

605 vs

616

610 m,p

604m

CBr

563 s

561 s

570

562 s.p

560~

CBr str(C1)

526

str(C*,

Cl)

548 VW 526 w

str(Cs)

525 m,p

CBr

481w

479 VW

?(C&J

425 m

425 w,p

CBr def(Cs)

400" 334 m

335 m,p

337 v

203

206 s,p

209 8

193 w 162 m,dp 148 w,dp

CBr def(Cs) CBr def$)

154 w

CBr def(C1) CBr def(Cs)

112 VW 72

91 m,dp

102 w 48 s 39 m 27 m

a C, band, this assignment is not sustained. We interpret the medium intensity band at 1245 cm-l in the solid i.r. spectrum, which has been left unassigned so far, as a combination band of 683 +561. It seems from the intensity that this band is a counterpart of the liquid band at 1259 cm-l, but this assignment was rejected by the normal coordi-

CBr def(C1) 7

163 v

174 m,p 160 135

CBr def$) CBr def(Cs)

2.72w

torsion(C1) lattice lattice lattice

nate calculations. This may be in Fermi resonance with the strong band at 1272 cm-‘. B. Culcwlations After the tentative assignment by comparing the solid and liquid spectra, a normal coordinate analysis was carried out. The molecular structure

A vibrational spectroscopic study on 1,1,2&ibromoethane

39

Table 2. Final values of force constants K%

4.412

HBrCBr

1.099

FBrBr

0.183

KCH

4.620

HHCBr

0.392

%Br

0.237

KCBr2

1.142

HHCC

0.340

%C

0.360

1.645

HCCBr

0.094

FCBr

Kcc

2.962

Qa

0.378

FHH

pCH

-0.173

Y(gaucbe)

0.647

k

'CBr

-0.147

TBr-Br

0.111

GBr-Br

0.148

TBr-Ii

0.242

%-H

0.020

TH-H

0.022

%li

0.069

0.117

CBr2 r-to=(G)

0.047

-0.119

cH2 r-to=(G)

0.041

K

CBr

CBr2 r-to=(T) CH2 r-to=(T)

The stretching force constants are in mdynA_’ mdyn 8, rad-‘.

was assumed to be as follows [lo]: r,

= 1.09 8,

rce, = 1.94

&&c = 112.5” &.-sr

rcc = 1.54

= 113.0

4 Hm = 109.5

The starting values of Urey-Bradley force constants were taken from CH3CH2Br [ll] and CBr, [12]. A set of computer programmes, GCCC, BGLZ and LSh4B [13] were used. To obtain a reasonable agreement with the observed frequenties, a few non-Urey-Bradley type of force constants were introduced. The final values of force constants are given in Table 2, and the calculated

0.741 0.300 -0.006

and the others are in

frequencies are shown in Table 3 together with the observed values. The calculated results are compared with the tentative assignment of TORGFUMSEN et al. [9] on this molecule and also with the calculated results on 1,1,2-trichloroethane by CHRISIAN et al. [2], a noticeable thing is the order of CH, wagging and CH bending vibrational frequencies. In our assignment the CH, wagging frequency is higher than the CH bending frequencies for both forms, while the reverse was suggested by the other authors. Although there is no absolute evidence for the .order of these frequencies, we propose our assignment on the following bases: (a) the preliminary calculations

Table 3. Observed and calculated frequencies trans

A'

A"

obs

C&llC

description

gauche obs

ca1c

description

3000

3000

CH str

3020

3020

CH2 ast

2959

2969

CH2 sse

3000

3000

CH str

1430

1427

CH2 sciss

2959

2959

CH2 sst

1259

1262

CH2 wag

1417

1418

CH2 sciss

1183

1186

CCH bend

1272

1270

CH2 "a!&

1001

1014

cc str

1219

1219

CCH bend

582

CBr str

1147

1144

CCH bend

526

514

CBr str

1120

1119

CH2 twist

425

428

CBr def

1042

1030

cc str

174

173

CBr def

a79

878

CH2 rock

112

86

CCBr def

688

690

CBr2 ast

3020

3020

CH2 ast

610

614

CBr

1272

1273

CCH bend

564

566

CBr2 ast

1184

str

CH2 twist

336

326

CBr def

853

C&l2rock

208

221

CBr def

610

603

CBr str

163

173

CBr def

274

278

CBr def

148

149

CBr def

91

85

torsion

80

torsion

40

S. SUZUKIand G. VERGOTIZN

using the values of force constants of CHJZH*Br and CBr, gave the order which corresponded to our assignment, (b) the calculations on 12 chlorinated ethanes using the general valence force field [14] also gave a higher frequency for the CH, wagging mode than the CH bending modes in C,-CHCl,CH,Cl. The difference in the order of these frequencies is reflected in the values of force constants. In chlorinated compounds, CHRISTIAN et al. have found the values of HHCC(0.6902, 0.7343) are greater than those of HHccl (0.5879, 0.6140) for both groups, -CH,Cl and -CHCl*, while SUZUKI et al. have found the value of HHcc (0.708) is smaller than that of HHM (0.729). The reason why CHRISTIAN et al. used different values for the different groups is, perhaps, because they did not include trans- and gauche- interaction terms. For 1,1,2-tribromoethane, using the Urey-Bradley force field, we have found that the value of HHcc (0.340) is smaller than that of HHCBr (0.392) as a consequence of the assignment. It is interesting to note that the same trend has been found for secondary bromides using the general valence force field [lS], although the direct comparison of numerical values cannot be made because of the difference in the type of molecular force fields employed. To obtain a better agreement between observed and calculated frequencies, interaction terms between the CH2 rock, the CBrz rock and torsion have been introduced. These are, together with trans- and gauche- interaction terms, non-UreyBradley type force constants used in this calculation. CONCLUSION

Examination of new experimental results on the solid and gaseous Raman spectra together with a 1

normal coordinate analysis has enabled the previous vibrational assignment to be corrected.

Acknowledgements-The authors wish to thank Dr J. M. BRUCE for preparing the sample and Professor Sir G. ALLEN for his interest in this work.

REFERENCES

[l] S. MIZUSHIMA, Structure of Molecules and Mema Roration. Academic Press, N.Y. (1954). [2] S. D. CHRISIIAN, J. GRUNDNES, P. KLAEBOE, C. J. NIELSEN and T. WOLBAEK, J. Mol. Smruct. 34, 33

(1976). [3] I. MJYAGAWA, [4] [S] [6] [7] [8] [9]

[lo]

[ll] [12] [13]

[14] [15]

Nippon Kagakuzasshi 75, 1162 (1954). H. R. BUYS, C. ALTONA and E. HAVINGA, Tetrahedron Leti. 32, 3067 (1967). F. HEATLEY and G. ALLEN, Mol. Phys. 16, ?7 (1969). E. WYN-JONES and W. J. ORVILLE-THOMAS, Trans. Faraday Sot. 66, 1597 (1970). F. E. MALHERBE and H. J. BERNSTEIN, J. Am. Chem. Sot. 74, 1859 (1952). L. KAHOVEC and J. WAGNER, 2. physik Chem. B47, 48 (1941). T. TORGRIM~ENand P. KLAF.BOE,Acra Chim. Scand. 24, 114.5 (1970). Tables of Interatomic Distances (Ed. by L. E. SUTMN), Special Publication No. 11, Chem. Sot. Swppl. (1962). S. SUZUKI, J.-L. Brus~s and R. GAUF&.S, J. Mol. Speciry 47, 118 (1973). T. S HIMANOUCHI, Nippon Kagakuzasshi 86, 261, 768 (1965). T. S HmfAN0UCT-q Computer programs for normal coordinate treatment of polyatomic molecules, Tokyo (1968). S. SUZUKI and A. B. Da, J. Mol. Strut. 32, 339 (1976). G. A. CROWDERand M. I~UNZE, Can. J. Chem. 55, 3413 (1977).