Vibrational spectra of cis-stilbene

Journal of

MOLECULAR STRUCTURE Journal

of Molecular

Structure

349 (199.5)

29-32

Vibrational spectra of cis-stilbene. J.F. Arenas, I.L. To&n, J.C. Otero and J.I. Marcos. Department of Physical Chemistry, University of Malaga, E-2907 1-MQaga, Spain. Infrared and Raman spectra of cis-stilbene have been recorded and then interpreted with SQMFF methodology by using scale factors directly transferred from benzene and ethylene. The computed spectrum allowed for the complete assignment of in-plane vibrations of this molecule. 1. INTRODUCTION Vibrational spectrum of cis-stilbene has been the subject of very few studies while that of trans- isomer is rather well known [l-6]. A semiempirical calculation of the force field of cisstilbene was carried out by Warshel but his results were not compared with the experiment [3]. Anyway, he stressed the difficulty of analyzing the spectrum because of the steric hindrance of two hydrogens of benzene rings which are responsible for the non-planarity of the molecule. As a consequence, symmetry should be C2 instead of C2h and therefore every molecular vibrations should be i.r. and Raman active while trans-stilbene follows the mutual exclusion rule quite well [l-6]. That operative symmetry C2 has been confirmed by the i.r. and Raman results by Bree and Zwarich [7] because anomalous depolaritation ratios have been observed in the low frequency region. The respective bands correspond to out-of-plane vibrations which should be depolarized if the molecular point group were Czv. Because of the complexity of the spectrum, those authors have assigned only a few fundamentals which could be easily correlated with analogous modes of trans-stilbene. In this work, we report results of the force field calculation concerning in-plane vibrations of cis-stilbene with SQMFF methodology [8]. We think that this particular method is the most convenient one for molecules of moderate complexity where direct transfer of scale factors from related molecules is effective. In this case, scale factors have been transferred from benzene and ethylene. 2. RESULTS

AND DISCUSSION

All our quantum mechanical calculations were carried out by using a 3-21G basis. First of all, geometries of ethylene, benzene and cis-stilbene have been optimized and thereafter the i.r. respective force fields have been computed by analytical differentation in the equilibrium position. Internal coordinates have been chosen according to Pulay’s recomendations [9]. Finally, force constants of benzene and ethylene have been scaled in order to fit the respective experimental frequencies. Table 1 shows the calculated frequencies for in-plane vibrations of benzene which have been computed by introducing four scale factors (SET I), i.e. one for each internal coordinate. It is evident that agreement between computed and experimental data is good except for vibration 14, what made necessary to introduce an additional scale factor involving interaction 0022-2860/95/$09.50

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force constans CC ortho-, meta- and para- (SET II). Table 2 shows final results for ethylene; in this case four scale factors involving diagonal force constants have been optimized too. Table 1 Observed and calculated wavenumbers (cm-l) for in-plane vibrations of benzene. Exp.['ol

cdC.1

2

3072

El,, 20 a,b Qg, 7 a&

3064 3057

3086 3072

Symmetry Alp

3056

calc.11

P.E.D.(%)

P.E.D.(%)

100 UK-H) . , 99 u(C-H)

3086 3072

100 u(C-H) 99 u&-H)

99 u(C-H) 100 u(C-H)

3056

99 u(C-H)

3047

100 u(C-H)

69 u(C-C), 24 &C-H)

1608

69 u(C-C), 23 &C-H)

1482

’

3056

3047

Qg, 8 a,b El,, 19 a,b

1599

1600

1482

1482

Azg,3

1350

1363

69 &C-H), 31 u(C-C) 100 &C-H)

1363

68 &C-H), 32 u(C-C) 100 &C-H)

B2u.14

1309

1215

54 &C-H), 48 u(C-C)

1307

84 u(C-C), 18 &C-H)

E2g, 9 0

1178

1182

80 &C-H), 20 u(C-C)

1185

81 &C-H), 21 u(C-C)

B2u, 15

1146

1129

84 u(C-C), 12 @C-H)

1167

84 S(C-H), 18 u(C-C)

El,,18

1037

1035

Blu, 12

1010

1000

69 u(C-C), 33 &C-H) 100 Gring

1036

69 u(C-C), 33 @C-H) 100 king

Alg,

993

994

990

606

611

100 U(C-C) 90 Gring

13

Blu,

0

1

E2g, 6 0

1000 611

100 U(C-C) 90 Gring

Table 2 Observed and calculated wavenumbers (cm-l) for in-plane vibrations of ethylene. Symmetry B2u Big Ag B3u

P.E.D. (%)

2) (C-H)

talc. 3110 3081

2) (C-H) 2) (C-H) 2) (C=C)

3039 3021 1628

96 v (C-H) 100 2) (C-H) 61 u (C=C), 38 2) (C-H)

6 (CH2)

1443

100 6 (CH2)

6 (CH2) r (CH2) r (CIW

1343

64 6 (CH2), 36 2) (C=C) 100 r (CH2) 100 r (CH2)

Exp.I”l 3105

Description

3086 3026

Ag B3u

3021 1630 1444

Ag

1342

Big l32u

1220 826

u (C-H)

1217 827

100 2) (C-H) 100 2) (C-H)

Scale factors have been transfered directly from benzene and ethylene to cis-stilbene. However, internal coordinates of the type ring-substituent (CX and CCC) have to be defined in this molecule; their respective scale factors have been fixed to be 0.8 with the exception of that involving u(C-X) which has been increased up to 0.91 to fit the experimental frequency of that vibration in the spectrum of trans-stilbene. Table 3 summarizes scale factors used to compute the theoretical spectrum of cis-isomer according to the C2 symmetry given by 3-21G basis.

31

Table 4 Calculated and observed in-plane vibrational frequencies (cm-l) of cis-stilbene. Mode A

IR 3079 m

3054 s

P.E.D. (%)

ModeB

P.E.D. (%)

3073

100 U&X-I)

3073

100 W-H)

3061 p, s

3060

98 U(C-H)

3060

99 W-H)

3049 dp, s

3047

98 u(C-H)

3047

99 u(C-H)

3040 84 U(C-H)e, 15 u(C-H) 3024 s

3030 dp, sh

3012 sh

3014 p, m

3036

87 u(C-H), 11 U(C-H)e

3036

98U(C-H)

3028

97 u(C-I-I)

3028

98 W-H)

3016

97 NC-H),

1629 p, vs

1656

56 u(C=C), 17 u(C-X)

1600s

1599 p, s

1615

64 u(C-C), 19 &C-H)

1617

66 u(C-C), 20 &C-H)

1576 m

1573 p, m

1588

66 u(C-C), 19 &C-H)

1591

69 u(C-C), 20 &C-H)

1498

58 &C-H), 34 u(C-C)

1452

55 &C-H), 33 u(C-C)

1402

69 &C-H), , 14 u(C-X)

1495 s 1490 s

1490 p, w

1494

63 &C-H), 33 u(C-C)

1449 s 1444s

1443 p, w

1406 m

1405 dp,vw

1336 VW 1333 ?, sh 1305 p, m 1234 p, m 1203 w

1448

60 &C-H), 34 u(C-C)

1339

84 &C-H)

1336

82 &C-H), 14 u(C-C)

1312

70 u(C-C), 14 NC-H),, 13 &C-H)

1295

85 u(C-C)

1244

46 u(C-C), 31 &C-H), 1203

33 u(C-X), 24 u(C-C), 15 &C-H)

1203 dp, m 1193 p, m

1191

82 &C-H), 17 u(C-C)

1190

81 &C-H), 18 u(C-C)

1180m

1182p,m

1173

78 &C-H), 21 u(C-C)

1172

80 &C-H), 19 u(C-H)

1156m

1149p, s

1la

23 u(C-C), 22 u(C-X), 18 &C_H)e,15 Gring

1078

57 u(C-C), 38 &C-H)

1082

54 W-C),

1074 s

416(C-H)

1029 s

1029 p, s

1026

70 u(C-C), 18 &C-H)

1027

66 u(C-C), 21 &C-H)

1001 w

1001 p, vs

998

43 Gring, 24 u(C-C)

995

62 Gring, 38 u(C-C)

619 w

619 dp, m

89 Gring

519 p, w

87 king 45 Gring, 14 GCCC

623

519 w

625 521

500

54 Gring, 11 u(C-X)

259

55 S(C-X), 14 ting

502m 261 p, m

248

28 ting,

20 GCCC,

20 &C-X)

32

Table 3 Optimized scale factors. Benzene Description Factor

Ethylene Description Factor 0.78 2) (C=C)

Benzene-Ethylene Description Factor 0.91 2) (C-X)

u (C-H)

o*83

U (C-C)

0.87

2) (C-H)

0.83

6 (C-X)

OS8

6 (C-H) 6 ring

0.78 0.76

6 (CH2) r (CH2)

0.77 0.77

6 (CCC)

O-8

2) (C-C)o,m,p

0.69

Generally speaking, the fit between experimental and computed frequencies is quite good. However, vibrations clearly involving either ring stretchings or ethylenic double bond stretching (1700 - 1500 cm-l region) are computed systematically in excess, what suggests that 3-21G underestimates molecular electronic delocalization. On the other hand, four fundamentals are calculated at frequencies higher than 1400 cm-l corresponding to the vibrations 19ab of benzene, which can be observed split in the infrared spectrum according to the symmetry C2. Likewise, vibration G(CHe) B, is easily assigned to the Raman depolarized band recorded at 1405 cm-l while the respective mode of symmetry A participates in several fundamentals of the 1300 cm-1 region. These vibrations exhibit two significant differences with respect to the trans- isomer. First, in cis-stilbene it is difficult to assign mode G(CHe) A to any single band while in trans-stilbene it is quite straightforward to identify it; secondly, separation between symmetric and antisymmetric modes is significantly higher in the spectrum of cis-stilbene what suggests a stronger coupling between modes when both CHe bonds are in cis position. Finally, it is to be stressed the excelent agreement between experimental and computed frequencies as well as the optimum prediction of the order of appearing of the fundamentals, as can be seen by correlating the observed polarized Raman bands with the calculated A fundamentals. Therefore, the assumed hypothesis of the transferibility of scale factors between these molecules is sufficiently proved.

Acknowledgement The authors would like to express their gratitude to the CICYT for the financial support for this work through Project # PB90/0806.

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