3-Methyl-3-butenenitrile and 3-bromo-2-methyl-1-propene; normal coordinate calculations based on valence force fields

Specrrochlmlca Acto, Vol

39A, No. 4, pp. 327-330,

0584-8539~83~0403270404103 oO,O

1983

C 1983 Pergamon Press Ltd

Prmted m Great Bntam

3-Methyl-3-butenenitrile and 3-bromo-2-methyl-1-propene; normal coordinate calculations based on valence force fields S.H. Department

of Chemistry,

Umversity

(Recewed Abstract-Normal

CH,=C(CH,wH,X,

SCHEI

of Trondheim,

NLHT

10 August

Rosenborg,

7000 Trondhelm,

Norway

1982)

calculations have been made for the actual conformers of the molecules X = CzN, Br. A valence force field of 37(34) force constants reproduced the 33(30)

coordinate

fundamental frequencies for X = C=N(Br) to an average devlatlon of 14( 11) cm- ‘. For 3-bromo-2-methyllpropene the calculation suggests a reassignment. According to tins, the existing experImenta data may be reinterpreted as 3-bromo-2-methyl-1-propene having 2 conformers, instead of 3 as suggested earher

INTRODUCTION

The 3-substituted propenes have been the subject of several vibrational spectroscopic studies [l-4], and normal coordinate analysis using symmetry force fields have been reported [3,5]. Recently also some vibrational spectra of molecules of the type CH,=C (CH+CH,X have appeared [G-8]. A symmetry force field was developed for 3-chloro-2-methyl-1-propene [8], while normal coordinate calculations have earlier been made for 3-methyl-3-butenenitrile [9] (hereafter abbreviated MBN) and 3-bromo-2-methyl-1-propene (BMP) using a valence force field. However, both these calculations were made for the anti conformer, torsion angle 5 = 180” (see Fig. 1). From microwave spectroscopy and electron diffraction conformational studies of molecules of the type CH,=CH-CH2X, X=Cl [lo, 111, X=Br[l2, 131 and X=GN[14, 151; it is clear that the actual conformers are syn, z = 0” and gauche, t = 120”. The same conformational situation has been found from electron diffraction study [ 151 and vibrational spectral data [7,8] for 3-chloro-2methyl-1-propene and MBN. Correspondingly, it would be expected that BMP exists mainly as gauche conformer with minor contribution of syn conformer. Thus, there may be some doubt as to whether this molecule exists as three conformers as earlier suggested [6]. A normal coordinate analysis for the actually exist-

Fig. 1. Numbermg of the carbon skeleton of 3-methyl-3butenemtrlle (X = C,=N) and 3-bromo-2-methyl-l-propene (X = Br). The conformer shown 1s syn, 7 = 0”.

ing conformers, may be desirable from an interpretative point of view; especially for BMP since the predominant conformer of this molecule most likely do not possess any symmetry.

CALCULATIONS In connection

with a structural

and conformational

of the type CH,=CH-CH2X, X=Cl, Br, GN[ll, 13, 151; quite simple valence force fields have been developed. Testing the force field of 3-chloro-1-propene for transferability to other molecules containing the fragment C=GC-Cl, gave encouraging results. Thus, it is likely that also for X=Br and CEN the valence force fields will possess enough transferability to serve as an interpretative aid for frequency assignment of related molecules. Valence force fields for 3-butenenitrile, 3-bromo-lpropene, propene and isobutene [ 151 were developed in a consistent way. They all contain as few interaction force constants as possible, while still reproducing the observed frequencies to an average deviation of 5 cm- ‘. The valence force fields for the above mentioned molecules were combined to the force fields given in Table 1 for MBN and BMP. One problem m doing such a calculation, will probably arise from extra strain in the angles around the C, atom m syn form compared to the 3-substituted propenes. Further, there may be additional gauche interaction between the CH, and CH,X groups. As to the torsional force constants, f,, the same values were used for MBN as for 3-butenenitrile [15]. To find suitable values for BMP, semiempirical molecular mechanics (MM) calculations were made for 3bromo-1-propene and BMP. Non-bonded potentials were in the form of Morse_potential[16]. Other parameters were identical to those used for MM calculations of bromoalkanes [17], except for the C+..Br potential function, for which a modified value of R, = 3.01 was used. This is a similar change as was made for conversion of C,,~..cl non-bonded potential to C,,~.Cl[18]. When the MM obtained gauchef, of study

of some

molecules

S. H.

328

SCHEI

and 3-bromo-2-methyl-l-propene. Valence force constants, in mdyn A- ’ and mdyn A (rad)) 2, given as gauche(syn) values

Table 1. 3-Methyl-3-butenenitrile

No.

Type

1 2 3 4 5 6 I 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 21 28 29 30 31 32 33 34 35 36 31 38

str.

C=C

c2-G

c,-x

GG C=N =C-H C,-H bend

C;-H C&-H H<,-H C=CC, C=CPC*

G&-c4 c,x=,-x

tors.

0.o.p. hn. bend str/str

str/bend

bend/bend

MBN

Coordinate*

CC,-H XC-H CC,-H HfZ-H HfZ-H C=C C,-c, C,AJ, =CH, =cc, C-CzN GG/C,-c, C,+&LG C,-H/C,-H C3-H/C3-H C=C/C=C-H c=C/c,-c,+ CC,/CC,-H C<.,/CC,,-H C,.4-c&J-c2G C,, ,&,/C,-C~-G C,-C,-H/C&Z-H C,-C,-H/X-C-H C-Cd-H/CC4-H

9.14 4.17 4.24 4.24 17.87 5.00 4.67 4.71 0.52 0.40 0.87(1.24) 0.98 0.73 0.73(1.37) 0.69 I 0.64 0.51 0.54 0.49 0.09 0.08 0.22 0.35 0.34 0.25 0.78 0.05 0.07 0.41 -0.43 0.23 0.38 0.32 0.43 -0.02

BMP 9.14 4.44 2.30 4.24 5.00 4.83 4.71 0.52 0.40 0.82(1.37) 0.98 0.73 0.89(1.63) 0.75 0.64 0.64 0.54 0.54 0.49 0.16(0.06) 0.08 0.22 0.35

0.78 0.05 0.05 0.41 - 0.43 0.59(0.36) 0.38 0.32 -O.OS( -0.02) 0.09(0.12) - 0.02

*For atom numbering see Fig. 1.

3-bromo-1-propene was multiplied by a factor 1.8, the experimentally obtained gauche root mean square amplitude of vibration was reproduced. Correspondingly, the MM obtained f, values of BMP were multiplied by the same factor, giving f = 0.16 and 0.06 mdyn A rad-* for gauche and syn respectively. For the geometries of the molecules, bond lengths and angles were transferred from 3-butenenitrile [ 151, 3-bromo-1-propene [13] and propene [19]. Calculated frequencies are listed in Table 2, along with the observed ones. The calculated frequencies of MBN deviate from observations by an average of 14 cm- I. The deviation for BMP is more uncertain, since the frequency assignments were given from less extensive data [6]. The correspondence between calculations and observations as suggested in Table 2, show an average deviation of 11 cm- ’ for the gauche conformer of BMP. For MBN the calculations reproduce the assignments[7] well. There are two deviations as large as 54cm-‘, v13 and v15, both partly involving C=C-H

bends and C-C stretches. The corresponding gauche v13 is calculated to agreement with the observation, whilegauche vr,, involving the same coordinates, is less well calculated. Some deviations must be expected for the frequencies associated with the carbon skeleton, and even these deviations are not larger than such a calculation as a whole can be of interpretative help. Such interpretative support may be needed for BMP. The calculations suggest frequency associated with a C-C stretch, as high as 1326cm-r. Also comparison to 3-chloro-2-methyl-1-propene [8] and MBN make it likely that the observations at 1285 (1260)cm-‘, v 1,, could be approximately described as CC stretch for gauche (syn) conformer. Frequencies of the same molecules [S, 91 correspond well to several of the present interpretations for BMP. For CH, and =CH, rock the correspondence to observations are somewhat uncertain. The two syn frequencies around 60&700 cm- ’ may be interpreted as C-Br stretch and C=C torsion. By giving an alternative interpretation in the areas 12W1300 and 60&7OOcm-‘, a two con-

“33

“32

v31 .~

“30

“29

“26 “27 “28

“25

vz4

v23

“20 “21 “22

v19

“16 “17 “1s

“15

“14

VI3

“IO “II “12

“9

“6 “7 “s

y5

Vt “3 v4

“1

Table

Cd.

3091 3082 2997 2981 2983 2962 2921 2911 2912 2890 2247 2251 1660 1655 1450 1450 1418 1411 1418 1418 1383 1376 1330 1311 1200 1254 1017 992 972 918 850 874 830 831 554 568 372 410 355 358 167 167 2976 2961 2947 2941 1450 1451 1218 1239 1045 1046 918 936 904909 685 696 423 429 355 378 210 77

obs.

17(21)

1 l(24)

19(19) 19(28) 9(17)

Potential energy distribution (%)

8(98) 3(91) 1(84) 19(91) 18(78) lO(58) 9(38) 17(22) 15(21) 15(46) 17(33) 15(39) 4(24) 17(75) 9W) 3(33) 4(47) 3(18) 13(26) 2(48) 14(19) 4(21) 12(26) 25(26) 13(42) 12(21) 25(40) 14(36) 8(100) 7(100) 19(92) 15(100) 17w 23(42) 15(27) 23(46) 15(44) 20(97) 24(88) 25w) 22(100) 21(91)

W9) W9) WW V8)

Vibrational frequencies (cm- ‘)

792 415 364 212

1227 1025

2956

812 573

980

1296 1270

obs.

2961 2942 1450 1220 1041 932 901 688 427 381 212 78 “30

“29

“28

“26 “27

“25

“20 “21 “21 “23 “24

“19

“I8

“14 “15 “16 “17

“13

“IO “I1 “12

y9

“8

“6 V7

“5

“4

“3

1450 1130 1010 910 880 723 395

815 604 462 375

1642 1456 1440 1420 1375 1285 1207 973

2940 2910

7(W 8(W 19(91) 15(50) 17(71) 23(76) 16(38) 20(71) 24(33) 22(95) 21(87)

210 100

1lW) 14(52)

1(85) 18(71) 19(86) lO(55) 17(27) 17(33) 15(72) 17(74) 9(52) 4(56) 3(30) 12(28)

8(78) 7(76) 8(98)

W9 66’9)

2992 2959 1439 1146 1033 921 882 710 409

1647 1455 1439 1418 1376 1326 1199 985 913 828 635 467 362 155

3086 2975 2961 2959 2893

3075 2970

“2

3081 2981 2962 2909 2890 2251 1650 1451 1401 1418 1354 1294 1276 991 916 858 776 558 393 361 VI

CA.

obs.

Vibrational frequencies (cm- ‘)

talc.

Vibrational frequencies

2(17)

14(16) 3(18)

4(20)

12(29)

13(23)

2(21) 15(15)

16(36)

2(25) 2(15) 20(28) 24(22) 13(27) 24(17)

19(25) 19(28) 16(34)

9W)

7(22) 8(23)

Potential energy distribution (%)

In terms of

673?

1160

636?

1260

1400

obs.

210 67

2991 2959 1439 1174 1027 928 885 686 429

1665 1462 1439 1420 1369 1286 1229 984 904 854 690 458 305 203

3087 2975 2961 2960 2893

CdC.

Vibrational frequencies

syn conformer

energy distribution

3-Bromo-2-methyl-1-propene

ones. Potential

gauche conformer

calculated frequencies compared to observed are given for the most abundant conformer

gauche conformer

3-bromo-2-methyl-1-propene, valence coordinates

3-Methyl-3-butenenitrile

and

syn conformer

2. 3-Methyl-3-butenenitnle

M

w

S. H. SCHEI

330

former interpretation seems more reasonable inclusion of a third conformer.

than

CONCLUSIONS

No attempt was made to refine force constants to fit observations better. Such an adjustment would be beyond the scope of this study. Valence force fields consisting of 24(23) distinct diagonal and 13(11) interaction valence force constants were used to calculate the 33(30) fundamental frequencies of MBN (BMP). Showing average deviations from observations ofl4andllcm‘, the calculations provide an example that quite simple valence force fields can be used for interpretative use when developed and transferred with care. REFERENCES

Cl1R.

D. MCLACHLAN

and R. A.

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PI

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M

(1976). A. 0. DIALLO, Spectrochim. Acta 36A, 799 (1980). [;j D. A. C. COMPTON, S. C. HSI and H. H. MANTSCH, J. phvs. Chem. 85. 3721 (1981). A. C. CO&TON, s. C. ‘HsI, H. H. MANTSCH and PI W. F. MURPHY, J. Raman Suectrosc. 13. 30(1982). A. 0. DIALLO, Spectrochim.Acta 35A, il8d (197$). E. HIROTA, J. Mol. Spectrosc. 35, 9 (1970). S. H. SCHEI, Q. SHEN and R. L. HILDERBRANDT, to be t-111 published. [12] Y. NIIDE, M. TAKANO~~~ T. SATOH, J. Mol. Spectrosc. 63, 108 (1976). 1131 S. H. SCHEIand Q. SHEN, J. Mol. Struct 81,269 (1982). [I41 K. V. L. N. SASTRY, V. M. RAoand S. C. DASS, Can. J. Phys. 46, 959 (1962). S. H. SCHEI, J. Mol. Struct., in press. [16j R. J. ABRAHAM and R. STBLEVIK, Chem. Phys. Lett. 58, 622 (1978). 1171 T. RYDLAND, Thesis, UnIversIty of Trondheim (1981). 8. THINGSTAD, Thesis, Umversity of Trondheim (1981) I. TOKUE, T FUKUYAMA and K KUCHITSU. J. Mel

b.’

PI Cl01

rif

Struct. 4, 255 (1969). Note added rn proof: In Table 1 there should constant, C=C-C,/C-C-X

be an additional interaction force of value 0.45 for syn conformer