Application of self-consistent-field ab initio calculations to organic molecules

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Abstract
The scale-factor method has been applied in order to derive the General Valence Force Field for ethene from ab initio (4–31 G) force constants. The use of five scale factors proved to be sufficient for the calculation of 50 harmonic frequencies of C2H4, C2D4, CH2CD2 and 13C analogues within narrow limits: the average deviation amounted to 3·0 cm-1 or 0·26 per cent. Adjustment of the five scale factors on 81 observed frequencies of C2H4, C2D4, CH2CD2, cis- and trans CHDCHD, C2H3D and C2HD3 resulted in an average error of 7·1 cm-1 or 0·42 per cent. Two serious discrepancies were noted between calculated and experimental data: v 6 and ω 6 (C2H4) were calculated 17 cm-1 too high, whereas the calculated 13C shift of ω 9 (C2D4) amounted to 5·1 cm-1 (observed Δω 9 = 6·4 ± 0·5 cm-1). Infra-red and Raman intensities have been obtained using the same basis set. Calculated infra-red intensities and electro-optical parameters show good agreement with experimental ones, in contrast to calculated Raman intensities and degrees of depolarization, which deviate largely from the experimental ones.