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

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.