Comparison of ab initio,CNDO/2 and experimental dipole moment derivatives for C2 hydrocarbons and formaldehyde

Dipole moment derivatives determined by ab initio and CNDO/2 calculations are compared with the corresponding data obtained from infrared intensities. For ethane, ethylene and formaldehyde, the recent results of quadratic force field calculations have been used to calculate experimental derivatives; for the latter two molecules, the individual intensities of certain overlapping bands were determined from the results of rovibrational analysis. The experimental dipole moment derivative with respect to the rocking symmetry coordinate, S₁₀, of ethylene has been found to be 0.03 D, as opposed to the value of ca. 0.4 D reported previously. CNDO results agree both in sign and magnitude with ab initio dipole moment derivatives.