Calculation of unique force constants for the dihalomethanes

Abstract

The force constants have been determined from the vibrational spectral data of dibromo- and dichloromethane and their deuterated analogs. In previous studies of these molecules different unitary transformations were used for different isotopic analogs and it was therefore not possible to calculate unique values for the symmetrized force constants. In this investigation the same unitary transformation was used for the normal, deuterated, and partially deuterated analogs so that it was possible to calculate unique values for 17 of the 22 symmetrized force constants in the most general quadratic potential energy function. Seven of these were determined by direct calculation instead of the usual method of successive approximations. From the results of these calculations and the vibrational spectral data for CH 2I 2 and CH 2F 2, six symmetrized force constants including those corresponding to the CH and C-halogen stretching vibrations were estimated for the diiodo- and difluoromethanes. It is shown that, because of the angular redundancy, it is possible to calculate unique values for only five valence force constatns in the most general valence type quadratic potential energy function. These five corresponding to the CH, C-halogen stretches and their interactions were calculated for the dichloro- and dibromomethanes and estimated for the difluoro- and diiodomethanes.