The empirical general harmonic force field of methylene chloride

Abstract The general harmonic force field of methylene chloride has been calculated without the necessity of imposing constraints, through the use of a set of 74 observables over eight isotopic species. These include vibration frequencies, 37 Cl and 13 C isotopic frequency shifts, and quartic centrifugal distortion constants. All force constants are well defined by the data with one exception, and this takes a value close to that predicted by transfer from methyl chloride, although with an uncertainty larger than itself. The force field is used to predict precise isotopic distortion constants, in terms of differences from the accurate experimental values for CH 2 35 Cl 2 and CD 2 35 Cl 2 , and to calculate Coriolis interaction constants which will be of assistance in high-resolution rovibration studies. Alternative definitions which may be used for the deformation coordinates of an XY 2 Z 2 molecule are considered. That which associates each deformation uniquely with either the XY 2 or XZ 2 group is clearly preferred.