A b i n i t i o MO calculation of the anharmonic force field of methyl fluoride and methyl chloride

Abstract

Several levels of ab initio MO calculations have been carried out to obtain the anharmonic symmetry force field of methyl fluoride and methyl chloride. The basis sets used were 4-31G* and 6-311G** for methyl fluoride, and 4-31G* and Dunning’s double zeta basis set augmented with polarization functions for H, C, and Cl atoms for methyl chloride. Also the effect of the configulation interaction was explored by using the Mo/ller–Plesset second-order perturbation theory. For each case, the molecular energy was calculated for about 230 or more different displacements to determine 55 force constants; 12 quadratic, 38 cubic, and 5 quartic force constants. The remarkable result concerned with the cubic force constants is that although the diagonal stretching force constants decrease on going from lower to higher levels of calculation, the differences between the off-diagonal force constants for different levels of calculation employed here are almost negligibly small. In fact even the force constants obtained through HFSCF calculation for 4-31G* basis set were found to be able to reproduce quite well the observed values of the spectroscopic constants of these molecules. Also a close similarity of the cubic and quartic force constants between the two molecules is noted.