Ab initio and LMO studies of integrated intensities of infrared absorption bands of polyatomic molecules. Part 2.—Chloroacetonitriles

Abstract

Minimal basis set ab initio(STO-6G) calculations are performed on the equilibrium dipole moments and dipole moment derivatives of the chloroacetonitriles CH_3–nCl_nCN (n= 0, 1, 2, 3) in order to explain the experimentally observed behaviour of the CN and CC stretching intensities in this series. New force fields are presented for acetonitrile and dichloroacetonitrile. A normal coordinate analysis is perfomed for the corresponding modes. The experimental minimum in the CN intensity for the monochloro-derivative is not found theoretically. Equilibrium dipole moment decomposition schemes together with bond moment analyses of the various dipole moment derivatives permit rejection of the existing literature hypotheses and explanations on the CN intensity evolution in the chloroacetonitriles. The essential factor which governs the computed CN intensity evolution was seen to be vibrationally and electronically localized in the CN bond. A careful examination of the displacements of the nitrogen–atom nucleus and the centroid of negative charges in the nitrogen lone pair LMO led to an extension of the concept of incomplete orbital following, which is well known in bending vibrations, to the case of stretching modes. The different behaviour of the CN bond during the CN stretching vibration in CH₃CN and CCl₃CN was seen via this technique, to be due to a reversal of sign of the µ_CN bond moment variation during molecular distortions along the CN normal coordinate.