Building a database of force constants based on scaled ab initio (SQM) results. I. Chlorobenzenes

Abstract

As part of a project for developing a database of harmonic force constants for organic molecules, the complete force fields for chlorobenzene, ortho-, meta-, para-dichlorobenzene and sym-trichlorobenzene have been determined, on the basis of ab initio Hartree—Fock calculations combined with empirical adjustments. The latter serve to correct for systematic errors in the theory, and are applied at two stages: the geometry is corrected by using empirical offset forces during the optimization; force constants are corrected by a few scale factors according to the SQM (scaled quantum mechanical) force field procedure. With scale factors taken over fixed from benzene and only two new scale factors introduced for the chlorobenzenes, experimental frequencies are reproduced with mean deviations of about 10 cm −1. Some controversial assignments, still present in the deuterated derivatives, are discussed. Theoretical IR and Raman intensities have also been calcuated and used as semiquantitative information to assist assignments.