BUILDING A DATABASE OF FORCE-CONSTANTS BASED ON SCALED AB-INITIO (SQM) RESULTS .1. CHLOROBENZENES

As part of a project for developing a database of harmonic force const ants for organic molecules, the complete force fields for chlorobenzen e, ortho-, meta-, para-dichlorobenzene and sym-trichlorobenzene have b een determined, on the basis of ab initio Hartree-Fock calculations co mbined with empirical adjustments. The latter serve to correct for sys tematic errors in the theory, and are applied at two stages: the geome try is corrected by using empirical offset forces during the optimizat ion; 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 reprod uced with mean deviations of about 10 cm-1. Some controversial assignm ents, still present in the deuterated derivatives, are discussed. Theo retical IR and Raman intensities have also been calculated and used as semiquantitative information to assist assignments.