EMPIRICAL FORCE-FIELD AND AB-INITIO CALCULATIONS ON DELOCALIZED OPEN-CHAIN CATIONS

Force-field calculations are reported for large delocalized cations. T he results for the geometries, heats of formation, and pi-electron den sities agree well with MP2(full)/6-31G ab initio calculations. Both m ethods give similar results for the distortion of the carbon skeletons of unsubstituted cations by hyperconjugating methyl groups. Because o f the SCF treatment of pi-interactions, the MMP2 force-field technique enables calculations of resonance energies in delocalized cations. Th e additional resonance stabilization produced by extending conjugation is directly related to the pi-charge on the carbon at which a vinyl g roup is substituted. The good agreement of MMP2 results for nonbonded resonance effects in large delocalized cationic pi-systems with ab ini tio data suggests that MMP2 can be used to study the influence of thes e interactions in cationic pi-systems too large to be calculated by co rrelated ab initio methods. (C) 1997 by John Wiley & Sons, Inc.