A NEW METHOD FOR EMPIRICAL FORCE-FIELD CALCULATIONS ON LOCALIZED AND DELOCALIZED CARBOCATIONS

A new empirical force field method for localized and delocalized carbo cations is described. Additional geometry parameters for carbocations were added to Allinger's MMP2 molecular mechanics program, which can t reat delocalized pi-systems. The effect of hyperconjugation in carboca tions is introduced via a quantum chemical term into force field calcu lations for the first time. The calculated heats of formation are in e xcellent agreement with a wide range of experimental data; the largest deviations are about 3.5 kcal/mol. The calculated structures agree ve ry well with those computed at correlated ab initio levels (MP2(full)/ 6-31G). The relative energies and geometries of different conformatio ns of representative carbocations also were in good agreement with MP4 /6-31G//MP2(full)/6-31G* results. (C) 1996 by John Wiley & Sons, Inc.