MODERN MOLECULAR MECHANICS AND AB-INITIO CALCULATIONS ON BENZYLIC ANDCYCLIC DELOCALIZED CATIONS

Calculations of optimized force field (MMP2 extended to carbocations) and ab initio (MP2/6-31G) geometries as well as pi-electron densities of various benzyl and cyclic delocalized cations agree well. The MMP2 heats of formation reproduce the available experimental values. MMP2 pi-resonance energies are consistent with those obtained by isodesmic equations from experimental and ab initio data. When carbon pi-charges are lower than 0.2, the influence of phenyl substituents is attenuate d. Thus, the triphenylmethyl cation resonance stabilization value (-41 .6 kcal/mol average for each phenyl ring) is much less than that of th e benzyl cation (-76.4 kcal/mol) and the benzhydryl cation (average st abilization value of -51.4 kcal/mol). MMP2 aromatic stabilization ener gy estimates of the benzyl and tropylium cations as well as benzene ag ree well with the assessments of aromaticity by the nucleus independen t chemical shift (NICS) criterion, which is based on the magnetic shie ldings computed at ring centers. The MMP2 method allows quantitative e valuations of homoconjugative interactions. The stabilization in the h omotropylium cation due to 1,7 homoconjugative overlap is estimated to be quite appreciable, -13.4 kcal/mol.