Abstract
Calculations of optimized force field (MMP2 extended to carbocations) and ab initio (MP2/6-31G*) geometries as well as π-electron densities of various benzyl and cyclic delocalized cations agree well. The MMP2 heats of formation reproduce the available experimental values. MMP2 π-resonance energies are consistent with those obtained by isodesmic equations from experimental and ab initio data. When carbon π-charges are lower than 0.2, the influence of phenyl substituents is attenuated. Thus, the triphenylmethyl cation resonance stabilization value (−41.6 kcal/mol average for each phenyl ring) is much less than that of the benzyl cation (−76.4 kcal/mol) and the benzhydryl cation (average stabilization value of −51.4 kcal/mol). MMP2 aromatic stabilization energy estimates of the benzyl and tropylium cations as well as benzene agree well with the assessments of aromaticity by the nucleus independent chemical shift (NICS) criterion, which is based on the magnetic shieldings computed at ring centers. The MMP2 m...
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