THERMODYNAMIC PROPERTIES OF CARBOCATIONS AND CARBANIONS - SOLVATION EFFECTS FROM AN ELECTROCHEMICAL AND THEORETICAL (AM1) STUDY OF SOME SUBSTITUTED BENZYL RADICALS

Gas phase ionization potentials (IP) and electron affinities (EA) for a number substituted benzyl radicals have been calculated using the AM1 method. There is a remarkably good correlation between the electrochemical oxidation potentials in acetonitrile and the calculated IP's (slope = 1.03, r = 0.995) and between the reduction potentials in acetonitrile and the calculated EA's (slope = 1.04, r = 0.996). These correlations, which include both monosubstituted radicals and alpha,para-disubstituted radicals, cover a potential range of ca. 2 eV for both the oxidations and reductions. The relationship between size and solvation energy of a carbocation in acetonitrile is demonstrated. Thus, the solvation free energy of the benzyl cation is ca. -40 kcal mol-1 while that for the methoxymethyl cation is -56 kcal mol-1. The data suggest that charge distribution, and not only size, is responsible for the observed differential solvation effects of the benzylic ions. It is concluded that the alpha-methoxybenzyl cations have solvation free energies that are at least 8 kcal mol-1 more exoergonic than either the benzyl or alpha-cyanobenzyl cations. This solvation effect is the result of a higher localized charge density on the benzylic group in the alpha-methoxy derivatives. Substituent-substituent interactions in the disubstituted ions are evaluated by using the electrochemical potentials to calculate the free energy change for isodesmic electron transfer reactions. Saturation effects are observed in the interaction of two stabilizing substituents (i.e. two methoxy groups on a cation, two cyano groups on a carbanion). Other substituent-substituent effects appear to be essentially additive.