Towards a general scheme for estimating the heats of formation of organic ions in the gas phase. Part II. The effect of substitution at charge-bearing sites

The effects on ionic heats of formation of the multiple substitution of -CH3, -OH, or -OCH3 groups, in place of H atoms, are described. It is shown that for all three substituents, good linear correlations exist between ΔHf(ion) and the logarithm of the number of atoms in the ion (i.e. a quantity proportional to ionic size). The changes in ΔHf(ion) resulting from successive substitutions at a charge-bearing atom or π-system essentially depend only upon the specific nucleophilic character of the group and the size of the resultant ion. Examples are given of the utility of these correlations in estimating new ionic heats of formation.