are obtained if the metal substitution does not alter the first shell too much.Examination of Table II shows the number of different factors that are important in determining the EPA. The Born estimate of the reaction field interaction is the most significant. The cavity radius increases by about 0.25 Á in going from ZnCA to CdCA. This leads to a relative stabilization of ZnCA in the protonated state while the high-pH complex has roughly constant Bom energy for all the dications. Consequently, the spread in the EPA is reduced by about 7 kcal/mol relative to the SCF cluster values. In one case, the higher moment contributions to the Born energy have a qualitative effect on the relative EPA values. The reaction field stabilization energy of the MnCA high-pH model is smaller than that for the larger CdCA cluster. This is because of the larger M-O-H angle found for Mn (see Table I), giving a more symmetric cluster with smaller dipole and quadrupole moments leading to a smaller reaction field energy.

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