How does isotopic substitution affect electron affinity? PM3 calculations on benzene and pyrene

The effect of isotopic substitution on the electron affinity (ΔEA) of benzene and pyrene was studied using semiempirical MO calculations. Zero-point energy calculations were performed for various 1H/2H- and 12C/13C-substituted benzenes and their corresponding radical anions and for partially deuterated pyrenes and pyrene dianions. PM3 turned out to be appreciably better than either AM1 or MNDO in calculating the vibrational frequencies of neutral benzene. PM3 results were also in good agreement with the experimental IR data for the benzene radical anion The calculated values for ΔEA are 50-60% of the experimentally observed values. For example, ΔEAcalc = 260 cal/mol for the benzene-h6benzene-d6 couple, while ΔEAexp = 442 cal/mol. This shows that effects of this small size can reasonably be reproduced. A study of the relation between the charge on the carbon atom at which substitution takes place and the size of ΔEA showed no obvious general correlation to be present. 22 refs., 3 figs., 4 tabs