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

Abstract

The effect of isotopic substitution on the electron affinity ({Delta}EA) of benzene and pyrene was studied using semiempirical MO calculations. Zero-point energy calculations were performed for various {sup 1}H/{sup 2}H- and {sup 12}C/{sup 13}C-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 {Delta}EA are 50-60% of the experimentally observed values. For example, {Delta}EA{sub calc} = 260 cal/mol for the benzene-h{sub 6}benzene-d{sub 6} couple, while {Delta}EA{sub exp} = 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 {Delta}EA showed no obvious general correlation to be present. 22 refs., 3 figs., 4 tabs.