Spin delocalization in the allyl and ethyl radicals

Abstract

An analysis is made of pi-electron spin delocalization into the methyl-group hydrogen pseudo pi orbital of an ethyl radical. Configuration-interaction molecular orbital theory with semiempirical integral parameters is employed to achieve a separation of the exchange polarization and the electron-transfer contributions. A set of calculations for the allyl-type radical (C3-C2=C1), which serves as a simpler model for the ethyl radical (C3-C2=H31), shows that essentially equivalent results are obtained from treatments with and without overlap. In the ethyl calculation without overlap, as well as in the allyl studies, it is found that both pi-electron exchange polarization and electron transfer are important, with the former contributing ∼60% and the latter ∼40% to the spin delocalization. A perturbation treatment is introduced to evaluate the dependence of the results on values of the integral parameters; it is shown that the relative contributions of exchange polarization and electron transfer are rather insensitive to the parameter values, although the two types of electron-transfer terms (into and out of the methyl group) are individually dependent on the electronegativity difference between the carbon pi orbital and the methyl group. The results demonstrate that neither a simple valence bond treatment (which is based on an exchange polarization mechanism) nor a simple Hückel calculation (which is constructed from electron-transfer terms) provides by itself a completely satisfactory description of spin delocalization in these systems.