Unrestricted Hartree—Fock calculations of methoxyl-substituted semiquinone radicals and the proton hyperfine coupling constants

Abstract

Using the ab initio unrestricted Hartree—Fock method, spin densities are calculated for various members of the family of methoxyl-substituted p-semi, respectively, p-hydrosemiquinones. The empirical McConnell relation has been applied to estimate from these results the proton isotropic hyperfine splittings measured in electron spin resonance (ESR) experiments. The principle of additivity, which in general accurately describes the changes in the hyperfine coupling constants is violated on substituting two neighboring ring protons in p-semiquinone by the bulky methoxyl groups according to the experimental results. The breakdown of this concept, however, is not observed when the theoretically calculated spin densities are used. Therefore, solvent effects cannot be excluded to explain the determined anomalous ESR in the case of two adjacent methoxyl substituents in the ring. Furthermore, the general problem of the interpretation of spin-densities obtained from an unrestricted Hartree—Fock wave function which is not an eigenstate of spin is discussed using a projection operator technique. From the results it is obvious that the UHF-wavefunction obtained for all systems almost corresponds to the pure doublet spin state.