The relationship between the molecular structure of semiquinone radicals and their g-values

The g-values of semiquinones are analyzed using molecular orbital (MO) theory. MO calculations clearly show the importance of molecular geometry and the local electronic environment, such as hydrogen bonds, on the g-values of semiquinones. In order to assess the magnitude of the effects, g-values are calculated using a modified method of Stone and Angstl. This new method uses the unrestricted Hartree-Fock (UHF) MNDO method with PM3 parameterization, is free of adjustable parameters, but necessarily incorporates several assumptions and approximations relevant to the use of UHF wavefunctions and excited states. For a number of semiquinone radical anions, differences between measured and calculated g-value components were as small as 1-5 x 10(-4). The effects of hydrogen bonds on g-values and spin densities are calculated. Hydrogen bonding distances obtained from the g-values an found to be in good agreement with ENDOR data. (C) 1997 Elsevier Science B.V.