Very high frequency EPR (95GHz) spectra of semiquinone radicals in frozen solution are reported. At this frequency, the increased Zeeman interaction readily resolves the anisotropic components of the g-matrix. The radical-solvent interactions are important in understanding the structural variations in the g-matrix. We have used a variety of solvents, both polar and nonpolar, to generate semiquinone radicals from various quinones and have measured the g-anisotropy. For the most part, the hyperfine splittings remain unresolved, even in deuterated solvents. Comparison of the g-matrix of semiquinone radicals in different solvents is quite important and hydrogen-bonding has been known to play a remarkable effect on specific components of the g-matrix. Thus the role of hydrogen bonds during the semiquinone radical formation is of particular importance to understand their formation in in vivo systems. There are consistent trends in the variation of the components of the g-matrix with respect to the nature of the solvents and structure. These trends can be understood in the light of SCF-MO (both ab initio and semi-empirical) calculations of the g-tensor, which show that the g-tensor is sensitive to both hydrogen bonding and ion-pair formation. These computations have become much more accurate and efficient during this grant year. This is an ongoing multi-year project.
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