| Abstract
| - A recently developed density functional approach has been used to carry out a systematiccomputational study of electronic g-tensors for a series of 1,4-semiquinone radical anions. Good agreementwith high-field EPR data in frozen 2-propanol is achieved only after taking into account the significantreduction of g-tensor anisotropy caused by hydrogen bonding to solvent molecules. The comparison ofvarious model systems for the first solvation shell suggests two hydrogen bonds from 2-propanol moleculesto each of the carbonyl groups of the radical anions, and one additional hydrogen bond to each of themethoxy groups in ubiquinone systems. 2-Propanol makes stronger hydrogen bonds than water and thusinfluences g-tensor anisotropy more strongly. Substituent effects at the semiquinone are reproducedquantitatively by the calculations. The g-tensor anisotropy is influenced significantly by the conformationsof methyl and methoxy substituents, with opposite contributions. Analyses and interpretations of theinterrelations between structure, bonding, and spectroscopic data are provided. The relevance of thecomputational results for the EPR spectroscopy of semiquinone radical anions in photosynthetic reactioncenters is discussed.
|
