Comparison of Spin Density Calculation Methods for Various Alkyl-Substituted 9,10-Anthraquinone Anion Radicals in the Solution Phase

EPR and ENDOR spectra were recorded for 2-methyl-9,10-anthraquinone (2-methylAQ), 2-ethylAQ, 2-tert-butylAQ and 2,3-dimethylAQ anion radicals in the solution phase. The EPR spectra were simulated with the help of ENDOR data. The experimental isotropic hyperfine coupling constants (IHFCs) were compared with calculated values from semi-empirical INDO, spin-restricted AM1/CI and B3PW91 density-functional methods. The best computational methods for the IHFCs were the semi-empirical AM1/CI method and the B3PW91 density-functional method with a large basis set.