Accurate computations of the methyl isotropic hyperfine coupling constants in 2-methyl-1,4-benzosemiquinone radical intermediate

The accuracy of the UB1LYP hybrid density functional as a method of calculating the isotropic hydrogen hyperfine coupling constants, aiso(1H), of a rapidly rotating methyl functional group in a semiquinone radical is investigated for the first time. The 2-methyl-1,4-benzosemiquinone anion is chosen as the simplest case. Its geometry is optimized as the torsion angle between the semiquinone ring plane and the methyl hydrogen atoms is varied in 5° intervals. From these extensive computations, the rotationally averaged coupling constants, 〈aiso(1H)〉Rot, are calculated and found to be in excellent agreement with experimental data.