The accuracy of the UB1LYP hybrid density functional as a method of calculating the isotropic hydrogen hyperfine coupling constants, a(iso)(H-1), 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 5degrees intervals. From these extensive computations, the rotationally averaged coupling constants, (a(iso)(H-1))(Rot), are calculated and found to be in excellent agreement with experimental data. (C) 2002 Elsevier Science B.V. All rights reserved.