Hyperfine Interaction Tensors of ¹3C Nuclei for Ring Carbons of Ubisemiquinone-10 Hydrogen Bonded in Alcohol Solvents

The anion radicals of ubiquinones-10 ¹³C chemically labeled at the C₅ or C₆ ring positions in alcohol have been studied by 1D and 2D ESEEM to define the hyperfine interaction tensors with the ¹³C nuclei. Analysis of the cross-peak line shapes and simulations of the spectra allowed us to conclude that the hyperfine tensors are characterized by an anisotropic component T ~6 MHz and an isotropic coupling a ~-3 MHz with support from DFT calculations. However, these values were found to be inconsistent with the shift of the sum combination harmonic in the four-pulse ESEEM spectra. Simulations resolve this apparent discrepancy by showing that the shift of the sum combination to lower frequency and its broadening can be accounted for by a distribution of the hyperfine couplings. A spread of the methoxy group conformations, as supported by previous experimental observations, is suggested as the mechanism influencing the distribution of the hyperfine couplings for the ring carbons.