Electron Spin Resonance Study of Oxygen-17 Enriched p-Benzosemiquinone Anion Radical

Role of zero-point vibrational corrections to carbon hyperfine coupling constants in organic π radicals

“…The methyl and benzyl radicals have been extensively studied previously 11,12,19,20,[22][23][24][49][50][51][52] and the importance of vibrational corrections to HFCCs in these radicals are by now well established. Among the anion radicals, only p-benzoquinone has been extensively investigated in the past, [53][54][55][56][57][58][59] as it represents the simplest possible model of a quinone-type electron acceptor in bacterial and plant photosystems. 12,53,54 However, to the best of our knowledge, vibrational effects on carbon HFCCs have so far have not been studied for any of the here selected anion radicals.…”

“…Among the anion radicals studied in this work, the pbenzoquinone anion radical has been the most extensively investigated by experimental and theoretical methods, [53][54][55][56][57][58][59][60] as it serves as the simplest model for quinone-type electron ac- ceptors in various biological systems, including bacterial and plant photosystems. In this system, the unpaired electron resides in a b 2g π -type orbital primarily localized on the oxygen atoms of the carboxyl groups, and due to this particular structure of the singly-occupied orbital, only carbons in the carboxyl group (see position one in Fig.…”

Hyperfine Coupling Tensors of the Benzosemiquinone Radical Anion from Car–Parrinello Molecular Dynamics

“…As the value of A iso for C ortho is also slightly overestimated (see below), the former reason appears more likely. Comparison of the dynamic and static values, for both the gas-phase and liquid states, indicates that neglecting dynamics decreases the 5 + COSMO] model). Hence, due to error compensation, static calculations are actually closer to experiment.…”

“…To a degree, such knowledge can be elicited from experimental data; for instance, comparison of A iso (the isotropic HFC constant) in protic and aprotic solvents indicates that hydrogen bonding causes spin density to shift from oxygen to ipso-carbon. [4,5] However, quantum chemical calculations provide the possibility of extracting further essential information. Indeed, from the earliest days of EPR measurements of these systems, theoretical calculations have been used to supplement and interpret the experimental data.…”

“…Two previous papers [29,30] focussed on the intramolecular and hydrogen-bonding dynamics and on g-tensors. Herein we present and analyse HFC data for our benzosemiquinone radical anion (BQC À ) simulation, and compare the results with both experimental [5,21,31] and previous computational data. [8-18, 20, 21] The two main goals of this work are 1) to provide a set of dynamically averaged benchmark results against which more approximate static calculations may be compared, and 2) to understand the effects of molecular and intermolecular motions on HFC tensors of such p radicals qualitatively and quantitatively on a microscopic level.…”