Electron Spin Resonance of Semiquinones: Spin-Density Distribution and Carbonyl Sigma—Pi Parameters

Abstract

The spin-density distributions in the 1,4-benzosemiquinone and 2,5-dioxy-1,4-benzosemiquinone ions have been estimated by measuring the proton and 13C splittings and using reasonable choices for the values of the sigma—pi parameters which relate the splittings to the spin densities for C–C and C–H bonds. The determination of the signs as well as the magnitudes of the 13C splittings was required to obtain the spin densities. With QCHH=−27.0 G in the relation aiH=QCHHρi π between proton splitting aiH and carbon-atom—pi-electron spin density ρiπ, the sigma—pi parameters for the 13C splitting at a carbonyl position were found to be QCOC=17.7 G and QOCC=−27.1 G. The values of QCOC and QOCC depend on the value assumed for QCHH. Contrary to earlier predictions made on the basis of molecular orbital calculations, the sign of the 13C splitting from the 2 position of the p-benzosemiquinone ion was found to be negative. This discrepancy results because the MO calculations were performed so as to reproduce proton splittings rather than the entire spin-density distribution, a procedure that is insensitive to the spin densities on the carbonyl carbon and oxygen atoms. New MO calculations were performed to fit the spin densities at all the positions in the radical. The results show that calculations fitted to proton splittings alone are inadequate for predicting spin densities at ``blind'' positions such as those at the carbon and oxygen atoms of a carbonyl group, or the nitrogen and oxygen atoms of a nitro group. Carbon-13 splittings were also observed in the anthrasemiquinone ion in both ethanol and 1,2-dimethoxyethane (DME) solutions. The sign of the 9-position 13C splitting changes from +0.70 G in ethanol to −0.47 G in DME solution. By using the sigma—pi parameters for the carbonyl group carbon atom, the spin densities were determined for this radical and MO calculations performed to reproduce the spin-density distributions. The variation of the splittings in the p-benzosemiquinone ion with solvent composition could be accounted for quantitatively by using the newly determined sigma—pi parameters.

Asymmetric binding of the primary acceptor quinone in reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26, probed with Q‐band (35 GHz) EPR spectroscopy

“…The spectral features of Q~-were shown to be much more similar to the radical anion of UQ-10 in frozen isopropanol than Q~- [1,2]. An inequivalence of the hydrogen bonds to the two carbonyl oxygens of QA was proposed from the crystal structure [3], which may be related to an asymmetric electron spin density distribution in Q[-, as distinct from the situation in isopropanol glass, where the electron spin density is (nearly) symmetrically distributed over the molecule [4]. Proton ENDOR of the methyl group of Q~-, however, provides only indirect information on the carbon spin densities.…”

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.…”

An estimation of the magnitudes of the contributions from spin polarization and from hyperconjugation to the hyperfine splitting constant of an hydroxylic proton

“…It is of interest, nevertheless, to calculate, from the spin density distributions used to evaluate Q, and Q,, the expected 13C hyperfine splitting constants, and to compare them with the experimental values. Using standard notation (24) a similar equation holding for adC. We take The monoprotonatedp-benzosemiquinone radical may be regarded as the 4-hydroxyphenoxy radical, so that atC may be calculated using QcoC = 8.9 G, eoCc = 16.4 G. Unfortunately, similar parameters to allow the evaluation of adC, a more crucial calculation in the present context, are not available.…”