ESR spectra and the behavior of substituted indophenoxyl radicals

ESR spectra and structure are compared for radicals formed from substituted indophenols by oxidation (substituents CH₃, i-C₃H₇, t-C₄H₉, t-C₅H₁₁, cyclohexyl). The unpaired electron interacts with the nitrogen and with the hydrogen in the ortho and meta positions of both benzene rings. A kinetic equation of first order with respect to the indophenol applies to the reaction of the latter with benzoyl peroxide. The loss of indophenoxyl radicals in benzene obeys an equation of second order in the radical concentration. The rate constants for radicals with various substituents indicate the radical stability, which falls greatly in going from ones with ortho-t-alkyl substituents to ones with less highly branched groups.     

2-Aryl substituted 3-oxophenalenoxyl radicals: π-Spin structures and properties evaluated by dimer structure

We recently demonstrated that 6- and 4-oxophenalenoxyl radicals possess high stability in air due to extensive π-spin delocalization and steric protection of active sites. In sharp contrast, 3-oxophenalenoxyl radicals were found to be not stable enough to be isolated as an open-shell species, and easily dimerized with each other at 2-positions. The radicals were generated by thermal bond cleavage of the dimers in a solution state and their electronic structures were probed by ESR techniques. In this study, we have revealed a sterically hindered structure of the dimer of 2-(p-methoxyphenyl)-3-oxophenalenoxyl in terms of both X-ray crystal structure analysis and variable temperature ¹H NMR studies. DFT calculations of the radicals indicate that π-spin densities dominantly exist at the 2-position, giving a rationale for the dimerized position of the radicals. These studies demonstrate that the lower stability of 3-oxophenalenoxyl is due to smaller π-spin delocalization into the whole phenalenyl skeleton than 6- and 4-oxophenalenoxyl radicals.     

ESR study of the effect of fluorinated alcohols on magnetic resonance parameters of spin-adducts of phosphoryl radicals with products of cycloaddition of substituted nitriloxides to C60

Regioselectivity of the addition of phosphoryl radicals to the products of cycloaddition of substituted nitriloxides to fullerene C₆₀ was studied by ESR spectroscopy. The effect of fluorine-containing alcohols on the magnetic resonance parameters of the isomers of spinadducts of phosphoryl radicals located at different distances from the five-membered heterocycle was studied. Complexation of the obtained isomeric spin-adducts with (CF₃(₃COH leads to a 3 to 5 G increase in the constant of hyperfine interaction with the phosphorus nucleus, which makes it possible to observe additional lines in the ESR spectrum. Deceased. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1808–1811, September, 1999.     

Investigation of antioxidant radicals by the electron paramagnetic resonance method

Journal of Structural Chemistry -     

Correlations between magnetic resonance parameters of ESR and NMR spectra

Empirically established correlations between magnetic resonance parameters of free radicals (g-factors, isotropic hyperfine coupling constans) and isostructural molecules (chemical shifts, isotropic spin-spin coupling constants) or structurally similar ligands in paramagnetic transition metal complexes (isotropic chemical shifts) are systematized and critically discussed. Quantum-chemical analysis of the suggested spin distribution damping coefficients in model systems and structurally similar chemical compounds is performed. Based on the results obtained, physicochemical interpretation of the observed correlations between the parameters of ESR and NMR spectra is given. Dedicated to the memory of Academician V. V. Voevodskii (to the 80th anniversary). Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1575–1583, September, 1997     

Understanding the electon paramagnetic resonance parameters of protein-bound glycyl radicals

The number of enzymes that require a glycyl-based radical for their function is growing. Here, we provide systematic quantum-chemical studies of spin-density distributions, electronic g-tensors, and hyperfine couplings of various models of protein-bound glycyl radicals. Similarly to what is found in a companion paper on N-acetylglycyl, the small g-anisotropy for this delocalized, unsymmetrical system presents appreciable challenges to state-of-the-art computational methodology. This pertains to the quality of structure optimization, as well as to the choice of the spin-orbit Hamiltonian and the gauge origin of the magnetic vector potential. Environmental effects due to hydrogen bonding are complicated and depend in a subtle fashion on the different intramolecular hydrogen bonding for different conformations of the radical. Indeed, the conformation has the largest overall effect on the computed g-tensors (less so on the hyperfine tensors). This is discussed in the context of different g-tensors obtained by recent high-field electron paramagnetic resonance (EPR) measurements for three different enzymes. On the basis of results of a parallel calibration study for N-acetylglycyl, it is suggested that the glycyl radical observed for E. coli anaerobic RNR may have a fully extended conformation, which differs from those of the corresponding radicals in pyruvate formate-lyase or benzylsuccinate synthase.