Analysis of static distributions in hydrogen hyperfine interactions in randomly oriented radicals in the solid state by using 2H electron spin echo envelope modulation spectroscopy: Conformational dispersion of β -2H coupling in the model tyrosyl radical

Abstract

The experimental analysis of static distributions in hydrogen hyperfine interactions in randomly-oriented organic radicals in the solid state by using 2H electron spin echo envelope modulation spectroscopic techniques has been examined systematically. The hyperfine interaction between the two β-methylene-2H nuclei and coupling π-spin density (ρπ) at ring carbon atom C1 in the tyrosine neutral radical trapped in a low temperature aqueous glass was addressed specifically. Stimulated echo envelope modulation generated by the microwave pulse-swapping sequence was collected for τ values of 176–1295 ns at external magnetic field strengths of 0.3258 and 0.3983 T. The spectra reveal weak (β-2Hw) and strong (β-2Hs) sets of hyperfine couplings. The envelope modulation depths and line shape responses to changes in τ and magnetic field strength could not be reproduced by simulations that incorporated discrete principal hyperfine tensors. Successful simulations were achieved by using two sets of distributed principal hyperfine tensors. The principal tensor distribution is caused by a variation in the isotropic coupling for β-2Hw and β-2Hs of 0–2.0 MHz and 2.0–6.8 MHz, respectively. The range of isotropic couplings corresponds to a distribution in the dihedral angles, θ, between the phenol ring normal and the Cβ–Hβ bonds of ‖θw‖=60°–90° and ‖θs‖=60°–30°. A common, θ-independent dipolar coupling constant for each rotamer yields a value for ρπ at C1 of 0.35±0.03. Relative rotamer populations in the distribution are given by the conformational weighting function required for exact reproduction of the line shapes. The quantitative relation between modulation amplitudes and the details of the electron–nuclear interactions that determine the hyperfine frequencies gives the 2H electron spin echo envelope modulation method unique merit for the detection and accurate description of static distributions of principal hyperfine tensors in randomly-oriented paramagnetic systems in the solid state.