ANALYSIS OF STATIC DISTRIBUTIONS IN HYDROGEN HYPERFINE INTERACTIONS IN RANDOMLY ORIENTED RADICALS IN THE SOLID-STATE BY USING H-2 ELECTRON-SPIN ECHO ENVELOPE MODULATION SPECTROSCOPY - CONFORMATIONAL DISPERSION OF BETA-H-2 COUPLING IN THE MODEL TYROSYL RADICAL

The experimental analysis of static distributions in hydrogen hyperfine interactions in randomly-oriented organic radicals-in the solid-state by using H-2 electron spin echo envelope modulation spectroscopic techniques has been examined systematically. The hyperfine interaction between the two beta-methylene-H-2 nuclei and coupling pi-spin density (rho(pi) at ring carbon atom C-1 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 tau values of 176-1295 ns at external magnetic field strengths of 0.3258 and 0.3983 T. The spectra reveal weak (beta-H-2(w)) and strong (beta-H-2(s)) sets of hyperfine couplings. The envelope modulation depths and line shape responses to changes in rand 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 tenser distribution is caused by a variation in the isotropic coupling for beta-H-2(w) and beta-H-2(s) 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, theta, between the phenol ring normal and the C-beta-H-beta bonds of \theta(w)\=60 degrees-90 degrees and \theta(s)\=60 degrees-30 degrees. A common, theta-independent dipolar coupling constant for each rotamer yields a value for rho(pi) at C-1 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 H-2 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. (C) 1995 American Institute of Physics.