A comparative analysis of powder ENDOR spectra of aromatic and aliphatic radicals by exact and 1st order simulation

Exact and perturbation methods were employed in simulations of powder ENDOR spectra to obtain the anisotropic hyperfine (hfc) and nuclear quadrupole (nqc) coupling constants of certain organic radicals of interest in fundamental research and in applications in radiation research, surface chemistry and biophysics. The principal hfc values of the ring protons and methyl substituents for several aromatic cations trapped in disordered matrices might be more accurate than those previously reported using regular EPR. Only one of the earlier assignments of the naphthalene cation spectrum was in acceptable agreement with the simulations. The proton couplings at the beta-position of alkyl radicals were deduced by simulations while the spectrum due to alpha-couplings with appreciable anisotropy was weak. Accurate simulation of the 14N (I = 1) spectra in bio-radicals was obtained by adjustment of the relative orientation of the principal hfc and nuclear quadrupole coupling (nqc) tensors as well as the principal values. Adjustment of the excitation width parameter employed in the software was also required in a few cases to improve the agreement with the experimental spectra. The hfc patterns due to matrix nuclei (1H and 7Li) around the radicals of X-irradiated samples were simulated to elucidate the nature of the trapping sites in materials used for EPR dosimetry. ENDOR simulation programs known to us are presented in an Appendix. The performance of the ENDORF2 program used in previous works was examined by comparison with exact treatment. Input and output examples, source and executable codes used in this work can be downloaded at https://github.com/EndorF2/Simulation.