Nitroxide Spin Labels—Magnetic Parameters and Hydrogen-Bond Formation: A High-Field EPR and EDNMR Study

In this work, we report on a detailed high-field EPR investigation of magnetic parameters of deuterated nitroxide radicals dissolved in deuterated frozen solvents of polar or unpolar, protic or aprotic character including ortho-terphenyl, methanol, propanol, glycerol, aniline, phenol, and water. The analysis of the high-resolution W-band (95 GHz) pulsed ELDOR-detected NMR (EDNMR) and 244 GHz cw EPR spectra yielded precise electron Zeeman, g_xx, ¹⁴N hyperfine, A_zz, and ¹⁴N quadrupole, P_zz, tensor components, which are most sensitive probes for the microenvironment of dissolved nitroxide radicals. Remarkable changes in the solvation networks were found in the various matrices. The solvent-dependent magnetic parameters of the nitroxide are discussed on the basis of previous semi-empirical and DFT quantum chemical calculations in an attempt to adequately model the nitroxide–matrix interactions. The obtained results demonstrate that the principal values of all the magnetic interaction parameters primarily depend on the nitroxide hydrogen-bond situation and the type of hydrogen-bond donor group of the solvent. The solvent bulk polarity, as described by the static dielectric constant, is of minor importance.