Temperature-dependent ESR hyperfine constants for nitroxides and orientational correlation time determination

A direct experimental observation of temperature-dependent electron-spin-resonance (ESR) hyperfine coupling constants has been made for TANOL nitroxide radicals in a polycarbonate polymer. This observation is based on the accurate rigid-limit spin Hamiltonian constants obtained from the ESR powder spectra of deuterated TANOL enriched with 15N isotope (I=1/2) (TANOLD–15N) at 77 K, and the isotropic hyperfine splitting factor obtained from the completely motionally shifted 15N ESR hyperfine lines at temperatures >445 K. From this reduction in the isotropic hyperfine splitting, it is determined that the hyperfine splitting factor ‖Az/gzμB‖ is reduced by 0.16 and 0.11 mT for TANOL–15N and TANOL–14N, respectively, when the temperature is increased from 77 to 445 K. The latter value for normal TANOL–14N is consistent with the temperature-dependent hyperfine splitting factor ‖Az/gzμB‖ deduced recently from the application of a new theoretical and experimental approach for studying slow-tumbling nitroxide radicals. It is shown that the temperature-dependent nitroxide hyperfine coupling constants will directly affect the measurements of slow orientational motional correlation times by conventional methods.