Temperature Dependence of Hyperfine Interaction for 15N Nitroxide in a Glassy Matrix at 10–210 K

Abstract

Principal 15 N hyperfine interaction (hfi) values in 15 N-substituted nitr- oxide spin probe 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-oxyl dissolved in nematic liquid crystal 4-pentyl-4 0 -cyanobiphenyl (5CB) were measured in a wide temperature range of 10-210 K, for 5CB frozen to a glassy state. X-band continuous- wave electron paramagnetic resonance (CW EPR) and pulse X- and Q-band 15 N electron-nuclear double resonance (ENDOR) techniques were employed. To avoid microwave saturation at low temperatures in CW EPR studies, a holmium complex Ho(Dbm)3Bpy (where Dbm is dibenzoylmethane and Bpy is 2,2 0 -bipyridine) was added. X- and Q-band 15 N-ENDOR data have shown that the nitroxide hfi tensor is axially symmetric. The combination of data from all techniques allowed us to obtain the temperature dependence of isotropic and anisotropic parts of the nitroxide hfi tensor. Above *100 K, a linear dependence of the anisotropic hfi value was observed, whereas below 30 K it was found to be nearly temperature independent. Such a behavior can be interpreted using the model of restricted orientational motions (librations) of a spin probe in a glassy matrix, with quantum effects occurring at low temperature (''freezing'' of the librations). The energy quantum for the libration motion estimated from the temperature dependence of hfi of the spin probe is 84 cm -1 . Low-frequency Raman spectra of 5CB were also obtained, which provided the mean vibrational frequency of 76 cm -1 for glassy 5CB.