Electron-nuclear double resonance study of molecular librations of nitroxides in molecular glasses: quantum effects at low temperatures, comparison with low-frequency Raman scattering.

Pulsed electron-nuclear double resonance applied to 15N nitroxide spin probes in molecular glasses is shown to be very sensitive to measurement of the A(XX) principal value of the hyperfine interaction tensor. For molecules experiencing fast restricted orientational motions (molecular librations), this provides a precise tool to determine the motion-averaged value. For nitroxides in glycerol and o-terphenyl glasses, the observed temperature dependence below 40 K may be readily interpreted as arising from quantum effects in librations, when the thermal energy of a librating molecule becomes comparable with the elementary quantum of the oscillator. The estimated elementary quanta for nitroxide librations, approximately 60 cm(-1) in glycerol and approximately 90 cm(-1) in o-terphenyl, are found to match the characteristic frequencies of the vibrational spectral densities seen in low-frequency Raman scattering for these glasses. Above approximately 80 K in glycerol and above approximately 120 K in o-terphenyl, the temperature dependences manifest a kink with a slightly smaller slope than at lower temperatures.