QUANTUM AND CLASSICAL DYNAMICS OF A METHYL-GROUP WITH TUNNELING FREQUENCY 3.4 MHZ STUDIED BY LOW-FIELD NMR

Nuclear magnetic resonance (NMR) investigations of methyl tunneling in 2,5-dimethyl-1,3,4-thiadiazole are reported. The tunneling frequency (v(t) = 3.39 MHz) was obtained using low field NMR spectroscopy. By me ans of rapid field cycling irradiation and relaxation measurements the probabilities of the absorption transitions, responsible for the spec tral lines in the low field NMR spectra, can be quantified. The result s obtained agree well with the calculated probabilities of rf induced transitions between the eigenstates of a methyl rotor with v(t) = 3.39 MHz. Measurements of the temperature dependence of v(t), the spin con version time tau(con) and an analysis of the proton spin lattice relax ation time T-1, the latter two revealing the correlation time tau(c), enabled the study of the methyl group dynamics over a temperature rang e encompassing both the quantum mechanical and the classical regimes. The dynamical data can be explained well with a threefold hindering ba rrier of height V-3/k(B) = 1175 K and are compared with existing theor etical models. (C) 1995 American Institute of Physics.