Quantum and classical dynamics of a methyl group with tunneling frequency 3.4 MHz studied by low field NMR

Abstract

Nuclear magnetic resonance (NMR) investigations of methyl tunneling in 2,5-dimethyl-1,3,4-thiadiazole are reported. The tunneling frequency (νt = 3.39 MHz) was obtained using low field NMR spectroscopy. By means of rapid field cycling irradiation and relaxation measurements the probabilities of the absorption transitions, responsible for the spectral lines in the low field NMR spectra, can be quantified. The results obtained agree well with the calculated probabilities of rf induced transitions between the eigenstates of a methyl rotor with νt = 3.39 MHz. Measurements of the temperature dependence of νt, the spin conversion time τcon and an analysis of the proton spin lattice relaxation time T1, the latter two revealing the correlation time τc, enabled the study of the methyl group dynamics over a temperature range encompassing both the quantum mechanical and the classical regimes. The dynamical data can be explained well with a threefold hindering barrier of height V3/kB = 1175 K and are compared with existing theoretical models.