Proton Spin-Lattice Relaxation from Classical and Tunneling Rotation of Methyl Groups in Thioacetamide

Abstract

The proton spin-lattice relaxation rate 1/ T 1 in solid thioacetamide were measured between 28.5 K and 200 K at two Larmor frequencies, 11.4 MHz and 25 MHz. The temperature dependence of 1/ T 1 was characterized by three maxima, two maxima observed around 90 K and 37 K showing the frequency dependence and the frequency independent one at 44 K. Taking into account two crystal-lographically inequivalent thioacetamide molecules, the temperature and Larmor frequency dependence of 1/ T 1 was explained by the contribution from the quantum mechanical and classical limit of tunneling rotation of the methyl groups around their C 3 axes.