Effect of Molecular Tunnelling on NMR Absorption and Relaxation in Solids

Abstract

About twenty years ago Tomita [1] showed that, as a result of the indistinguishability of protons, only those spin states exist in solid methane which allow the molecular wave function to be antisymmetric under the exchange of any two protons or, equivalently, symmetric under any real rotation which maps the CH₄ molecule into itself. He demonstrated that CH₄ molecules in different “spin isomer” states have very different NMR absorption spectra. The most characteristic isomeric spectrum is the delta function-like spectrum of the meta isomer (total spin two) which differs greatly from the broad and structured semiclassical absorption line. Since the spatial ground state of the CH₄ molecule combines with the meta spin isomer, it was expected that the absorption line of solid methane would remain very narrow at the lowest temperatures. This symmetry narrowing was observed in several lattices in which molecules or atomic groups are weakly hindered from reorienting [1, 2].