Rotational tunnelling in methylpyridines as studied by NMR relaxation and inelastic neutron scattering

Abstract

Nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) techniques have been applied to study the phenomenon of methyl group tunnelling of various picolines and lutidines in the crystalline state over a wide range of temperatures. In most of these materials, two maxima—one is field independent—and two different apparent activation energies in the proton spin lattice relaxation rate have been observed. The NMR-T1 results are discussed in terms of Haupt’s model, where nuclear relaxation occurs via successive coupling of the spins with the tunnelling CH3 rotator and the phonon system. The meaning of correlation times and associated activation energies, and the connection between classical and quantum mechanical theory are considered. By INS, the tunnel splittings and the energy differences between the lowest torsional states have directly been determined. For 3,5-dimethylpyridine the tgemperature dependence of the tunnel splitting has been measured; there is agreement between this data and the NMR average tunnel frequency. Potential curves have been derived using tabulated eigenvalues. The INS results confirm Haup’s model.