Neutron scattering transitions between spin symmetry species of methyl groups

Methyl tunnelling, reorientation and NMR relaxation in solid acetates

Rotational excitations of methyl groups in six solid acetates have been investigated by 1H nuclear magnetic resonance (NMR) relaxation time measurements at 15 MHz and 30 MHz and at temperatures between 10 K and the melting point. Hindering barriers between 1.6 kJ/mol and 3.7 kJ/mol have been found that could be correlated to the tunnelling frequencies observed by inelastic neutron scattering. A consistent description of the relaxation rate dependences from the classical regime at high temperatures to the quantum-mechanical regime at low temperatures is possible by Haupt's equation. The rotational potentials are mainly determined by inter-molecular interaction with an important influence of water of crystallization, if present.

Anomalous proton relaxation, rotational tunnelling and barriers to methyl group rotation in solid acetyl halides

Rotational excitations of methyl groups attached to carbonyl in solid acetic acid, acetyl fluoride, acetyl chloride and acetyl bromide have been investigated by 1H nuclear magnetic resonance (NMR) relaxation times and field-cycling measurements at two frequencies and various temperatures. The tunnel splittings have been found to occur between 3.3 and 0.08 mu eV making quantum effects important for the relaxation behaviour. For the acetyl halides, similar tunnelling and NMR frequencies lead to an anomalous-looking temperature dependence of the relaxation rates. A consistent description by Haupt's equation is possible. The rotational potentials have been derived from the data and compared with those obtained from microwave spectra of the corresponding isolated molecules. The hindering potential is purely three-fold and the barrier is dominated by the functional group.