CH3 Rotational Tunnelling in Alkali Acetates and Correlation between NMR Spin-Lattice Relaxation and Neutron Scattering Data

Abstract

NMR spin-lattice relaxation rates l / T1 have been measured at 15 and 30 MHz and various temperatures in polycristalline sodium and potassium acetate. The results for CH3COONa are characteristic of methyl group tunnelling with a frequency which is very much larger than the NMR Larmor frequency. Derived values of the torsion energy (1.1 kJ/mol) and the ground state tunnel splitting (5.6 meV) agree with data obtained from inelastic neutron scattering by Clough et al. The shape of the hindering potential and the activation energy for classical reorientation (3.2 kJ/mol) have been determined. Since the relaxation rate of CH3COOK is a superposition of at least two mechanisms, in this case the results are less accurate and refer to one type of CH3 rotor only: activation energy 5.4 kJ/mol, tunnel splitting 0.23 meV. Comparison is made between tunnel splittings and classical activation energies for the acetates and various other materials: strong correlations are demonstrated and discussed.