Methyl tunnelling in solid diacetyl

Abstract

The techniques of dipole-dipole driven low-field NMR spectroscopy have been employed to measure the tunnelling frequency, ν t, of methyl groups in solid diacetyl (2,3-butanedione) at 4 K. The value of ν t is 74±1 kHz from which the height of the barrier to reorientation (assumed to have three-fold symmetry) is deduced to be 1845 K (15.3 kJ mol −1). The intermolecular contribution to the barrier has been calculated by evaluating the lattice sum of pairwise interatomic potentials as a function of methyl group orientation. This analysis reveals that the predominant contribution to the barrier arises from the methyl group of a neighbouring molecule. Temperature dependence measurements of the spin-lattice relaxation time have also been employed to study the dynamics of methyl reorientation in diacetyl and in the related molecule acetonylacetone (2,5-hexanedione).