Pressure dependence of methyl tunnelling in solid diacetyl: an extension to the methyl thermometer model

Abstract

Methyl tunnelling has been measured in solid diacetyl at low temperatures as a function of pressure using low field NMR spectroscopy. In addition the associated thermally activated rotation at higher temperatures has been measured as a function of pressure by spin-lattice relaxation time (T 1) analysis. The tunnel frequency decreases exponentially from 76 kHz at atmospheric pressure to 9 kHz at 6·25 kbar. The T 1 data can be correlated with the tunnel frequency according to the methyl thermometer model, extending the model to lower tunnelling frequencies than previously demonstrated. A small sixfold symmetric term in the potential hindering rotation can be deduced from the data. The overall height of the hindering potential and its shape are compared with the potential calculated from the crystal structure.