THE PRESSURE-DEPENDENCE OF METHYL TUNNELING IN ACETYLACETONE

Pressure dependence measurements are reported on methyl tunnelling in acetylacetone and an isotopic derivative in which the hydrogen bond in the ENOL ring is deuterated. The effect of deuteration is to increase the intra-molecular component of the potential which hinders the reorientation of neighbouring methyl groups. Application of pressure, however, results in a decrease in the inter-molecular component of the hindering potential in both samples and the methyl group tunnelling frequency increases with compression. Above 3 kbar the principal tunnel peak no longer shifts with change in pressure but intensity is transferred progressively to new peaks. This structure is typical of coupled methyl groups and two models, coupled pairs and infinite coupled chains, are considered.