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 reor ientation of neighbouring methyl groups. Application of pressure, howe ver, results in a decrease in the inter-molecular component of the hin dering potential in both samples and the methyl group tunnelling frequ ency increases with compression. Above 3 kbar the principal tunnel pea k no longer shifts with change in pressure but intensity is transferre d progressively to new peaks. This structure is typical of coupled met hyl groups and two models, coupled pairs and infinite coupled chains, are considered.