The pressure dependence of tunnelling of coupled pairs of methyl groups in lithium acetate dihydrate

The pressure dependence of tunnelling of coupled pairs of methyl groups in lithium acetate dihydrate has been investigate using the neutron scattering spectrometer IRSIS. Methyl tunnelling spectra recorded at a temperature of 5 K and pressures up to 9.5 kbar, reveal structure which has been attributed to coupling. The data, which extend an earlier investigation by Heidemann et al. (1987) have been fitted to a time-independent Hamiltonian, and the fit parameters include the potential barriers of the individual methyl groups and the coupling potential between them. The magnitudes of both parameters increase with increasing pressure and their pressure dependence has been analyzed to provide information on the inter-molecular forces.