Proton spin conversion of coupled methyl groups in lithium acetate studied by inelastic neutron scattering

The temperature dependence of the tunneling spectrum of methyl groups in lithium acetate dihydrate has been studied in the temperature range between 1.2 and 8.0 K by inelastic neutron scattering. The results unambiguously prove that it is to a first order approximation correct to describe the tunneling motions by a model of coupled CH₃ pairs which are isolated from each other. However, from the fact that the tunneling frequencies shift to higher values with decreasing spin temperature, we conclude that coupling effects are important not only between nearest neighbour CH₃ groups. Quantitatively we can describe the observations by a model of coupled pairs with a fixed value for the interaction potentialW₃ and a variable single particle potentialV₃ which depends linearly on the concentration of the spin symmetry species.