Neutron scattering transitions between spin symmetry species of methyl groups

Abstract

The methyl tunnel splittings have been measured by inelastic neutron scattering at 4K for ammonium and lithium acetates. They are 16.2+or-0.2 and 250+or-10 mu eV respectively. An interesting difference between the two neutron energy transfer spectra is the relative intensities of the inelastic and quasi-elastic peaks. These are measured at a temperature at which the quasi-elastic peak is several times broader than the elastic peak so that the two can be separated. For ammonium acetate the inelastic and quasi-elastic peaks are of similar intensity, while in lithium acetate the quasi-elastic peak is relatively very weak. This is explained by a calculation which shows that a big difference between the intensities of the two types of peak is characteristic of almost freely rotating groups or of groups hindered by a potential of six-fold symmetry as in lithium acetate. A three-fold hindering potential tends to equalise the intensities as observed for ammonium acetate.