Neutron scattering transitions between spin symmetry species of methyl groups

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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.