Abstract
In a simple model of (mechanically and magnetically) isolated methyl groups with mutualintramolecular dipolar coupling between the nuclear magnetic moments of the three protons the conversion rate (i.e. for the symmetry changing transitionsE a,b ↔A) is investigated. It turns out, that within the model of a fixed rotational axis only half of theE-symmetric rotor states can convert intoA-symmetric ground states. The usual coupling to the phonon bath of thermal lattice excitations is considered in lowest order perturbation theory, and used to describe the temperature dependence of shift and broadening of tunnelling-and librational lines by inelastic neutron scattering (INS). For the two possible types of this phonon coupling,breathing andshaking, a different temperature dependence of the conversion rate is obtained. The temperature dependence of the INS-spectra and of the conversion rate are predicted to be mutually interdependent. Only in case of a non zero phonon coupling of breathing type a non vanishing conversion rate at zero temperature is found; this rate is estimated to be proportional toω 3 .
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