Symmetry-species conversion in CD3 systems

Abstract

The rates for symmetry-species conversion of CD3 groups are calculated using a model in which the interaction between the quadrupolar moment of the deuterons with electric-field gradient at the site of the nucleus causes symmetry-changing transitions. Just the same phonons are considered for energy conservation as are used to describe the temperature dependence of inelastic neutron scattering experiments. For the conversion rate, a similar temperature dependence is found as has already been obtained for CH3. For temperatures around the tunnelling energy, a behaviour is predicted for CD3 that is different from the behaviour in protonated systems according to all theories known to the authors. In comparison with the corresponding protonated species embedded in the same surroundings, the conversion rate at elevated temperatures turns out to be larger by orders of magnitude. Only the low-temperature conversion rate is suppressed owing to the lack of resonance phonons at the usually smaller tunnelling frequency in CD3. The relative increase of the conversion rate with deuteration due to Raman processes is predicted to be independent of temperature but strongly dependent on the height of the orientational potential in the case of shallow potentials. If the tunnelling energy is smaller than 25 mu eV, the conversion rates increase by a constant factor of approximately 10 compared to the protonated species at a given temperature.