Partially deuterated methyl groups in molecular solids: the system CHD2NO2 with a threefold symmetric barrier

By partial deuteration of a methyl group in a threefold symmetric potential the nuclear spin states are no longer correlated to the rotational states, resulting in a fast establishment of thermal equilibrium between the rotational levels (even at low temperature), enabling a calorimetric study of the low lying energy levels. For CHD2NO2 these data confirm the localisation of the CHD2 group, as already seen by infrared and Raman spectroscopy at low temperatures. Combining the optical and specific heat data allows an improvement of the energy level diagram, consistent with the NMR result. (author)