POTENTIAL SEEN BY THE METHYLS OF THE TRIB ROMO-MESITYLENE

The crystalline form of tribromo-mesitylene at room temperature belong s to the triclinic space group P1BAR. The isolated molecule has three- fold symmetry; for that reason, it is an ideal probe to distinguish be tween the intra-molecule and the inter-molecular contributions to the potential V(s) (THETA), hindering the methyl quantum rotors, since eac h one of the three methyls which are equivalent in the isolated molecu le are located at non equivalent crystal positions. This is confirmed by neutron tunnelling spectroscopy. For the computation of V(s) (THETA ), inter-molecular interactions were obtained from optimized Buckingha m-type atom-atom potentials; we show that these potentials are inadequ ate for calculating intra-molecular interactions which instead were ob tained by a quantum chemistry method. The global calculated V(s) (THET A) gives tunnelling frequencies in reasonable accordance with the expe riment.