METHYL TUNNELING IN PURE AND MATRIX-ISOLATED HALOGENOMESITYLENES

The trihalogenomesitylenes, C6(CH3)3X3, where X = Cl, Br, I crystallis e in the triclinic P1BAR space- group, such that the three methyl grou ps in one molecule see different environments. Indeed, we have detecte d three tunnelling and three torsional transitions in the pure compoun ds. In the case of a 3.7% mixture of protonated tribromomesitylene in a matrix of hexabromobenzene, we have detected only one tunnelling tra nsition at 102 mueV; from this, we conclude that the three methyl grou ps of the TBM molecule in the dilute mixture are equivalent in what is a relatively symmetrical environment and as a consequence also in the isolated molecule. A solid solution of 6% fully protonated trichlorom esitylene in a matrix of fully deuterated TCM gives three transitions similar to those in the pure compound, indicating that there is no str ong coupling between the different methyl groups. We also present Rama n and FIR spectra of pure TBM giving the phonon frequencies at the GAM MA point. Some IR absorption maxima fall near maxima of the neutron sp ectra, indicating coupling between some phonons and the torsional tran sitions. So as to give us a model of the tribromomesitylene system we have optimised atom-atom Buckingham-type coefficients so as to reprodu ce the experimental phonon frequencies. These coefficients were used t o calculate the intermolecular contribution V(ex)(theta) to V(s)(theta ), the potential hindering the methyl groups. The intramolecular contr ibution V(in)(theta) was found using the quantum chemistry program AM1 . The total hindering potential V(s)(theta) so obtained allowed us to calculate the principal dynamic features of the methyl rotors in reaso nable agreement with the experiments.