HIGH-PRESSURE NMR-STUDY OF METHYL-GROUP TUNNELING IN DIMETHYL SULFIDE

The rotational tunnel splittings of pairs of methyl groups in dimethyl sulphide have been determined by low field proton NMR as a function o f hydrostatic pressure up to 4.5 kBar. Two principal splittings are cl early observed, one being equal to 750 kHz for all pressures, the othe r decreasing exponentially from 100 kHz at atmospheric pressure to 40 kHz at 4.0 kBar. For the first time, tunnelling sidebands of a sideban d in the NMR spectrum have been seen. The temperature dependence of th e spin-lattice relaxation time has also been measured as a function of pressure and has revealed double minima at higher pressure. The resul ts have been analysed using both weak and strong methyl coupling model s.