Methyl tunnelling and rotational potentials in solid xylenes and fluorotoluenes

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Abstract
CH3 rotational excitations have been investigated in solid xylenes and fluorotoluenes using inelastic neutron scattering (INS) and nuclear resonance (NMR) techniques. For two of the six materials the temperature dependence of the tunnel line (shift and broadening) has been compared with the density of states and the slopes of the NMR-T, data. The rotational potentials have been derived from tunnel splittings, librational excitations and activation energies. The tunnel energies have been found to occur between 0.026 and 26 mu eV. In one case nonequivalent methyl groups have been identified. As compared with results from isolated molecules, the potentials of ortho isomers are distinguished by a considerable intramolecular contribution, while the rotational potential of the para and meta compounds are governed by intermolecular interactions. The hindering potential calculated for p-xylene on the basis of pair potentials is inconsistent with the experimental results; reasons are discussed.