Effect of isotopic dilution in the methyl tunnelling spectra of molecular crystals

The rotational tunnel and torsion frequencies of crystalline nitromethane and toluene have been investigated using inelastic neutron scattering. The effects on these frequencies of complete deuteration, selective deuteration (in the case of toluene) and dilute mixing of the protonated species in a deuterated sample are reported. Small shifts in the tunnel frequencies of the protonated species dissolved in the deuterated matrix can be partially understood on the basis of lattice contraction upon deuteration. More surprising is the magnitude of the increase in the intrinsic energy width of both the tunnel and the torsional energy levels upon dilution.