Inelastic neutron scattering of methyl tunnelling in isotopic mixtures of dibromomesitylene

In dibromomesitylene, only the methyl group denoted Me2 and surrounded by two bromine atoms, is a quasi free rotor. In isotopic mixtures the Me2 tunnelling gap is almost insensitive to variations in temperature and to isotopic dilution. The Me2 hindering potential remains the same as in the pure material with a main component cos6θ. Because there is no coupling between the methyl groups, dibromomesitylene provides an illustration of the ‘Single Particle Model’ in rotational tunnelling. An explanation is given why Density Functional Theory calculations fail to represent the characteristics of tunnelling produced by the Schrödinger equation.