A process known as ‘molecular surgery’ enables the synthesis of the fullerene C60 enclosing a small molecule such as H2, H2O, HF and CH4. The caged molecules in these ‘molecular endofullerenes’ are quantum rotors showing the isolation and free rotation expected in a low pressure gas, but in an easily handled form and high effective concentration. In molecular endofullerenes the energies of the quantised translational states may be measured by inelastic neutron scattering and provide a detailed test for models of non-bonding interactions.In this proposal we aim to measure the energy levels of the hydrogen isotopomer HD enclosed in C60, denoted HD@C60. HD provides advantages over H2 as the different mass of the two atoms causes substantial mixing between the rotational and translational quantum states, and all rotational states may be occupied.