Light molecules inside the nanocavities of fullerenes and clathrate hydrates: inelastic neutron scattering spectra and the unexpected selection rule from rigorous quantum simulations

The translation-rotation (TR) dynamics and spectroscopy of light molecules, e.g. H (Formula presented.), HD, HF, and H (Formula presented.) O, inside nanoscale cavities such as those of fullerenes and in clathrate hydrates, is dominated by strong nuclear quantum effects (NQEs) to a degree that is without parallel among realistic molecular species. The NQEs include the large TR zero-point energy, quantisation of the translational centre-of-mass motions of the guest molecule, the coupling of various angular momenta in the system, and nuclear spin isomerism. They leave rich and intriguing fingerprints in the inelastic neutron scattering (INS) spectra arising from the transitions between the TR levels of the systems studied. Here we describe the major methodological advances made in the past decade, in both bound-state and scattering calculations that, when combined, have led to the novel and powerful approach for rigorous quantum simulations of the INS spectra a diatomic molecule, homo- and heteronuclear, inside a nanocavity of an arbitrary geometry. As illustrated by several demanding applications, these simulations have been indispensable, and very successful, for the assignment and interpretation of the measured INS spectra. Very surprisingly, this effort has also resulted in the completely unexpected, precedent-setting discovery of the INS selection rule for diatomic molecules in near-spherical nanocavities, overturning the widely accepted view that the INS has no selection rules.