A simple confined rotor model to describe the ro-translational dynamics of water endofullerenes and to assign the ro-vibrational spectra of solid H2O@C60

Abstract

A simple Confined Rotor Model (CRM) is used to assign the thirty-nine ro-vibrational transitions observed through the HOH bending and OH stretching ranges of the solid H2O@C60 mid-infrared (MIR) spectra reported in the companion paper [Chartrand et al., J. Chem. Phys. 162, 144312 (2025)]. Assuming that the intramolecular vibrations of the water molecules are separable from their rotational and translational motions, the CRM Hamiltonian describes confinement of H2O within C60 as an eccentric, but otherwise isotropic, 3D harmonic oscillator and as an asymmetric rigid rotor. The topology of the effective confinement potential is constrained using seven transitions observed in the HOH bending range of the MIR spectra of solid H2O@C60, yielding an effective force constant, k = (11.86 ± 0.03) J m−2, and an eccentricity, dCI = (7.55 ± 0.07) pm, in good agreement with DF-LMP2/cc-pVDZ results. While twenty-one broad and overlapping spectral features arising from hot band transitions were described and tentatively assigned by Chartrand et al., some of them appear very strongly perturbed compared to the gas phase. Using the CRM, it is shown that the conspicuous shifts displayed by certain very specific pairs of ro-vibrational transitions provide evidence for confinement-induced rotation–translation coupling between the orientational and positional degrees-of-freedom of endohedral water, resulting in a strong mixing between very specific pairs of ro-translational eigenstates of translational and rotational character. The CRM is shown to provide a satisfactory description of all observed ro-vibrational transitions along with a compelling interpretation for the complex confinement-induced quantum nuclear dynamics of endohedral water as revealed by the rotational and ro-vibrational spectra of solid H2O@C60.