Caging effects in the low-temperature rotational spectra of endohedral diatomic molecules at C60 fullerene

Caging effects in the low-temperature (T=50 K) rotational spectra of CO@C60, LiF@C60, and LiH@C60 are investigated by first deriving an effective Hamiltonian to describe the coupled rotation of both, cage and confined molecule. Our results prove the close connection between the degree of anisotropy of the molecule–cage interaction and the features in the rotational spectra. If the anisotropy is weak, as in CO@C60, the spectra are dominated by the free rotation of the confined molecule. The stronger anisotropy in LiF@C60 and LiH@C60 produces more complex spectra dominated by librational motions and hindered rotations. The cage rotation plays a fundamental role in the spectral features, enhancing the CO free rotation in CO@C60, and broadening the librational bands in LiF@C60 and LiH@C60.