Interaction potential and infrared absorption of endohedral H₂ in C₆₀ Available to Purchase

We have measured the temperature dependence of the infrared spectra of a hydrogen molecule trapped inside a C₆₀ cage, H₂@C₆₀, in the temperature range from 6 to 300 K and analyzed the excitation spectrum by using a five-dimensional model of a vibrating rotor in a spherical potential. The electric dipole moment is induced by the translational motion of endohedral H₂ and gives rise to an infrared absorption process where one translational quantum is created or annihilated, ΔN = ±1. Some fundamental transitions, ΔN = 0, are observed as well. The rotation of endohedral H₂ is unhindered but coupled to the translational motion. The isotropic and translation–rotation coupling part of the potential are anharmonic and different in the ground and excited vibrational states of H₂. The vibrational frequency and the rotational constant of endohedral H₂ are smaller than those of H₂ in the gas phase. The assignment of lines to ortho- and para-H₂ is confirmed by measuring spectra of a para enriched sample of H₂@C₆₀ and is consistent with the earlier interpretation of the low temperature infrared spectra [Mamone et al., J. Chem. Phys. 130, 081103 (2009)].