Coupled translation-rotation eigenstates of H2 in C60 and C70 on the spectroscopically optimized interaction potential: Effects of cage anisotropy on the energy level structure and assignments

We have developed a quantitatively accurate pairwise additive five-dimensional (5D) potential energy surface (PES) for H-2 in C-60 through fitting to the recently published infrared (IR) spectroscopic measurements of this system for H-2 in the vibrationally excited nu = 1 state. The PES is based on the three-site H-2-C pair potential introduced in this work, which in addition to the usual Lennard-Jones (LJ) interaction sites on each H atom of H-2 has the third LJ interaction site located at the midpoint of the H-H bond. For the optimal values of the three adjustable parameters of the potential model, the fully coupled quantum 5D calculations on this additive PES reproduce the six translation-rotation (T-R) energy levels observed so far in the IR spectra of H-2 @ C-60 to within 0.6%. This is due in large part to the greatly improved description of the angular anisotropy of the H-2-fullerene interaction afforded by the three-site H-2-C pair potential. The same H-2-C pair potential spectroscopically optimized for H-2 @ C-60 was also used to construct the pairwise additive 5D PES of H-2 (nu = 1) in C-70. This PES, because of the lower symmetry of C-70 (D-5h) relative to that of C-60 (I-h), exhibits pronounced anisotropy with respect to the direction of the translational motion of H-2 away from the cage center, unlike that of H-2 in C-60. As a result, the T-R energy level structure of H-2 in C-70 from the quantum 5D calculations on the optimized PES, the quantum numbers required for its assignment, and the degeneracy patterns which arise from the T-R coupling for translationally excited H-2 are all qualitatively different from those determined previously for H-2 @ C-60 [M. Xu et al., J. Chem. Phys. 128, 011101 (2008). (C) 2009 American Institute of Physics. [DOI: 10.1063/1.3152574]