The Al+-H2 cation complex:: Rotationally resolved infrared spectrum, potential energy surface, and rovibrational calculations

The infrared spectrum of the Al+-H-2 complex is recorded in the H-H stretch region (4075-4110 cm(-1)) by monitoring Al+ photofragments. The H-H stretch band is centered at 4095.2 cm(-1), a shift of -66.0 cm(-1) from the Q(1)(0) transition of the free H-2 molecule. Altogether, 47 rovibrational transitions belonging to the parallel K-a=0-0 and 1-1 subbands were identified and fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The results suggest that Al+-H-2 has a T-shaped equilibrium configuration with the Al+ ion attached to a slightly perturbed H-2 molecule, but that large-amplitude intermolecular vibrational motions significantly influence the rotational constants derived from an asymmetric rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 3.03 A, decreasing by 0.03 A when the H-2 subunit is vibrationally excited. A three-dimensional potential energy surface for Al+-H-2 is calculated ab initio using the coupled cluster CCSD(T) method and employed for variational calculations of the rovibrational energy levels and wave functions. Effective dissociation energies for Al+-H-2(para) and Al+-H-2(ortho) are predicted, respectively, to be 469.4 and 506.4 cm(-1), in good agreement with previous measurements. The calculations reproduce the experimental H-H stretch frequency to within 3.75 cm(-1), and the calculated B and C rotational constants to within similar to 2%. Agreement between experiment and theory supports both the accuracy of the ab initio potential energy surface and the interpretation of the measured spectrum.