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

Abstract

The rotationally resolved infrared spectrum of the Na(+)-H(2) cation complex is recorded in the H-H stretch region (4067-4118 cm(-1)) by monitoring the production of Na(+) photofragments. Altogether 42 lines are identified, 40 of which are assigned to K(a)=1-1 transitions (associated with complexes containing ortho-H(2)) and two tentatively assigned to K(a)=0-0 transitions (associated with complexes containing para-H(2)). The K(a)=1-1 subband lines were fitted using a Watson A-reduced Hamiltonian, yielding effective spectroscopic constants. The band origin is estimated as 4094.6 cm(-1), a shift of -66.6 cm(-1) with respect to the Q(1)(0) transition of the free H(2) molecule. The results demonstrate that Na(+)-H(2) has a T-shaped equilibrium configuration with the Na(+) ion attached to a slightly perturbed H(2) molecule but that large-amplitude vibrational motions significantly influence the rotational constants derived from the asymmetric rigid rotor analysis. The vibrationally averaged intermolecular separation in the ground vibrational state is estimated as 2.493 A, increasing slightly (by 0.002 A) when the H(2) subunit is vibrationally excited. A new three-dimensional potential energy surface is developed to describe the Na(+)-H(2) complex. Ab initio points calculated using the CCSD(T) method and aug-cc-pVQZ basis set augmented by bond functions are fitted using a reproducing kernel Hilbert space method [Ho et al., J. Chem. Phys. 104, 2584 (1996)] to give an analytical representation of the potential energy surface. Ensuing variational calculations of the rovibrational energy levels demonstrate that the potential energy surface correctly predicts the frequency of the nu(HH) transition (to within 2.9 cm(-1)) and the dissociation energies [842 cm(-1) for Na(+)-H(2)(para) and 888 cm(-1) for Na(+)-H(2)(ortho)]. The B and C rotational constants are slightly underestimated (by 1.7%), while the vibrationally averaged intermolecular separation is overestimated by 0.02 A.