Spectroscopy and potential energy surface of the H₂–CO₂ van der Waals complex: experimental and theoretical studies

A 4-D ab initio potential energy surface is calculated for the intermolecular interaction of hydrogen and carbon dioxide, using the CCSD(T) method with a large basis set. The surface has a global minimum with a well depth of 212 cm⁻¹ and an intermolecular distance of 2.98 Å for a planar configuration with both the O–C–O and H–H axes perpendicular to the intermolecular axis. Bound state calculations are performed for the H₂–CO₂ van der Waals complex with H₂ in both the para and ortho spin states, and the binding energy of paraH₂–CO₂ (50.4 cm⁻¹) is found to be significantly less than that of orthoH₂–CO₂ (71.7 cm⁻¹). The surface supports 7 bound intermolecular vibrational states for paraH₂–CO₂ and 19 for orthoH₂–CO₂, and the lower rotational levels with J ≤ 4 follow an asymmetric rotor pattern. The calculated infrared spectrum of paraH₂–CO₂ agrees well with experiment. For orthoH₂–CO₂, the ground state rotational levels allowed by symmetry are found to have (K_a, K_c) = (even, odd) or (odd, even). This somewhat unexpected fact enables the previously observed experimental spectrum to be assigned for the first time, in good agreement with theory, and indicates that the orientation of hydrogen is perpendicular to the intermolecular axis in the ground state of the orthoH₂–CO₂ complex.