Ab initio structure and vibration-rotation dynamics of the formyl and isoformyl cations, HCO+/HOC. J Chem Phys 2019;150:154307. [PMID: 31005073 DOI: 10.1063/1.5089718]

Structural and vibrational characterization of HCO+ and Rg-HCO+, Rg = {He, Ne, Ar, Kr, and Xe}

The structures of the formyl ion (HCO⁺) and its rare gas tagged counterparts (Rg-HCO⁺, Rg = He, Ne, Ar, Kr, and Xe) were studied at the coupled-cluster singles, doubles, and perturbative triples [CCSD(T)]/aug-cc-pVTZ level of theory and basis set. A linear structure for these tagged complexes was predicted. The Rg binding energies for Rg-HCO⁺ are also examined at the CCSD(T) level. It was found that the binding interaction increases from He-HCO⁺ to Xe-HCO⁺. A multilevel potential energy surface built at the CCSD(T) and second-order Møller-Plesset perturbation levels of theory were used to study these species' vibrational spectra. By changing the Rg in the first-solvation shell for HCO⁺, the Fermi resonance interaction between the first H⁺ bend overtone and the asymmetric and symmetric H-C-O stretches can be modulated. This Fermi resonance modulation is demonstrated by examining a series of rare gas solvated HCO⁺.

Rotational Transitions of HOC+ Induced by Collision with He at Low Temperatures

The HOC⁺ molecule has for long been detected in several regions of the interstellar medium (ISM). The collisional ro-vibrational rate coefficients of this molecule with the most common colliders in the ISM are then required for applying nonlocal thermal equilibrium models. However, this molecule has a low bending frequency (249 cm^-1), and the use of the rigid rotor approximation is therefore limited to low collision energies. Also, the complete determination of the ro-vibrational rate coefficients of HOC⁺ in collision with He requires including the bending motion in the analytical model of the potential energy surface (PES) of the system. The first goal of this work is then to develop the first rigid bender four-dimensional PES for the interaction between HOC⁺ and He. To this aim, a large grid of ab initio energies are computed at the CCSD(T)-F12b/aug-cc-pVQZ level of theory and an analytical representation of the PES is obtained using a combination of least square and reproducing kernel Hilbert space procedures. The global minimum of this PES is found to be reached for a linear configuration of the complex. In the second part of this study, rigid rotor close-coupling calculations are performed at low collision energy, and the calculated rate coefficients are compared to those previously determined for the collisions of He with its HCO⁺ isomer.