Ab initio study of gas phase HClSO2: Building up the molecular complexity of candidate UV-absorbers in Venusian clouds

Sulfur-chlorine-oxygen-containing species have an indispensable role in chemistry in the Venusian atmosphere. Here, the detailed electronic structure and spectroscopy of the [H, Cl, S, O, O] molecular system are explored using Density Functional Theory, coupled cluster theory [CCSD(T)], and multireference configuration interaction theories [MRCISD+Q]. Equilibrium geometries, rotational constants, harmonic vibrational frequencies, and thermodynamic data of the ground state isomers of [H, Cl, S, O, O] are presented. The meta-stable van der Waals system (complex 1; HCl + SO2) is identified as the lowest energy isomer, possessing a noncovalent O⋯H interaction between SO2 and HCl fragments. The thermodynamics and kinetics of dissociation of HClSO2 isomers into HCl and SO2 fragments are explored in detail. From computations, complex 1 is not thermodynamically stable in the upper cloud of the Venusian atmosphere where UV absorbers are present, as it is predicted to spontaneously dissociate into HCl and SO2 fragments at temperatures above 100 K. Calculated vertical excitation energies of the low-lying isomers using equation-of-motion CCSD (EOM-CCSD) and MRCISD+Q levels of theory are also reported. Our analysis showed that all excited state transitions from the ground state of the lowest energy isomers were below 280 nm and thus might not be candidates for the UV absorber(s). As the lowest energy isomer of [H, Cl, S, O, O] is a van der Waals complex, we expect larger van der Waals clusters with more favorable intermolecular interactions than complex 1 to be plausible candidates for the near-UV absorber(s) observed in the Venusian atmosphere.