ClSO4 in Venus sulfur–chlorine chemistry: Radical localization, vibrational signatures, and near-UV/visible spectrum

Venus photochemistry couples SO2 variability, chlorine activation, sulfuric-acid cloud formation, and the still-unassigned near-UV absorber, yet several sulfur-chlorine intermediates in this network remain molecularly undefined. ClSO4 is one of them: it has been invoked as a chlorine-assisted route toward SO3 and H2SO4, but its structure and spectroscopy have not been established. We report a focused ab initio characterization of neutral doublet ClSO4 centered on explicit validation of its electronic structure. Multireference calculations with two independent active spaces locate the same O-bound chlorine sulfate radical and show that the relevant structure is not strongly multiconfigurational. An independent second-order perturbation-theory optimization recovers the same bond-localized topology, but the coupled-cluster minus second-order perturbation correction substantially refines the radical-bearing S-O distance, showing that second-order perturbation theory alone is not sufficient for final structural parameters. Final structural parameters are therefore obtained from a composite geometry, including explicit-correlation to account for basis-set extension, post-second-order correlation, and core-valence increments, whereas the vibrational analysis uses a second-order perturbation-theory force field and second-order vibrational perturbation treatment. The resulting vibrational spectrum contains a compact set of intense infrared markers, and the electronic absorption spectrum extends into the near-UV/visible region. These results establish ClSO4 as a chemically plausible and spectroscopically accessible intermediate in Venus sulfur-chlorine chemistry.