Oxygen-free sulfur–chlorine chains under Venus-relevant reducing conditions: Structures, vibrations, and electronic excitations from Cl2S to Cl2S4

Sulfur-chlorine-oxygen chemistry is central to Venus-relevant reducing environments and has been widely studied, but the oxygen-free sulfur-chlorine compounds accessible within that chemical space remain much less characterized beyond the smallest members. Here, Cl2S, Cl2S2, Cl2S3, and Cl2S4 are treated as a calibrated progression of chlorine-capped, oxygen-free sulfur chains. The experimentally anchored Cl2S and Cl2S2 systems validate the local S-Cl and S-S descriptions before extension to the higher homologues, with good agreement with experiment in both cases; for Cl2S2, the calculations also confirm the high-energy pyramidal minimum proposed in a previous study. For Cl2S3, two open helical minima are separated by a torsional barrier of about 9.0 kcal mol-1, and a pyramidal local minimum is also characterized. For Cl2S4, the open chain remains more stable than the compact cyclic/transannular and pyramidal alternatives. Across the series, intense computed IR fundamentals concentrate in the 490-520 cm-1 terminal/backbone stretching region, while the experimentally assigned Raman bands near 440-450 cm-1 are consistent with substantial combination/overtone contributions, by analogy with S2Cl2. Excited-state calculations give weak low-energy ultraviolet onsets and place most oscillator strength deeper in the ultraviolet, defining a molecular reference frame for reduced sulfur-chlorine chemistry under Venus-relevant conditions.