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.