Speciation and Thermodynamics of Sulfur Vapor
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Sulfur vapor at temperatures of between 200 and 1000 °C consists of all molecules with 2–10 atoms, some of which exist as two or more isomers . Extensive mass spectrometric investigations and vapor pressure measurements provided reliable thermodynamic data for the various dissociation reactions S8 ⇌8/ n S n . Earlier quantum-chemical calculations yielded data partly in conflict with these experimental findings, but the most recent high-level ab initio MO and DFT calculations confirmed more or less the best experimental results. These theoretical data may now be considered as the most trustworthy. At low temperatures S8, S7, and S6 are dominating in saturated sulfur vapor, the concentration of S4 passes through a maximum at 900 K, S3 is always more abundant than S4, and S2 is the dominating species above 1100 K. The S4 molecule is of cis-planar geometry but under non-equilibrium conditions the trans-planar isomer has also been observed. Triplet chain molecules S n ·· as well as branched rings S n =S and other isomers are too unstable to contribute more than 1% in each group to the equilibrium vapor composition at any temperature.
KeywordsSulfur clusters Quantum-chemical calculations Stability Isomers Spectra
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