Speciation and Thermodynamics of Sulfur Vapor

Abstract

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.