Thermal Stability of Thionyl Chloride and the Sulfinyl Chloride Radical Determined by Cl-S Bond Energy

Thionyl chloride (Cl2SO) is a widely used chlorinating reagent in industrial and scientific applications, owing to its ability to provide two chlorine atoms while producing SO2 as an easily removable by-product. However, limited attention has been given to the mechanism of how Cl2SO releases its chlorine atoms because the intermediate sulfinyl chloride (ClSO) radical was assumed to be thermally unstable with an estimated Cl-S bond energy of ~5 kcal mol1. In this work, we determined the Cl-S bond energy of the ClSO radical by measuring its ionization energy using photoionization mass spectrometry interfaced with synchrotron radiation. High-level ab initio calculations were performed to support the derived bond energy. The Cl-SO bond energy was found to be 54.2 ± 0.9 kcal mol1, a factor of 10 larger than the previous estimations. Moreover, thermodynamic calculations indicated that the thermal decomposition rate of ClSO was negligible. Further kinetic simulations revealed that the ClSO concentrations could be orders of magnitude higher than previously assumed. These findings highlight the unconsidered importance of the ClSO radical in industrial applications of Cl2SO. A better understanding and control of ClSO chemistry could improve reaction efficiency and reduce Cl2SO waste for semiconductor fabrication, battery development, and pharmaceutical research, as well as planetary/exoplanetary atmospheres where chlorine and sulfur oxide are present.