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

Thionyl chloride (Cl2SO) is widely used as a chlorinating reagent in both industrial and scientific applications due to its ability to provide two chlorine atoms while generating sulfur dioxide as an easily removable by-product. However, relevant studies on the role of the intermediate sulfinyl chloride (ClSO) radical in Cl2SO applications are sparse as the 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, further supported by performing high-accuracy extrapolated ab initio thermochemistry calculations. The Cl-SO bond energy was found to be 54.2 ± 0.9 kcal mol1, which was a factor of 10 larger than the previous estimations, indicating that the thermal decomposition rate of ClSO was negligible. As a result, kinetic simulations revealed that the ClSO concentrations can be orders of magnitude higher than previously assumed in a variety of applications. This highlights the unconsidered importance of ClSO chemistry. A better control of ClSO radicals could improve 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.