Identification and Photochemistry of the Mercaptomethyl Radical

The mercaptomethyl radical (·CHSH) is a higher-energy isomer of the methylthio radical (CHS·) that has been proposed as an important intermediate in atmospheric and interstellar sulfur chemistry. Herein, we report the spectroscopic identification of ·CHSH during the UV (365 nm) photolysis of CHS· in a solid Ar-matrix at 10 K. Upon subsequent irradiation at 266 nm, the dehydrogenation of ·CHSH to yield CS via the intermediacy of the elusive thioformyl radical (HCS·) has also been observed. The characterization of ·CHSH and HCS· with matrix-isolation IR and UV-vis spectroscopy is supported by C-isotope labeling and quantum chemical calculations at the CCSD(T)-F12a/cc-pVTZ-F12 level using configuration-selective vibrational configuration interaction theory (VCI). The disclosed photochemistry of ·CHSH provides new insight into understanding the chemical evolution of organosulfur molecules in the interstellar medium (ISM).