Identification and Photochemistry of the Mercaptomethyl Radical

The mercaptomethyl radical (center dot CH2SH) is a higher-energy isomer of the methylthio radical (CH3S center dot) that has been proposed as an important intermediate in atmospheric and interstellar sulfur chemistry. Herein, we report the spectroscopic identification of center dot CH2SH during the UV (365 nm) photolysis of CH3S center dot in a solid Ar-matrix at 10 K. Upon subsequent irradiation at 266 nm, the dehydrogenation of center dot CH2SH to yield CS via the intermediacy of the elusive thioformyl radical (HCS center dot) has also been observed. The characterization of center dot CH2SH and HCS center dot with matrix-isolation IR and UV-vis spectroscopy is supported by C-13-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 center dot CH2SH provides new insight into understanding the chemical evolution of organosulfur molecules in the interstellar medium (ISM).