Identification and Photochemistry of the Mercaptomethyl Radical.

The mercaptomethyl radical (·CH 2 SH) is a higher-energy isomer of the methylthio radical (CH 3 S·) that has been proposed as an important intermediate in atmospheric and interstellar sulfur chemistry. Herein, we report the spectroscopic identification of ·CH 2 SH during the UV (365 nm) photolysis of CH 3 S· in a solid Ar-matrix at 10 K. Upon subsequent irradiation at 266 nm, the dehydrogenation of ·CH 2 SH to yield CS via the intermediacy of the elusive thioformyl radical (HCS·) has also been observed. The characterization of ·CH 2 SH and HCS· with matrix-isolation IR and UV-vis spectroscopy is supported by 13 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 ·CH 2 SH provides new insight into understanding the chemical evolution of organosulfur molecules in the interstellar medium (ISM).