Spectroscopic identification of the ammonia-mercapto radical complex

The elusive hydrogen-bonded radical complex (˙SH⋯NH₃) consisting of ammonia (NH₃) and a mercapto radical (˙SH) has been generated through the 193 nm laser photolysis of the molecular complex between NH₃ and hydrogen sulfide (H₂S) in solid Ar- and N₂-matrixes at 10 K. The identification of ˙SH⋯NH₃ with matrix-isolation IR spectroscopy and UV-vis spectroscopy is supported by ¹⁵N- and D-isotope labeling experiments and quantum chemical calculations at the B3LYP-D3(BJ)/6-311++G(3df,3pd) level of theory. In line with a large red shift of -172.2 cm^-1 for the frequency of the S-H stretching mode observed in ˙SH⋯NH₃ (cf. free ˙SH), the radical ˙SH acts as a hydrogen donor, and NH₃ acts as an acceptor. According to the calculations at the CCSD(T)/aug-cc-pVTZ level, the SH⋯N bonded structure ˙SH⋯NH₃ (binding energy D_e = 3.9 kcal mol^-1) is more stable than the isomeric amidogen radical complex HSH⋯˙NH₂ (D_e = 2.8 kcal mol^-1) by 16.6 kcal mol^-1. This is in sharp contrast to the photochemistry of the closely related HOH⋯NH₃ complex, since the water-amidogen radical complex HOH⋯˙NH₂ (D_e = 5.1 kcal mol^-1) was generated under similar photolysis conditions, whereas the ammonia-hydroxyl radical complex ˙OH⋯NH₃ (D_e = 7.9 kcal mol^-1) is higher in energy by 9.3 kcal mol^-1.