Spectroscopic identification of the ammonia-mercapto radical complex.

The elusive hydrogen-bonded radical complex (˙SH⋯NH 3 ) consisting of ammonia (NH 3 ) and a mercapto radical (˙SH) has been generated through the 193 nm laser photolysis of the molecular complex between NH 3 and hydrogen sulfide (H 2 S) in solid Ar- and N 2 -matrixes at 10 K. The identification of ˙SH⋯NH 3 with matrix-isolation IR spectroscopy and UV-vis spectroscopy is supported by 15 N- 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⋯NH 3 ( cf. free ˙SH), the radical ˙SH acts as a hydrogen donor, and NH 3 acts as an acceptor. According to the calculations at the CCSD(T)/aug-cc-pVTZ level, the SH⋯N bonded structure ˙SH⋯NH 3 (binding energy D e = 3.9 kcal mol -1 ) is more stable than the isomeric amidogen radical complex HSH⋯˙NH 2 ( 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⋯NH 3 complex, since the water-amidogen radical complex HOH⋯˙NH 2 ( D e = 5.1 kcal mol -1 ) was generated under similar photolysis conditions, whereas the ammonia-hydroxyl radical complex ˙OH⋯NH 3 ( D e = 7.9 kcal mol -1 ) is higher in energy by 9.3 kcal mol -1 .