The gas-phase generation and spectroscopic identification of nitrile sulfides by thermolysis of 1,2,5-thiadiazole
precursors was attempted, but in all cases the thiadiazoles were found to produce sulfur and the corresponding
nitrile. This prompted an investigation by ab initio and density functional calculations for the equilibrium
geometries, stabilities, and decomposition mechanisms of several nitrile sulfides (XCNS, where X = H, F,
Cl, CN, CH3). Equilibrium geometries obtained from calculations at the B3LYP, MPn(n = 2−4), QCISD,
QCISD(T), CCSD, and CCSD(T) levels with moderate to large basis sets indicate that the molecules have
linear heavy atom geometries. The exception is the fluoro derivative, which is bent with a calculated barrier
to linearity of 889 cm-1 (B3LYP/cc-pVTZ). The nitrile sulfides are predicted by the B3LYP method to be
stable in the dilute gas phase, whereas in the condensed phase they are suggested to be very unstable due to
bimolecular decomposition. The mechanism of this loss process is complicated by various sulfur transfer and
cyclization reactions between decomposition intermediates, with the predicted stable products being sulfur,
nitriles, and thiadiazoles. The first step of the bimolecular decomposition is either a cycloaddition to thiofuroxan
or a sulfur transfer with simultaneous S2 loss to nitriles.