Structure and Stability of Small Nitrile Sulfides and Their Attempted Generation from 1,2,5-Thiadiazoles

Abstract

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.

Structure, spectroscopy, and thermal decomposition of 5-chloro-1,2,3,4-thiatriazole: a He I photoelectron, infrared, and quantum chemical study

“…For comparison, the ''tail to tail'' reaction between two ClCNS molecules has also been computed. We note that this latter reaction was investigated by us earlier at the B3LYP/6-31G** level [18] and results are in agreement with the present work. Calculations predict similar decomposition mechanism for all of these investigated bimolecular reactions, with the .…”

First Isolation and Spectroscopic Observation of Thiofulminic acid (HCNS)

“…to photolysis, flash vacuum pyrolysis of this precursor in the gas phase produces only HCN and sulfur. [12] HCNS in the matrix cannot be photodecomposed by 254 nm UV light or by the visible components of the high-pressure mercury lamp. Broad-band UV irradiation, however, destroyed the molecule and the absorption bands of HCN increased.…”

The electronic states of 1,2,5-thiadiazole studied by VUV absorption spectroscopy and ab initio configuration interaction methods

“…A B3LYP pp * triplet state has been calculated [23], with excitation energy 4.22 eV, to be compared with a singlet state (discussed below) at 6.15 eV (f = 0.27); the possibility of saddle points was not investigated, and the triplet seems likely to be a saddle point from the present results.…”

“…As mentioned above, there do not appear to be experimental studies of triplet states for 1,2,5-thiadiazole, but fragmentation during high temperature thermal decomposition [23] has been attributed to its triplet states. A DFT (B3LYP) planar (pp * ) triplet state (excitation energy 4.22 eV) has been compared with its corresponding singlet state (6.15 eV) [24,25], but the present non-planar adiabatic Tables 4 and 6 shows that the triplet states are generally less complex mixtures of leading CSF.…”