Formation of Silicon Analogues of Thio-isocyanic Acid, HNSiS, and Its Isomers by Neutral−Neutral Reactions of the Fragments:  A Computational Study^†

Here, we report the detailed potential energy surface (PES) of the singlet and the triplet [H,N,Si,S] generated using the DFT-based G3B3 methodology and ab initio-based CCSD/6-311G(d,p)//MP2/6-311+G(d,p) methods. The PES reveals a lot of stationary states, thus indicating the possible existence of stable species. While the most stable molecules of the carbon analogues, isocyanic acid, HNCO, and thio-isocyanic acid, HNCS, have been detected in interstellar space, these silicon species have not yet been reported. Considering the potential importance of silicon in interstellar space, this is of interest. The barriers for isomerization and dissociations for these have been calculated, and they ascertain the stability. Thus, the most stable isomer on this surface, HNSiS (¹A‘) requires about 50 kcal/mol for the lowest isomerization, and the lowest dissociation is 71 kcal/mol. On this surface, some intersystem crossings between the singlets and the triplets occur, and the geometries at these crossing points have also been determined. It is suggested that these silicon analogues of the important thio-fulminic acid and thio-isocyanic acid are stable species, which may be experimentally generated at the laboratory level. The neutral−neutral reactions of the fragments leading to the stable isomers have also been mapped to suggest their possible formation in the interstellar space.