Structures and stability of SCBO+/− and SBCO+/−: prediction of very short yet classical triple bonding of sulfur

Compounds containing the sulfur triple bonding have continued to attract chemists’ attention. In this article, with an attempt to predict intrinsically stable species with sulfur-related triple bonding, we report a thorough computational study of two charged systems [B,C,O,S]+ and [B,C,O,S]− at the CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(3df,2p)+ZPVE level for singlet and triplet potential energy surfaces, aug-cc-pVTZ-B3LYP, M06-2X, and CCSD(T) levels for critical structures, as well as the CCSD(T)/aug-cc-pVQZ and G4 levels for adiabatic bond dissociation energy (ABDE). A total of 26 isomers and 25 transition states were located. The cationic and anionic [B,C,O,S] have the singlet and triplet ground states, respectively. For both systems, the former low-lying isomers are the linear SCBO+/− 01 and SBCO+/− 02, both of which contain the S≡X (X = C, B) bonding and are kinetically very stable against interconversion and fragmentation. With the increased valence electron number in the order of SCBO/SBCO+ < SCBO/SBCO < SCBO/SBCO−, the SX bond distance elongates as 1.5653 < 1.6126 < 1.6924 A for X = B and 1.4715 < 1.5319 < 1.6100 A for X = C at the M06-2X/aug-cc-pVTZ level. Notably, SBCO+ bears the shortest S≡B bond known to date, while SCBO+ bears the shortest S≡C bond among the known classical and non-protonated compounds (the well-known F3SCCF3 and F3SCSF5 have been termed as “nonclassical” because of their unusually low ABDE of SC bond). Future mass spectroscopic studies are greatly appealed for the characterization of the cationic and anionic SCBO+/− 01 as well as SBCO+/− 02.