Theoretical Investigation On Structure And Stability For Sulfur-containing Triple Bond Compounds

The multiple bonding associated with heavier main group elements hascontinuouly attracted chemists′attention. Due to the low tendency to form p-p πconjugation, heavier main group elements are usually reluctant to form multiplebonding, and the related compounds are relatively rare in comparison to those forlighter main group elements. Only by taking advantage of uniqued designing tricksand stabilization strategies do the isolation and characterization of such exoticcompounds become feasible. Their detailed structural and bonding analysis hasgreatly challenged the widely accepted principles of chemical bonding. Of the group16elements, oxygen forms some stable triply bonded compounds. This tendency ismuch less pronounced with sulfur, and compounds with the sulfur-containing triplebond are particularly scarce. On the one hand, it due to the synthetic difficulty infinding suitable ligand; on the other hand, triply sulfur-bonded polyatomic structuresundergo the easy transformation into the corresponding doubly sulfur-bondedcounterparts. Therefore, designing and synthesis of stable sulfur-containing triplebond compounds is important issue. The obtained results are summarized as follows:1) The known polyatomic sulfur–carbon triply bonded molecules are usually notthe global minima. Here, we report a potential energy surface investigation of atetra-atomic molecule [S,C,B,O] in both doublet and quartet states. The B3LYP andM06-2X methodologies with6-311/G(3df,2p) and aug-ccpVTZ basis sets wereapplied for geometrical optimization and CCSD(T)/aug-cc-pVTZ for single-pointenergy calculations. The thermodynamically most stable isomer is the linear SCBO01(0.0kcal/mol). Kinetically, SCBO01is separated from the other isomers andfragments by the rather high barriers of at least44.7kcal/mol. In particular, isomerSCBO01contains a typical sulfur–carbon triple bond based on the systematicanalysis from the structure, vibrational frequency, molecular orbital, Wiberg bondindex, and adiabatic bond dissociation energy. In addition, there exists a secondlow-lying isomer, i.e., linear SBCO02(7.3kcal/mol) with S≡B triple bonding, whose kinetic stability is governed by its fragmentation to2SB+1CO (30.4kcal/mol). Theremaining isomers are either kinetically unstable with low conversion barriers orenergetically very high lying. We propose that the simple two-body associationbetween SC and BO, SB and CO pairs can preferentially lead to the formation andstabilization of SCBO01and SBCO02, respectively. The isomer SCBO01, which isthe global structure and extraordinarily stable against both isomerization andfragmentation, strongly deserves future laboratory studies.2) Triply sulfur-bonded compounds are still rare, due to either the lack of suitablegeneration precursors or the conversion instability toward doubly sulfur-bondedstructures. A detailed computational study was performed on the structures andstability of various [Be,C,O,S] isomers at the coupled cluster singles doubles (tripleexcitations)(CCSD(T)/aug-cc-pVTZ//B3LYP/6-311+G(d)+ZPVE level to predictintrinsically stable isomers with triply bonded sulfur. The molecular orbital, bonddistance, and harmonic vibrational frequency analysis were carried out ataug-cc-pVTZ-B3LYP, M06-2X, and CCSD(T) levels to investigate the bonding natureof linear structures. It was shown that two low-lying isomers are linear SBeCO01(0.0kcal/mol) and SBeOC02(15.7kcal/mol), both of which possess the S≡Be triplebonding. The Lewis acid–base association of SBe+CO can barrierlessly form01and02, with the former more abundant, while the insertion reaction of SCO+Be mightgenerate more02than01via photochemical processes. By contrast, formation of theS≡C-bearing isomer SCBeO04(39.4kcal/mol) seems unlikely due to its higherenergy and less kinetic competition than that of01and02, via either simpleassociation or insertion reactions. The new stable isomers SBeCO01and SBeOC02add to the number of S≡Be triply bonded species. Their unique structures and variedbranching ratios under association and insertion processes deserve futureexperimental study.3) In this article, with an attempt to predict intrinsically stable species withsulfur-related triple bonding, we report a thorough computational study of twocharged systems [B,C,O,S]+and [B,C,O,S]–at theCCSD(T)/aug-ccpVTZ//B3LYP/6-311+G(3df,2p)+ZPVE level for singlet and tripletpotential energy surfaces, aug-cc-pVTZB3LYP, M06-2X, and CCSD(T) levels forcritical structures, as well as the CCSD(T)/aug-cc-pVQZ and G4levels for adiabaticbond dissociation energy (ABDE). A total of26isomers and25transition states werelocated. 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+/–01and SBCO+/–02, both of which contain the S≡X(X=C, B) bonding and arekinetically very stable against interconversion and fragmentation. With the increasedvalence electron number in the order of SCBO/SBCO+<SCBO/SBCO<SCBO/SBCO–,the SX bond distance elongates as1.5653<1.6126<1.6924for X=B and1.4715<1.5319<1.6100for 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 shortestS≡C bond among the known classical and non-protonated compounds (thewell-known F3SCCF3and F3SCSF5have been termed as ‘‘nonclassical’’ because oftheir unusually low ABDE of SC bond). Future mass spectroscopic studies are greatlyappealed for the characterization of the cationic and anionic SCBO+/–01as well asSBCO+/–02.