Theoretical study on structures and stability of SiC₂S isomers

The structures, energetics, spectroscopies, and isomerization of singlet and triplet SiC₂S isomeric species are explored at various levels. At the B3LYP/6-311G(d) level, a total of 29 minimum isomers and 37 interconversion transition states are located. At the higher CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest lying isomer is the linear SiCCS ¹1 with a resonance structure mainly between Si=C=C=S and Si≡C-C≡S. In contrast to the C₃S molecule that has no kinetically stable singlet species besides CCCS, SiC₂S has another two kinetically very stable species, i.e., the SiCC three-membered ring with exocyclic SSi bonding ¹5 at 23.2 kcal/mol, and the SiCSC four-membered ring with CC cross bonding ¹7 at 44.3 kcal/mol. Particularly, isomer ¹5 possesses considerable Si≡S and C≡C triple bonding character and can be viewed as a charge-transfer species containing SiS⁺ and C₂^-. Isomer ¹7 has a very short CC crossed bond due to the existence of two sets of π-bonding orbital and the weak electron-accepting ability of silicon. The remaining singlet and all the triplet species except triplet SiCCS ³1 either have much higher energy or much lower kinetic stability and have little chance of existence. The bonding nature and possible formation strategies of ¹1, ¹5, and ¹7 are discussed.