The structures, energetics, spectroscopies, and isomerization of singlet and triplet SiC_2S 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 ~1I with a resonance structure mainly between Si = C = C = S and Si ≡ C - C ≡ S. In contrast to the C_3S molecule that has no kinetically stable singlet species besides CCCS, SiC_2S 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 C_2~-. 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 ~3I 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.
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