Theoretical study on structures and stability of NC2S isomers

The structures, energetics, spectroscopies, and stabilities of the doublet NC2S radical are explored at density functional theory and ab initio levels. Sixteen minimum isomers are located connected by 38 interconversion transition states. At the CCSD(T)/6-311+G(2df)//QCISD/6-311G(d)+ZPVE level, the lowest-lying isomers are the Renner-Teller pair state isomers NCCS 1(I) (0.0 kcal/mol) with a bent structure and NCCS 1(II) (0.9) with a quasi-linear form. Their calculated rotational constants and dipole moments are consistent with recent experimental observations and theoretical studies. Additionally, three new chainlike isomers with high kinetic stability are predicted to be possible candidates for future experimental and astrophysical detection. They are bent CNCS 2(I) (21.0), quasi-linear CNCS 2(II) (24.7) and linear CCNS 4 (66.3), among which CNCS 2(I) and 2(II) are also Renner-Teller pair state isomers. In addition, the bent CSCN 3 (51.1), bent CCSN 6 (107.1) and cyclic S-cCCN 7 (31.7) can be considered as metastable isomers with slightly lower kinetic stability. The bonding natures of the most relevant isomers are analyzed. The calculated results are compared with those of the analogue NC2O radical. Possible formation strategies of the relevant isomers in laboratory and space are also discussed in detail.