Si2CN: A stable nitrogen-containing radical with cyclic ground state

The structures and isomerization of Si2CN species are explored at density functional theory and ab initio levels. Fourteen minimum isomers are located connected by 23 interconversion transition states. At the coupled-cluster single double (CCSD)(T)/6-311+G(2df)//QCISD/6-311G(d) +zero-point vibrational energies level, the thermodynamically most stable isomer is a four-membered ring form cSiSiCN 1 with Si–C cross bonding. Isomer 1 has very strong C–N multiple bonding characters, formally suggestive of a radical adduct between Si2 and CN. Such a highly π-electron localization can effectively stabilize isomer 1 to be the ground state. The second low-lying isomer is a linear form SiCNSi 5 (9.8 kcal/mol above 1) with resonating structure among |Si=C⋅–N=Si|, ⋅|Si=C=N=Si|, and |Si=C=N–Si|⋅ with the former two bearing more weight. The species 1 and 5 have very high kinetic stability stabilized by the barriers of at least 25 kcal/mol. Both isomers should be experimentally or astrophysically observable. In light of ...