Theoretical study on structures and stabilities of [H₃,Si,C,N] isomers

A detailed theoretical investigation of the singlet [H₃,Si,C,N] potential energy surfaces including 26 minimum isomers and 45 interconversion transition states is reported at the Gaussian-3//B3LYP/6-31G(d) level. The three lowest-lying isomers H₃SiCN 1 (0.0 kcal/mol), H₃SiNC 2 (4.9 kcal/mol), and H₃CNSi 3 (7.4 kcal/mol) are also kinetically stable with the least conversion barriers of 23.4-46.5 kcal/mol, whereas the high-lying H₃CSiN 4 (55.9 kcal/mol) is metastable with a barrier of 14.7 kcal/mol. This is consistent with available matrix spectroscopic identifications from the parent compound H₃CSi(N₃₎₃. Additionally, 14 new isomers with considerable kinetic stability are predicted. Among these, 7 chainlike isomers, SiCNH₃ 5 (85.3 kcal/mol), H ₂CNSiH 8 (27.2 kcal/mol), H₂CSiNH 9 (49.4 kcal/mol), H₂NCSiH 10 (56.5 kcal/mol), H₂NCHSi 11 (55.0 kcal/mol), H₂N-c-CHSi 12 (55.0 kcal/mol), and H₂NSiCH 13 (62.6 kcal/mol), and 4 three-membered ring isomers c-CH₂SiNH 22 (29.0 kcal/mol), c-NH₂SiCH 23 (49.6 kcal/mol), c-SiH₂NCH 25 (34.2 kcal/mol), and c-SiHCHNH 26 (50.6 kcal/mol) each have the conversion barriers of 18-37 kcal/mol. In particular, the 4 species 10,11,12, and 23 can be easily converted to each other, and all of them could coexist. The remaining 3 isomers H₂SiCNH 15 (45.7 kcal/mol), H₂SiNCH 17 (45.2 kcal/mol), and c-SiH₂CNH 24 (61.0) have lower conversion barriers of 11-13 kcal/mol and can be considered as metastable species. All of the predicted 14 new isomers could exist as stable or metastable intermediates under various conditions, such as the irradiation of Si,C,N-containing compounds or the SiH₂ + HCN/HNC, Si + H₂CNH, and SiC + NH₃ reactions. Finally, the structural and bonding analysis indicate that the newly found isomers contain various properties that are of chemical interest (e.g., silylene, Si≡C triple bonding, intra- or interdonor-acceptor bonding, and cumulenic Si bonding). This is the first detailed theoretical study on the potential energy surfaces of the series of hydrogenated Si,C,N-containing molecules. The knowledge of the present trihydrogenated SiCN isomerism could provide useful information for more highly hydrogenated or larger Si,C,N-containing species.