Ab initio and DFT energetics of silylenic X-CNSi (X=H, F, Cl, and Br)

Singlet-triplet energy splitting for 24 silylenic reactive intermediates, X-CNSi (where X=H, F, Cl and Br), are compared and contrasted at 11 levels of theory: B1LYP/6-31++G**, B3LYP/6-31++G**, B1LYP/6-311++G**, B3LYP/6-311++G**, MP3/6-31G*, MP3/6-311++G**, MP2/6-31+G**, MP2/6-311++G**, MP4 (SDTQ)/6-311++G**, QCISD(T)/6-311++G** and CCSD(T)/6-311++G**. Each X-substituted silylenic species may either be singlet (s) or triplet (t), with one of the following three structures: 3-X-2-aza-1-silacyclopropenylidene (1s-X, 1t-X); [(X-imino)methylene]silylene (2s-X, 2t-X); and X-cyanosilylene (3s-X, 3t-X). For all X-CNSi species studied, orders of singlet-triplet energy separations (ΔEs-t,X), appear as a function of electro-negativity (F>Cl>Br>H). For the six H-CNSi isomers (X=H), stability order is: 3s-H>1s-H>2t-H>3t-H>2s-H>1t-H. Likewise, stability order for the six isomers with X=F, is: 3s-F>3t-F>1s-F>1t-F>2s-F>2t-F. For X=Cl, the order of stability is: 3s-Cl>1s-Cl>3t-Cl>2t-Cl>1t-Cl>2t-Cl. Finally, the order of stability for six isomers of Br-CNSi is: 3s-Br>3t-Br>1s-Br>2s-Br>2t-Br>1t-Br. The lowest energy minimum, among all 24 species scrutinized, appears to be the singlet acyclic 3s-X. Triplet silylene 2t-H is suggested to be more stable than its corresponding 2s-H at MP3, MP2 and DFT levels of theory. Comparisons between relative stabilities; multiplicities and geometrical parameters of 1-3 are discussed.