Infrared Spectra and Density Functional Calculations for Singlet CH₂SiX₂ and Triplet HC–SiX₃ and XC–SiX₃ Intermediates in Reactions of Laser-Ablated Silicon Atoms with Di‑, Tri‑, and Tetrahalomethanes

Reactions of laser-ablated silicon atoms with di-, tri-, and tetrahalomethanes in excess argon were investigated, and the products were identified from the matrix infrared spectra, isotopic shifts, and density functional theory energy, bond length, and frequency calculations. Dihalomethanes produce planar singlet silenes (CH₂SiX₂), and tri- and tetrahalomethanes form triplet halosilyl carbenes (HC–SiX₃ and XC–SiX₃). The Si-bearing molecules identified are the most stable, lowest-energy product in the reaction systems. While the C–Si bond in the silene is a true double bond, the C–Si bond in the carbene is a shortened single bond enhanced by hyperconjugation of the two unpaired electrons on C to σ*­(Si–X) orbitals, which contributes stabilization through a small amount of π-bonding and reduction of the HCSi or XCSi angles. The C–Si bond lengths in these carbenes (1.782 Å for HC–SiF₃) are between the single-bond length in the unobserved first insertion intermediate (1.975 Å for CHF₂–SiF) and the double-bond length in the silene (1.704 Å for CHFSiF₂). The silicon s²p² and titanium s²d² electron configurations produce similar primary products, but the methylidyne with Ti has a bond to carbon stronger than that of the halosilyl carbene.