Si is a metalloid that readily donates or shares its four valence electrons to form chemical bonds, combining with many other element to form various compounds. ⁵ Unlike carbon it sometimes accept additional electrons to form hypervalent products with five or six bonds. ⁶ Recent matrix spectroscopic studies have shown that silicon, like transitionmetals, readily undergoes C-X (X = H, halogen) insertion and following X migration from C to Si in reactions with methane and halomethanes, generating small methyl silylenes, silaethenes, and silyl carbenes without a trace of hypervalent products. ⁷ Maier et al . have identified CH ₃ -SiH and CH ₂ =SiH ₂ in reactions of Si + CH ₄ from the matrix spectra and also proposed a reaction path (initial C-H insertion by Si and following H migration), ² which is in fact similar to that for reaction of a transition-metal with methane. ^7,8 They have later reported production of halogenated derivatives (CH ₃ -SiX and CH ₂ =SiHX) in reactions of Si with methyl halides. ³ Schreiner et al . have observed HC-SiH ₃ from matrix ESR and infrared spectra in reaction of C( ³ P) with SiH ₄ and the ESR hyperfine structures indicate that the silyl carbene owns a triplet ground state like other carbenes (CX ₂ , X = H, halogen). ⁴ Cho and Andrews have shown that the silaethene is the primary product in reactions of dihalomethanes and the silyl carbenes in reactions of tri- and tetrahalomethanes. ⁷ These Si bearing products from its s ² p ² electron configuration are in fact similar to those from Ti (s ² d ² ) reactions, but the Ti methylidynes (XC÷TiX ₃ ) own a much stronger C-Ti bond by effective electron donation from C to Ti’s low lying d-orbitals. ^2−4,7,8 The previous studies have shown that Ti also generates CH ₃ -MH and CH ₂ =MH ₂ in reactions with CH ₄ , but they produce CH ₂ =MX ₂ with CH ₂ X ₂ and triplet methylidynes (HC÷CX ₃ and XC÷MX ₃ ) with tri- and tetrahalomethanes. ^7,8 Substitution of H with halogen increases the stability of the higher oxidation-state compounds due to the stability of M-X bond. Density functional theory (DFT) calculations ⁹ were carried out for the Si bearing products and transition states using the Gaussian 09 package, ¹⁰ the B3LYP hybrid density functional, ¹¹ and the 6-311++G(3df, 3pd) basis sets for H, C, F, and Si. ¹² Geometries were fully relaxed during optimization, and the optimized geometry was confirmed by vibrational analysis, all real frequencies for a stable conformation and a single imaginary frequency for a transition state. Each transition state was linked to its reactant and product by intrinsic reaction coordinate (IRC) calculation. ¹ Natural bond orbital (NBO) calculations ¹³ were also done to examine the bond properties of the Si compounds and electron-donations between the MO’s. The vibrational frequencies were calculated analytically. In the calculation of binding energy for a Si product, the zero-point energy is included. Due to the law of spin conservation and the ground states of C and Si ( ³ P), these Si reactions most probably occur in a triplet potential surface. . 1 illustrates the energies of the products and transition states relative to the reactants [Si( ³ P) + CH ₄ and Si( ³ P) + CH ₃ F]. These results reveal that both the C-H and C-F insertion reactions by atomic Si have to go over a considerable barrier, requiring extra photon energy to provide enough kinetic energy to the reactants in a cold matrix. It is consistent with the observed results that the Si products were generated during photolysis after original deposition. ²⁻⁴ TS1, TS2, and TS3 in the methane reaction are considerably higher than the reactants [107, 126, and 177 kJ/mol higher than Si( ³ P) + CH ₄ as shown in . 1(a) ], indicating that the reaction becomes increasingly more difficult as moving toward its end [SiH ₄ + C( ³ P)]. The previous studies have also shown that while the insertion compound (CH ₃ - SiH) is the primary reaction product on UV photolysis (λ = 185 or 254 nm), and the silaethane is detected after additional UV irradiation (λ = 254 nm), but HC-SiH ₃ is not observed. ² The silyl carbene is provided later in reaction of SiH ₄ + C( ³ P), ⁴ which is 372 kJ/mol higher in energy than Si( ³ P) + CH ₄ . This reaction [SiH ₄ + C( ³ P) → HC-SiH ₃ (T)] is evidently barrierless as shown in . 1(a) [and . 4(a) ] while the next reaction step from HC-SiH ₃ (T) to CH ₂ -SiH ₂ (T) requires a substantial activation energy (151 kJ/mol), leading to provision of the silyl carbene (HC-SiH ₃ ). ⁴ Inter-system crossing is supposed to take place once CH ₃ -SiH and CH ₂ =SiH are generated; the singlet methyl silylene and silaethene are 153 and 171 kJ/mol more stable than the reactants. Maier et al . have also reported that the observed vibrational characteristics of the methyl silylene and silaethene correlate with the predicted values for their singlet states. ² On the other hand, all our attempts to optimize the geometry of singlet HC-SiH ₃ ended up with the structure of CH ₂ =SiH ₂ , suggesting that singlet HC-SiH ₃ is not a meaningful energy minimum. The observed vibrational characteristics and ESR hyperfine structures also correlate with those predicted for triplet HC-SiH ₃ . ⁴ In reaction of Si( ³ P) + CH ₃ F, TS1, TS2, and TS3 are considerably lower than those in the Si( ³ P) + CH ₄ system, 56, -65, -37 kJ/mol higher than the reactants [ . 1(b) ]. Therefore, C-F insertion by Si to form CH ₃ -SiF is expected to be easier, but the following conversions to CH ₂ -SiHF and to HC-SiH ₂ F to be more difficult. The F bearing Si products are relatively more stable than the corresponding analogues in the Si( ³ P) + CH ₄ system due to the strong Si-F bond (540 kJ/mol). ¹⁴ Maier et al . have observed a Si adduct (CH ₃ F···Si) in the original deposition spectrum, which easily converts to CH ₃ -SiF by short UV or visible photolysis. ³ CH ₂ =SiHF was observed later on much longer irradiation with λ = 366 nm. The fluorosilyl carbene (HC-SiH ₂ F), which is expected to be more difficult to provide as shown in . 1(b) , has not been reported. However, the present results indicate that provision of HC-SiH ₂ F is probably possible via reaction of C( ³ P) + SiH ₃ F, parallel to the case of HC-SiH ₃ , where the silyl carbene is produced in reaction of laser-ablated graphite plume with a SiH ₄ /Ar gas mixture on the surface of a cold window (1-12 K) by Schreiner et al . ⁴ As shown in . 1(b) and 4(b) , the Si-H insertion reaction by C is apparently barrierless and continuous downhill to HC-SiH ₂ F, while the reaction from HC-SiH ₂ F to CH ₂ -SiHF has to go over a considerable barrier (161 kJ/mol), making the fluorosilyl carbene a stable conformation. CH ₃ -SiF and CH ₂ =SiHF are also expected to undergo system crossing to their singlet ground states, which are 182 and 134 kJ/mol more stable than the triplet counter parts. Maier et al . have also reported that the observed vibrational characteristics of the fluorided products correlate with the computed values for the singlet species. ³ The silyl carbene, on the other hand, has a triplet ground state. Our attempts for geometry optimization of singlet HC-SiH ₂ F all lead to the structure of singlet CH ₂ =SiHF, indicating that this singlet fluorosilyl carbene is also not a meaningful energy minimum. . 2 and 3 illustrate our IRC computation results for the reaction steps in the course of Si( ³ P) + CH ₄ →HC-SiH ₃ (T) and those for the fluorided counterparts, showing that each step is a smooth conversion over its barrier. 4 shows the energy variations in conversions of HC-SiH ₃ to C( ³ P) + SiH ₄ (a) and HC-SiH ₂ F to C( ³ P) + SiH ₃ F (b), both of which are barrierless and continuous uphill. . 2 - 4 also show that the product of a previous reaction step is the reactant of the next reaction step, the entire reaction path being a series of smooth conversions from one to another product. This suggests that the generally accepted reaction mechanism (initial C-H or C-F insertion and subsequent H migrations) is most probably the true reaction course, which connects the saddle points in the triplet potential surface. The structures of triplet CH ₃ -SiH and CH ₃ -SiF in . S1 and S2 are similar to those of the singlet counterparts, ^2,3 whereas triplet CH ₂ -SiH ₂ and CH ₂ -SiHF own non-planar structures due to lack of C-Si π-bond (natural C-Si bond orders ¹³ 0.99 and 0.98), which contrasts the more usual planar or near planar geometry in the singlet states. ^2,3,7,8 The structure of triplet HC-SiH ₃ is almost identical to the reported structure, ⁴ carrying a bent HCSi moiety (160.5°) due to the unpaired electrons staying on C. Other carbenes also have a bent structure (e.g. 135.2° for CH ₂ at the same level of theory). The non-bonding electrons reportedly have considerable contribution to the C-Si bond (π characters), leading to a shorter C-Si bond ( Figs . S1 and S2). ⁷ In conclusion, the Si bearing products and transition states in the triplet reaction paths from Si( ³ P) + CH ₄ to C( ³ P) + SiH ₄ and from Si( ³ P) + CH ₃ F to C( ³ P) + SiH ₃ F have been examined. Each transition state is linked by IRC computation to its reactant and product, confirming that the reaction path is a series of smooth conversions from one to another reported product. ²⁻⁴ Unlike other reaction steps, conversion of silyl carbene to C( ³ P) + SiH ₃ X (X = H or F) is a continuous, barrierless uphill. Therefore, the unreported fluorosilyl carbene may be prepared in reaction of atomic C with SiH ₃ F, similar to the case of HC-SiH ₃ . ⁴ The silyl carbenes have a triplet ground state, leading to a bent HCSi moiety. The triplet methyl silylene and silaethenes with unusual non-planar structures are supposed to undergo a system crossing to the singlet ground state, whose vibrational characters correlate with the experimentally observed values. ²⁻⁴