The gas-phase reaction between silylene and 2-butyne: kinetics, isotope studies, pressure dependence studies and quantum chemical calculations

Abstract

Time-resolved kinetic studies of the reactions of silylene, SiH₂, and dideutero-silylene, SiD₂, generated by laser flash photolysis of phenylsilane and phenylsilane-d₃, respectively, have been carried out to obtain rate coefficients for their bimolecular reactions with 2-butyne, CH₃CCCH₃. The reactions were studied in the gas phase over the pressure range 1–100 Torr in SF₆ bath gas at five temperatures in the range 294–612 K. The second-order rate coefficients, obtained by extrapolation to the high pressure limits at each temperature, fitted the Arrhenius equations where the error limits are single standard deviations:log(k_H^∞/cm³ molecule⁻¹ s⁻¹) = (−9.67 ± 0.04) + (1.71 ± 0.33) kJ mol⁻¹/RTln10log(k_D^∞/cm³ molecule⁻¹ s⁻¹) = (−9.65 ± 0.01) + (1.92 ± 0.13) kJ mol⁻¹/RTln10Additionally, pressure-dependent rate coefficients for the reaction of SiH₂ with 2-butyne in the presence of He (1–100 Torr) were obtained at 301, 429 and 613 K. Quantum chemical (ab initio) calculations of the SiC₄H₈ reaction system at the G3 level support the formation of 2,3-dimethylsilirene [cyclo-SiH₂C(CH₃)C(CH₃)–] as the sole end product. However, reversible formation of 2,3-dimethylvinylsilylene [CH₃CHC(CH₃)SiH] is also an important process. The calculations also indicate the probable involvement of several other intermediates, and possible products. RRKM calculations are in reasonable agreement with the pressure dependences at an enthalpy value for 2,3-dimethylsilirene fairly close to that suggested by the ab initio calculations. The experimental isotope effects deviate significantly from those predicted by RRKM theory. The differences can be explained by an isotopic scrambling mechanism, involving H–D exchange between the hydrogens of the methyl groups and the D-atoms in the ring in 2,3-dimethylsilirene-1,1-d₂. A detailed mechanism involving several intermediate species, which is consistent with the G3 energy surface, is proposed to account for this.