Direct Gas-Phase Kinetic Studies of Silylene Addition Reactions:  SiH₂ + C₃H₆, SiH₂ + i-C₄H₈, and SiMe₂ + C₂H₄. The Effects of Methyl Substitution on Strain Energies in Siliranes

Abstract

Time-resolved studies of the title reactions have been carried out over the pressure range 1−100 Torr (in SF6 bath gas) and at temperatures in the range 293−600 K, using laser flash photolysis techniques to generate and monitor the silylenes, SiH2 and SiMe2. All three reactions showed evidence of pressure dependence, consistent with third-body assisted association reactions to form silirane products. Extrapolation of the pressure-dependent rate constants gave the following Arrhenius parameters: SiH2 + C3H6, log(A/cm3 molecule-1 s-1) = −9.79 ± 0.03, Ea (kJ mol-1) = −1.9 ± 0.3; SiH2 + C4H8, log(A/cm3 molecule-1 s-1) = −9.91 ± 0.04, Ea (kJ mol-1) = −2.5 ± 0.3; SiMe2 + C4H8, log(A/cm3 molecule-1 s-1) = −12.12 ± 0.02, Ea(kJ mol-1) = −8.5 ± 0.2. These parameters are consistent with fast, nearly collision-controlled processes for SiH2 but a tighter transition state for SiMe2. Rice, Ramsperger, Kassel, Marcus theory (RRKM) modeling, based on consistent transition states for silirane decomposition, and employing a weak collisional deactivation model, gave good fits to the pressure-dependent curves for each system, provided an appropriate value of Eo (fitting parameter) was used for each reaction. The kinetic results are consistent with an electrophilically led addition mechanism, although methyl substitution in the alkene hardly affects the rate constants. The RRKMderived Eo values have been used to derive reaction enthalpies which are in reasonable agreement with values obtained by ab initio calculations at the G2 (MP2,SVP) level. The experimental ΔH° values yield strain energies of 190, 196, and 216 kJ mol-1 for 2-methyl-, 2,2-dimethyl-, and 1,1-dimethylsilirane, respectively. Compared to the strain enthalpy of 167 kJ mol-1 for silirane itself, this shows that methyl substituents in the silirane products substantially increase the strain energies. Theory supports this. Disciplines Chemistry CommentsReprinted (adapted) M. S. Gordon Department of Chemistry, Iowa State UniVersity, Ames, Iowa 50011-3111ReceiVed: April 22, 1998; In Final Form: June 16, 1998 Time-resolved studies of the title reactions have been carried out over the pressure range 1-100 Torr (in SF 6 bath gas) and at temperatures in the range 293-600 K, using laser flash photolysis techniques to generate and monitor the silylenes, SiH 2 and SiMe 2 . All three reactions showed evidence of pressure dependence, consistent with third-body assisted association reactions to form silirane products. Extrapolation of the pressuredependent rate constants gave the following Arrhenius parameters: SiH 2 + C 3 H 6 , log(A/cm 3 molecule -1 s -1 ) ) -9.79 ( 0.03, E a (kJ mol -1 ) ) -1.9 ( 0.3; SiH 2 + C 4 H 8 , log(A/cm 3 molecule -1 s -1 ) ) -9.91 ( 0.04, E a (kJ mol -1 ) ) -2.5 ( 0.3; SiMe 2 + C 4 H 8 , log(A/cm 3 molecule -1 s -1 ) ) -12.12 ( 0.02, E a (kJ mol -1 ) ) -8.5 ( 0.2. These parameters are consistent with fast, nearly collision-controlled processes for SiH 2 but a tighter transition state for SiMe 2 . Rice, Ramsperger, Kassel, Marcus theory (RRKM) m...

Gas phase kinetic and theoretical studies of reactions of germylenes and dimethylstannylene

“…The same is true for the corresponding SiH 2 additions. 76, 78 The system GeH 2 + acetylene. The room temperature rate constant for this reaction in the gas phase was mea sured using both DMGCP 23 and PhGeH 3 24 as precur sors (Table 1).…”

Gas‐Phase Kinetics of Chlorosilylene Reactions II. ClSiH + C2H4: Absolute Rate Measurements and Quantum Chemical and RRKM Calculations for the Prototype π Addition Reaction

“…[31] The more negative activation energy for SiMe 2 + C 2 H 4 could imply the intermediacy of a complex. [6] This is discussed in more detail by Al-Rubaiey et al [35] and is not repeated here.…”

“…It is illuminating, however, to compare the data with those of other silylene reactions with C 2 H 4 . [13,[33][34][35] A comparison of rate constants at 298 K is shown in Table 8 and a comparison of Arrhenius parameters in Table 9. Table 8 shows that at 298 K, ClSiH is slightly less reactive than SiH 2 , by factors of 2.6 at 1.3 kPa and 2.3 (AE 0.3) at infinite pressure, compared with our earlier work, [13] although these factors are smaller when compared with the recent study of Friedrichs et al [33] Table 7.…”

Effect of pressure on the gas-phase chemistry when using monomethylsilane and dimethylsilane in hot-wire chemical vapor deposition

“…The addition of acetylene into their reaction mixture suppressed the formation of these two major products because acetylene traps methylsilylene, preventing the formation of free radicals. The reaction of :SiH 2 and C 2 H 2 has been studied extensively 26,27 and the adduct product is believed to be a silacyclopropene species. In another study by Haas and Ring,23 it was demonstrated that the reaction between :SiH 2 and C 2 H 2 took place either by direct insertion into a C−H bond or by addition to the C−C triple bond, resulting in the formation of silylacetylene.…”