Time-resolved studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with ethene. The reaction was studied in the gas phase, over the pressure range 1-100 Torr (in SF6 bath gas) at five temperatures in the range 298-595 K. The reaction of SiH2 with CzH4 is pressure dependent, consistent with a third-body assisted association reaction to form silirane. While silirane was too labile to be detected, vinylsilane, another possible product, was ruled out by G C analysis. Extrapolation of the pressure dependent rate constants gave the Arrhenius equation: log(k"lcm3 molecule-' s-l) = (-9.97 f 0.03) + (2.9 f 0.2) kJ mol-'/RT In 10. These parameters are consistent with a fast, nearly collision-controlled, association process. RRKM modeling, based on a consistent transition state for silirane decomposition, gave a good fit to the pressure dependent curves. The calculations employed a weak collisional deactivation model and confirmed (within f 1 2 kJ mol-') the most recent theoretical value for AH'f(si1irane) of 124 kJ mol-'. An enthalpy surface for the reaction highlights the importance of entropy, rather than enthalpy (or energy), in determining the breakdown pathway of silirane.

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...

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Abstract. Silylenes (silanediyls) have made an important impact on organosilicon chemistry even if it is of more recent foundation than carbenes in organic chemistry and much less complete. These species are highly reactive intermediates. They play a central role in the chemical vapour deposition (CVD) of various silicon-containing thin films which have a technological importance in microelectronics as well as in the dry etching processes of silicon wafers. Spectroscopic methods have been developed to observe these species, a necessary pre-requisite to their direct monitoring. In this work, deuterated phenylsilane precursor, PhSiD 3 was chosen for SiD 2 because its analogue phenylsilane, PhSiH 3 proved to be a good precursor for SiH 2 and the high quality decay signals observed revealed that SiD 2 be readily detected from PhSiD 3 and that if other decomposition pathways (e.g. PhSiD + D 2 ) are occurring, they do not effect measurements of the rate constants for SiD 2 . The absorption spectrum of SiD 2 formed from the flash photolysis of a mixture of PhSiD 3 and SF 6 at 193nm were found in the region 17384-17391 cm -1 with strong band at 17387.07 cm -1. This single rotational line of p Q 1 was chosen to monitor SiD 2 removal. Time-resolved studies of SiD 2 have been carried out to obtain rate constants for its bimolecular reactions with D 2 . The reactions were studied over the pressure range 5-100 Torr (in SF 6 bath gas) at four temperatures in the range 298-498K. Single decay from 10 photolysis laser shots were averaged and found to give reasonable first-order kinetics fits. Second order kinetics were obtained by pressure dependence of the pseudo first order decay constants and substance D 2 pressures within experimental error. The reaction was found to be weakly pressure dependent at all temperatures, consistent with a third-body mediated association process. In addition, SiH 2 + H 2 reaction is approximately ca. 60% faster than SiD 2 +D 2 reaction. Theoretical extrapolations (using Lindemann-Hinshelwood model and Rice, Ramsperger, Kassel and Marcus (RRKM) theory) were also carried out and obtained data fitted the Arrhenius equations.