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Author(s):   Becerra, R.; Cannady, J.P.; Walsh, R.
Title:   Reactions of Silylene with Unreactive Molecules. I: Carbon Dioxide; Gas-Phase Kinetic and Theoretical Studies
Journal:   J. Phys. Chem. A
Volume:   106
Page(s):   4922 - 4927
Year:   2002
Reference type:   Journal article
Squib:   2002BEC/CAN4922-4927

Reaction:   CO2 + SiH2CO + H2Si=O
Reaction order:   2
Temperature:   339 - 681 K
Pressure:  0.67 bar
Rate expression:   1.29x10-12 [cm3/molecule s] e-16360 [±1230 J/mole]/RT
Uncertainty:   2.0
Category:  Experiment
Data type:   Absolute value measured directly
Pressure dependence:   Rate constant is pressure independent
Experimental procedure:   Static or low flow - Data taken vs time
Excitation technique:   Flash photolysis (laser or conventional)
Time resolution:   In real time
Analytical technique:   Vis-UV absorption
Comments:   SiH2 formed by 193 nm excimer laser photolysis of Phenylsilane, typically 3-10 mtorr PhSiH3 in 0-500 torr CO2. Also used added SF6 at times to increase the total pressure. SiH2 detected with a CW dye laser at 17259.5 cm-1. Measured over temperature range 339-681 K.

Measured rate of reaction of SiH2 (silylene) with CO2. Goes to SiH2O (silanone) plus CO. Also considered product channel SiO + CH2O (formaldehyde). Did G2 calculations to elucidate reaction pathways, intermediates, and transition states. Found that SiO + CH2O pathway although exothermic by about 66 kJ/mol was kinetically disfavored going through a transition state with a barrier of about 24 kJ/mol and a low A factor. In contrast, the SiH2O + CO channel proceeds through a barrierless addition of SiH2 to CO2 followed by a modest insertion step of about 12 kJ/mol to form chemically activated oxacyclopropanone ?SiH2C(O)O)- which is about 73 kJ/mol downhill, which then can rearrange and eliminate CO through a local transition state (about 16 kJ/mol relative to oxacyclopropanone, but ?7 kJ/mol downhill from reactants). The measured temperature dependence of the overall reaction had an apparent activation energy of about 16 kJ/mol, in good agreement with the computed transition state of about 12 kJ/mol for insertion step.

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Rate constant values calculated from the Arrhenius expression:

T (K)k(T) [cm3/molecule s]
339 3.89E-15
350 4.67E-15
375 6.79E-15
400 9.42E-15
425 1.26E-14
450 1.63E-14
475 2.05E-14
500 2.52E-14
525 3.04E-14
550 3.60E-14
575 4.21E-14
600 4.86E-14
625 5.54E-14
650 6.25E-14
675 6.99E-14
681 7.17E-14