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Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen

Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen
Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen
Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O2. 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 297–600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation:log(k/cm3 molecule–1 s–1)=(–11.08 ± 0.04)+(1.57 ± 0.32 kJ mol–1)/RT ln10The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H2SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O + SiO. The decomposition of the intermediate cyclo-siladioxirane, via O–O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T ? S process in H2SiOO. This process has a small spin–orbit coupling matrix element, consistent with an estimate of its rate constant of 1 × 109 s–1 obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2+ O2 and SiCl2+ O2.
absolute rate constants, molecular-orbital methods, potential-energy surface, temperature-dependence, addition-reaction, basis sets, ab-initio, prototype, atoms, si
1463-9076
2900-2908
Becerra, Rosa
766758a2-9c71-4cab-9523-2874dfd5f84a
Bowes, Sarah-Jane
5085ec12-423d-4945-801c-2a30695d5d21
Ogden, J. Steven
5a99fd42-75e7-4f7b-a347-ac2b1717b8b3
Cannady, J. Pat
1b7d2bf5-1694-4d9f-8bc5-aab0783e9f76
Adamovic, Ivana
94618891-4f47-46d2-bea2-9b63e079a6a9
Gordon, Mark S.
0f9a17f3-2b29-4e03-85d5-95eed062f94b
Almond, Matthew J.
992a3a7b-3cbe-4818-828f-e0e6b876207f
Walsh, Robin
aacc9be2-5b7b-401b-9305-bb6cca0822a0
Becerra, Rosa
766758a2-9c71-4cab-9523-2874dfd5f84a
Bowes, Sarah-Jane
5085ec12-423d-4945-801c-2a30695d5d21
Ogden, J. Steven
5a99fd42-75e7-4f7b-a347-ac2b1717b8b3
Cannady, J. Pat
1b7d2bf5-1694-4d9f-8bc5-aab0783e9f76
Adamovic, Ivana
94618891-4f47-46d2-bea2-9b63e079a6a9
Gordon, Mark S.
0f9a17f3-2b29-4e03-85d5-95eed062f94b
Almond, Matthew J.
992a3a7b-3cbe-4818-828f-e0e6b876207f
Walsh, Robin
aacc9be2-5b7b-401b-9305-bb6cca0822a0

Becerra, Rosa, Bowes, Sarah-Jane, Ogden, J. Steven, Cannady, J. Pat, Adamovic, Ivana, Gordon, Mark S., Almond, Matthew J. and Walsh, Robin (2005) Time-resolved gas-phase kinetic and quantum chemical studies of the reaction of silylene with oxygen. Physical Chemistry Chemical Physics, 7 (15), 2900-2908. (doi:10.1039/b504760a).

Record type: Article

Abstract

Time-resolved kinetic studies of the reaction of silylene, SiH2, generated by laser flash photolysis of phenylsilane, have been carried out to obtain rate constants for its bimolecular reaction with O2. 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 297–600 K. The second order rate constants at 10 Torr were fitted to the Arrhenius equation:log(k/cm3 molecule–1 s–1)=(–11.08 ± 0.04)+(1.57 ± 0.32 kJ mol–1)/RT ln10The decrease in rate constant values with increasing temperature, although systematic is very small. The rate constants showed slight increases in value with pressure at each temperature, but this was scarcely beyond experimental uncertainty. From estimates of Lennard-Jones collision rates, this reaction is occurring at ca. 1 in 20 collisions, almost independent of pressure and temperature. Ab initio calculations at the G3 level backed further by multi-configurational (MC) SCF calculations, augmented by second order perturbation theory (MRMP2), support a mechanism in which the initial adduct, H2SiOO, formed in the triplet state (T), undergoes intersystem crossing to the more stable singlet state (S) prior to further low energy isomerisation processes leading, via a sequence of steps, ultimately to dissociation products of which the lowest energy pair are H2O + SiO. The decomposition of the intermediate cyclo-siladioxirane, via O–O bond fission, plays an important role in the overall process. The bottleneck for the overall process appears to be the T ? S process in H2SiOO. This process has a small spin–orbit coupling matrix element, consistent with an estimate of its rate constant of 1 × 109 s–1 obtained with the aid of RRKM theory. This interpretation preserves the idea that, as in its reactions in general, SiH2 initially reacts at the encounter rate with O2. The low values for the secondary reaction barriers on the potential energy surface account for the lack of an observed pressure dependence. Some comparisons are drawn with the reactions of CH2+ O2 and SiCl2+ O2.

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Published date: 2005
Keywords: absolute rate constants, molecular-orbital methods, potential-energy surface, temperature-dependence, addition-reaction, basis sets, ab-initio, prototype, atoms, si

Identifiers

Local EPrints ID: 20715
URI: http://eprints.soton.ac.uk/id/eprint/20715
ISSN: 1463-9076
PURE UUID: d08f9aad-76db-4911-b437-697acbab0a85

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Date deposited: 01 Mar 2006
Last modified: 15 Jul 2019 19:25

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Contributors

Author: Rosa Becerra
Author: Sarah-Jane Bowes
Author: J. Steven Ogden
Author: J. Pat Cannady
Author: Ivana Adamovic
Author: Mark S. Gordon
Author: Matthew J. Almond
Author: Robin Walsh

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