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Experimental and theoretical evidence for homogeneous catalysis in the gas-phase reaction of SiH2 with H2O (and D2O): a combined kinetic and quantum chemical study

Experimental and theoretical evidence for homogeneous catalysis in the gas-phase reaction of SiH2 with H2O (and D2O): a combined kinetic and quantum chemical study
Experimental and theoretical evidence for homogeneous catalysis in the gas-phase reaction of SiH2 with H2O (and D2O): a combined kinetic and quantum chemical study
Time-resolved kinetic studies of the reaction of silylene, SiH2, with H2O and with D2O have been carried out in the gas phase at 297 K and at 345 K, using laser flash photolysis to generate and monitor SiH2. The reaction was studied independently as a function of H2O (or D2O) and SF6 (bath gas) pressures. At a fixed pressure of SF6 (5 Torr), [SiH2] decay constants, k(obs), showed a quadratic dependence on [H2O] or [D2O]. At a fixed pressure of H2O or D2O, k(obs) Values were strongly dependent on [SF6]. The combined rate expression is consistent with a mechanism involving the reversible formation of a vibrationally excited zwitterionic donor-acceptor complex, H2Si...OH2 (or H2Si...OD2). This complex can then either be stabilized by SF6 or it reacts with a further molecule of H2O (or D2O) in the rate-determining step. Isotope effects are in the range 1.0-1.5 and are broadly consistent with this mechanism. The mechanism is further supported by RRKM theory, which shows the association reaction to be close to its third-order region of pressure (SF6) dependence. Ab initio quantum calculations, carried out at the G3 level, support the existence of a hydrated zwitterion H2Si...(OH2)(2), which can rearrange to hydrated silanol, with an energy barrier below the reaction energy threshold. This is the first example of a gas-phase-catalyzed silylene reaction.
silylene addition-reaction, potential-energy surface, absolute rate constants, temperature-dependence, ab-initio, silicon-hydrogen, oxygen-hydrogen, prototype, bonds, dimethylsilylene
0002-7863
6816-6824
Becerra, Rosa
766758a2-9c71-4cab-9523-2874dfd5f84a
Goldberg, Nicola
6bd11b0f-0f42-4484-a427-8d7bd5b11644
Cannady, J. Pat
1b7d2bf5-1694-4d9f-8bc5-aab0783e9f76
Almond, Matthew J.
992a3a7b-3cbe-4818-828f-e0e6b876207f
Ogden, J. Steven
5a99fd42-75e7-4f7b-a347-ac2b1717b8b3
Walsh, Robin
aacc9be2-5b7b-401b-9305-bb6cca0822a0
Becerra, Rosa
766758a2-9c71-4cab-9523-2874dfd5f84a
Goldberg, Nicola
6bd11b0f-0f42-4484-a427-8d7bd5b11644
Cannady, J. Pat
1b7d2bf5-1694-4d9f-8bc5-aab0783e9f76
Almond, Matthew J.
992a3a7b-3cbe-4818-828f-e0e6b876207f
Ogden, J. Steven
5a99fd42-75e7-4f7b-a347-ac2b1717b8b3
Walsh, Robin
aacc9be2-5b7b-401b-9305-bb6cca0822a0

Becerra, Rosa, Goldberg, Nicola, Cannady, J. Pat, Almond, Matthew J., Ogden, J. Steven and Walsh, Robin (2004) Experimental and theoretical evidence for homogeneous catalysis in the gas-phase reaction of SiH2 with H2O (and D2O): a combined kinetic and quantum chemical study. Journal of the American Chemical Society, 126 (21), 6816-6824. (doi:10.1021/ja049373g).

Record type: Article

Abstract

Time-resolved kinetic studies of the reaction of silylene, SiH2, with H2O and with D2O have been carried out in the gas phase at 297 K and at 345 K, using laser flash photolysis to generate and monitor SiH2. The reaction was studied independently as a function of H2O (or D2O) and SF6 (bath gas) pressures. At a fixed pressure of SF6 (5 Torr), [SiH2] decay constants, k(obs), showed a quadratic dependence on [H2O] or [D2O]. At a fixed pressure of H2O or D2O, k(obs) Values were strongly dependent on [SF6]. The combined rate expression is consistent with a mechanism involving the reversible formation of a vibrationally excited zwitterionic donor-acceptor complex, H2Si...OH2 (or H2Si...OD2). This complex can then either be stabilized by SF6 or it reacts with a further molecule of H2O (or D2O) in the rate-determining step. Isotope effects are in the range 1.0-1.5 and are broadly consistent with this mechanism. The mechanism is further supported by RRKM theory, which shows the association reaction to be close to its third-order region of pressure (SF6) dependence. Ab initio quantum calculations, carried out at the G3 level, support the existence of a hydrated zwitterion H2Si...(OH2)(2), which can rearrange to hydrated silanol, with an energy barrier below the reaction energy threshold. This is the first example of a gas-phase-catalyzed silylene reaction.

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More information

Published date: 2 June 2004
Keywords: silylene addition-reaction, potential-energy surface, absolute rate constants, temperature-dependence, ab-initio, silicon-hydrogen, oxygen-hydrogen, prototype, bonds, dimethylsilylene

Identifiers

Local EPrints ID: 20128
URI: http://eprints.soton.ac.uk/id/eprint/20128
ISSN: 0002-7863
PURE UUID: 5151f47a-77d1-432b-89c3-f4201e7dad43

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Date deposited: 23 Feb 2006
Last modified: 15 Jul 2019 19:26

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