Silicon cation and anion chemistry in a fuel-rich, methane–oxygen flame

Abstract

. Can. J. Chem. 70, 1069 (1 992). Silicon cations and anions in a fuel-rich, premixed, methane-oxygen flame at atmospheric pressure doped with 0.01 mol% of trimethylsilane were observed by sampling the flame through a nozzle into a mass spectrometer. Twelve cations were observed which can be grouped into five series: SiOH'.nH20 (n = 0-2); SiOCH3+.nH20 (n = 0-2); Si(OH)3'.nH20 (n = 0-2); cations by nucleophilic substitution (e.g., Si(OH)(CH3)2(H20)+); and carbonaceous aromatic cations (c-HS~CH=CH' and c-HSiCH=CCH,+). Similarly, five anions were observed as members of two series: H,Si03-(x = 0, I) and H,SiO,-(x = 1-3). The chemical ionization reactions for the formation of these ions are discussed in detail, including proton transfer and also methyl cation transfer, three-body addition, nucleophilic substitution (SN2) of both the ions themselves and also their neutral silicon precursors, and H-atom abstraction reactions. The neutral silicon chemistry in the flame is dominated by SiO, but evidence was obtained from both the cation and the anion chemistry for the presence of HSiO(OH), silanoic acid; SiO(OH)2, metasilicic acid; and Si(OH),, orthosilicic acid. The silicon ion chemistry differs markedly from the normal carbon ion chemistry that occurs naturally in the undoped methane-oxygen flame; the silicon ions show a strong tendency towards Si-0 bond formation. Consideration is given to the probable structures of the various silicon cations and anions observed. On a observe des anions et des cations silicies dans la flamme de primklange d'un melange de methane-oxygkne riche en methane la pression atmosphkrique et dope avec 0,1% de trimethylsilane, en injectant, a I'aide d'une buse, des Cchantillons de flamme dans un spectromktre de masse. On a observe 12 cations que l'on peut regrouper en cinq categories : SiOH' .n H 2 0 (n = 0-2); SiOCH,+.n H 2 0 ( n = 0-2); Si(OH),+ .n H 2 0 (n = 0-2); des cations obtenus par substitution nucleophile (par exemple : Si(OH)(CH3)2(HIO)'); et des cations carbones aromatiques (c-HS~CH=CH+ et c-HS~CH=CCH,'). D'une f a~o n analogue, on a observe cinq anions appartenant a deux series : H,Si03-(x = 0, 1); et H,SiO, (x = 1-3). On discute en dktail des rkactions d'ionisation chimique conduisant a ces ions. Ces riactions comprennent : le transfert de proton et de cation methyle, l'addition a trois espkces, la substitution nuclkophile (SN2) des deux anions eux-mCmes et de leur precurseur silicie et les reactions d'abstraction d'hydrogene. La chimie du silicium neutre, dans la flamme, est dominee par le SiO, mais on a obtenu des preuves de la presence de l'acide silanoique, HSiO(OH), de l'acide mktasilicique, SiO(OH),, et de l'acide orthosilicique, Si(OH),, a partir de la chimie des cations et des anions. La chimie du silicium differe nettement de la chimie d l'ion carbone normal qui se forme naturellement dans la flamme du melange methane-oxygene non dope; les ions silicium exhibent une forte tendance vers la formation de liaison Si-0. On formule des hypotheses sur les structures probables de...

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Thermochemistry of the reaction of silicon Si+(2P) with methanol: a G2 molecular orbital study

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