Abstract
A reatividade em fase gasosa das alcoxissilanas do tipo Me 4-n Si(OEt) n (n = 1-3) foi investigada por ressonância ciclotronica de ions por transformada de Fourier (FT-ICR) com o intuito de se caracterizar os mecanismos de ataque nucleofilico nesses importantes precursores de novos materiais. Nucleofilos como F - , MeO - e EtO - reagem rapidamente e de forma preferencial atacando o atomo de Si, formando a especie pentacoordenada de Si. Esta em seguida sofre processos de eliminacao iniciados, ou por um ion metidio nascente Me - , ou um ion etoxido EtO - , que por sua vez, podem abstrair um proton gerando carbânions ou siloxidos. Carbânions do tipo X(Me)3-nSi(OEt)nCH2 - (X = F, MeO, EtO e n = 1-3) e siloxidos do tipo X(Me)4-nSi(OEt)n-1O - (X = F, MeO, EtO e n = 2-3) podem ser facilmente dissociados por excitacao multifotonica no infravermelho induzida por um laser de CO 2 gerando uma grande variedade de siloxidos simples e silicatos analogos a especies intermediarias encontradas nas primeiras etapas de processos sol-gel. Para as alcoxissilanas que contem mais grupos etoxido, uma reacao competitiva analoga a uma eliminacao E2 e observada, onde o nucleofilo abstrai um hidrogenio β de um grupo etoxila, eliminando etileno. A cinetica, distribuicao relativa de produtos e termoquimica dessas reacoes tambem sao apresentadas para alguns casos especificos. The gas-phase reactivity of the Me 4-n Si(OEt) n (n = 1-3) alkoxysilanes was investigated by Fourier transform ion cyclotron resonance (FT-ICR) technique in order to characterize the fundamental mechanisms associated with nucleophilic attack on these important precursors of new materials. Typical nucleophiles such as F - , MeO - and EtO - react readily and preferentially by attack at the silicon center with formation of a pentacoordinated siliconate that undergoes elimination processes initiated by either a nascent methide ion, Me, or an ethoxide ion, EtO, that can abstract a proton to yield either a carbanion or a siloxide-type anion. Carbanions of the type X(Me) 3-n Si(OEt) n CH 2 - (X = F, MeO, EtO and n = 1-3) and siloxide ion of the type X(Me)4-nSi(OEt)n-1O - (X = F, MeO, EtO and n = 2-3) can be easily dissociated by infrared multiphoton excitation with a CO2 laser to give rise to a large variety of simple siloxide and silicate-type anions that are reminiscent of intermediate species in the early stages of sol-gel processes. For the higher-ethoxy containing substrates, a competing reaction is observed that is analogous to an E2 elimination reaction in which the nucleophile abstracts a β-proton from the ethoxy group leading to elimination of ethylene. The kinetics, relative product distribution and thermochemistry of these reactions are also reported for some specific cases.
Highlights
The chemistry of alkoxysilanes has attracted considerable attention in the last 30 years because these substrates are: (i) notorious precursors of a wide varietyA first-principle approach toward mapping out the fundamental reactivity of alkoxysilanes has relied on experiments within the realm of gas-phase ion chemistry under low-pressure conditions
Our studies extended to reactions with O- because of the potential interest in the outcome of these reactions in plasma enhanced vapor deposition (PECVD) processes
The gas-phase reaction of simple nucleophiles with ethoxymethylsilanes has been shown to proceed primarily by an addition-elimination mechanism in which the nucleophile initially becomes attached to the silicon center
Summary
The chemistry of alkoxysilanes has attracted considerable attention in the last 30 years because these substrates are: (i) notorious precursors of a wide varietyA first-principle approach toward mapping out the fundamental reactivity of alkoxysilanes has relied on experiments within the realm of gas-phase ion chemistry under low-pressure conditions. EtO–/MeSi(OEt)[3] system EtO- was observed to react by two different channels as shown in reaction 5 along with the product distribution. This proton shift must be the lowest dissociation channel for these ions because no other competing IRMPD processes are observed; (ii) The sequential dissociation results in the progressive hydrolysis of the ethoxy groups attached to the silicon center and generate anions that are common species in the successive condensation reactions of sol-gel type processes.
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