Solvolysis reactions of cation +SiCl3: Experiment and Theory / Reações de solvólise em fase gasosa do cátion +SiCl3: experimento e teoria

AUTOR(ES)
DATA DE PUBLICAÇÃO

2010

RESUMO

Polyhalogenated silicenium ions, X3Si+ (X = F, Cl), are common fragment ions in the mass spectra of polyhalogenated silanes obtained by electron ionization. These ions are powerful electrophiles and are believed to play a role in plasma enhanced corrosion processes and plasma enhanced chemical vapour deposition processes. In this dissertation, we present some new results on the gas-phase reactivity of the +SiCl3 ion with a number of simple n electron donor bases such as water, alcohols, ammonia, amines and some π electron donor bases. Ion-molecule reactions were characterized experimentally by Fourier transform ion cyclotron resonance mass spectrometry (FTICR) at pressures in the 10-8 Torr range. Reactions were followed as a function of trapping time of the ions in the cell of the spectrometer and this allowed for the identification of subsequent reactions of the primary product ions. The energy diagram and structure of the different silicenium ions were also characterized by computational chemistry using both ab initio and density functional theory methods in order to understand the mechanism of these reactions. +SiCl3 reacts rapidly in gas phase with various neutral substrates through processes similar to solvolysis in which the neutral substrate adds onto the silicenium ion followed by elimination of HCl. In some cases, complete solvolysis is observed with substitution of all three chlorine atoms. The calculations show that reactions proceed by initial addition of the electrophile onto the electron center of the neutral substrates giving rise to stable adducts. The transition state for these reactions involve a 1,3 hydrogen migration and the calculated energy for these transition states is less than the energy of the isolated reactants, a fact that is common to fast gas-phase ion-molecule reactions. Similar calculations for +CCl3 reveal similar solvolysis reactions to be energetically unfavourable, and in fact these reactions are not observed experimentally. Secondary reactions have been observed for the product ions ClnSi(OH)3-n+ (n = 1, 2 and 3), obtained from successive hydrolysis of +SiCl3, with the parent SiCl4 neutral. These secondary condensation reactions yield ionic species containing a siloxane type linkage (-Si-O-Si-). The theoretical calculations suggest that is that these secondary condensation reactions initially proceed via chloronium ion intermediate, R1-Cl+-R2 , followed by formal Cl transfer and rearrangement to a siloxane type structure.

ASSUNTO(S)

solvolysis solvólise gas-phase ressonância ciclotrônica de íons por transformada de fourier computational chemistry fase gasosa ion/molecule reactions silicenium cations cátions silicênios fourier transform ion cyclotron resonance mass spectrometry química computacional reações íon/molécula

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