Heterolytic Bond Dissociation in Water: Why Is It So Easy for C4H9Cl But Not for C3H9SiCl?

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• 作者：Peifeng Su ; Lingchun Song ; Wei Wu ; Sason Shaik ; Philippe C. Hiberty
• 刊名：Journal of Physical Chemistry A
• 出版年：2008
• 出版时间：April 3, 2008
• 年：2008
• 卷：112
• 期：13
• 页码：2988 - 2997
• 全文大小：129K
• 年卷期：v.112,no.13(April 3, 2008)
• ISSN：1520-5215

文摘

The recently developed (Song, L.; Wu, W.; Zhang, Q.; Shaik, S. J. Phys. Chem. A 2004, 108, 6017-6024)valence bond method coupled to a polarized continuum model (VBPCM) is used to address the long standingconundrum of the heterolytic dissociation of the C-Cl and Si-Cl bonds, respectively, in tertiary-butyl chlorideand trimethylsilyl chloride in condensed phases. The method is used here to compare the bond dissociationin the gas phase and in aqueous solution. In addition to the ground state reaction profile, VB theory alsoprovides the energies of the purely covalent and purely ionic VB structures as a function of the reactioncoordinate. Accordingly, the C-Cl and Si-Cl bonds are shown to be of different natures. In the gas phase,the resonance energy arising from covalent-ionic mixing at equilibrium geometry amounts to 42 kcal/mol fortertiary-butyl chloride, whereas the same quantity for trimethylsilyl chloride is significantly higher at 62kcal/mol. With such a high value, the root cause of the Si-Cl bonding is the covalent-ionic resonance energy,and this bond belongs to the category of charge-shift bonds (Shaik, S.; Danovich, D.; Silvi, B.; Lauvergnat,D.; Hiberty, P. C. Chem.- Eur. J. 2005, 11, 6358). This difference between the C-Cl and Si-Cl bondscarries over to the solvated phase and impacts the heterolytic cleavages of the two bonds. For both molecules,solvation lowers the ionic curve below the covalent one, and hence the bond dissociation in the solventgenerates the two ions, Me₃E⁺ Cl^- (E = C, Si). In both cases, the root cause of the barrier is the loss of thecovalent-ionic resonance energy. In the heterolysis reaction of Si-Cl, the covalent-ionic resonance energyremains large and fully contributes to the dissociation energy, thereby leading to a high barrier for heterolyticcleavage, and thus prohibiting the generation of ions. By contrast, the covalent-ionic resonance energy issmaller for the C-Cl bond and only partially contributes to the barrier for heterolysis, which is consequentlysmall, leading readily to ions that are commonly observed in the classical S_N1 mechanism. Thus, the reluctanceof R₃Si-X molecules to undergo heterolysis in condensed phases and more generally the rarity of freesilicenium ions under these conditions are experimental manifestations of the charge-shift character of theSi-Cl bond.