posted on 2008-04-03, 00:00authored byPeifeng Su, Lingchun Song, Wei Wu, Sason Shaik, Philippe C. Hiberty
The recently developed (Song, L.; Wu, W.; Zhang, Q.; Shaik, S. J. Phys. Chem. A2004, 108, 6017−6024)
valence bond method coupled to a polarized continuum model (VBPCM) is used to address the long standing
conundrum of the heterolytic dissociation of the C−Cl and Si−Cl bonds, respectively, in tertiary-butyl chloride
and trimethylsilyl chloride in condensed phases. The method is used here to compare the bond dissociation
in the gas phase and in aqueous solution. In addition to the ground state reaction profile, VB theory also
provides the energies of the purely covalent and purely ionic VB structures as a function of the reaction
coordinate. 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 for
tertiary-butyl chloride, whereas the same quantity for trimethylsilyl chloride is significantly higher at 62
kcal/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 bonds
carries 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 solvent
generates the two ions, Me3E+ Cl- (E = C, Si). In both cases, the root cause of the barrier is the loss of the
covalent-ionic resonance energy. In the heterolysis reaction of Si−Cl, the covalent-ionic resonance energy
remains large and fully contributes to the dissociation energy, thereby leading to a high barrier for heterolytic
cleavage, and thus prohibiting the generation of ions. By contrast, the covalent-ionic resonance energy is
smaller for the C−Cl bond and only partially contributes to the barrier for heterolysis, which is consequently
small, leading readily to ions that are commonly observed in the classical SN1 mechanism. Thus, the reluctance
of R3Si−X molecules to undergo heterolysis in condensed phases and more generally the rarity of free
silicenium ions under these conditions are experimental manifestations of the charge-shift character of the
Si−Cl bond.