Silicon-hydrogen bond dissociation enthalpies

The negligible effect of methyl groups on silicon-hydrogen bond dissociation enthalpies in methylsilanes is supported by simple correlations which only involve enthalpy of formation data. For example, when AH°(Me SiH, g) are plotted against AH°(Me SiH, g) (n = 0-3), the excelient linear fitting. Equation 10, implies " that the enthalpy of Reaction 11 is constant, which in turn... 

The thermochemical correlations, the bond terms, and the unshielded core potential method all support a negligible effect of methyl groups on silicon-hydrogen bond dissociation enthalpies. This constancy is also in keeping with the hydridic nature of Si-H bonds in silanes (23). The effect of other substituents is, by comparison, poorly defined (19), although some results have recently become available. For example, it has been shown that silylation destabilizes Si-H bonds D(H SiSiH -H) = 361 8 kJ/mol (19),... 

If, in silane, one hydrogen is replaced by a phenyl group, the silicon-hydrogen bond dissociation enthalpy is only reduced by about 9 7 kJ/mol (19). This is in sharp contrast with the decrease observed for the carbon analogues, where the large stabilization energy of benzyl radical makes D(Me-H)-D(PhCH -H) = 71 4 kJ/mol. The effect of a second phenyl group in the hydrocarbon family is also known, D(Ph CH-H) = 340 8 kJ/mol (27,28) and there is a recent estimate for D(Ph C-H), 338 13 kJ/mol (28). 

The calculation of silicon-hydrogen bond dissociation enthalpies follows the determination of AH and relies on literature values for D(t-BuO-OBu-t) and D(t-BuO-H) (22.), Equation 17. 

The constancy of silicon-hydrogen bond dissociation enthalpies in methyl substituted silanes is supported by a thermochemical analysis which does not rely on the enthalpies of formation of the corresponding silyl radicals. 

The replacement of hydrogen or methyl groups by phenyl groups induces a considerable decrease in D(Si-H). The use of Ph SiH as a reducing agent for alkyl bromides was suggested by the low silicon-hydrogen bond dissociation enthalpy in this molecule. 

Table V. Silicon-Hydrogen and Carbon-Hydrogen Bond Dissociation Enthalpies in Phenylsilanes and Phenylmethanes Data in kJ/mol...

Table I includes the relative bond dissociation enthalpies obtained for some group 14 hydrides by photoacoustic calorimetry,7 10 The data demonstrate that, for the trialkyl-substituted series, the bond strengths decrease by 6.5 and 16.5 kcal/mol on going from silane to germane and to stannane, respectively. The silicon-hydrogen bonds can be dramatically weakened by successive substitution of the Me3Si group at the Si-H functionality. A substantial decrease in the bond strength is also observed by replacing alkyl with methylthio groups.

A recent analysis of thermochemical data based on relationships between enthalpies of formation and the unshielded core potential of a moiety X, V ( ), throws some light into the behavior of silicon-hydrogen bonds in methyls lanes. Consider first the case of carbon-hydrogen bonds. The enthalpy of Reaction 12, which is shown to be a linear function of V iZ ) can be expressed in terms of bond dissociation enthalpies. Equation 13. 

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