"Front Matter". In: Organosilanes in Radical Chemistry - Index of

Bond Dissociation Enthalpies 23
2.2.3 THEORETICAL DATA
The continuous development and implementation of molecular orbital theory ab
initio methods have enlarged the applications to this area too. Indeed, the impact
of theoretical calculations in thermochemistry is substantial. Experimental
groups often use calculations as a supplement to the interpretation of their
results. In this section we will mention a few recent and representative studies
that are directly associated with the bond dissociation energies of silanes. Early
theoretical investigations of the Si w H bond strength in silanes have been summarized
[13].
The most reliable calculations so far are relative bond dissociation energy
studies by means of the isodesmic Reaction (2.8). DH2:8 can be reliably calculated
even at modest levels of theory because errors arising from deficient basis sets and
incomplete corrections for electron correlation largely are canceled. The DH2:8 for
R ¼ Me and n ¼ 0 is calculated as 13.5 kJ/mol at the MP3/6–31G * level in excellent
agreement with the experimental finding [14]. MP4SDTQ/6–31G * level of
theory was used to study the effects of substituents on DH(XSiH2 w H) by means
of the isogyric reaction shown in Reaction (2.9) [15]. The results indicated that
electropositive substituents with low-lying empty orbitals (Li, BeH, and BH2)
decrease the Si w H bond strengths by 30–50 kJ/mol. A 12 kJ/mol decrease in bond
strength from H3Si w HtoH3SiSiH2 w H and a difference of 34 kJ/mol between
H3Si w Hand(H3Si) 3Si w H were also computed.
R3 nSiH1þn þ H3Si: !R3 nSiHn: þ H3SiH (2:8)
XH2Si: þ H4Si !XSiH3 þ H3Si: (2:9)
Phenyl substitution was calculated to decrease the Si w H bond strength by
6 kJ/mol using the isogyric Reaction (2.8) for R ¼ Ph and n ¼ 2 at the HF/STO-
3G * and MP2/STO-3G * levels [16]. These calculations further indicated that the
presence of a second phenyl group, i.e. Reaction (2.8) with R ¼ Ph and n ¼ 1,
has no additional effect. UB3LYP/6–31G and ROMP2/6–311þþG(d,2p)
methods were used to calculate the Si w H bond strengths of 15 para-substituted
silanes p-Z w C6H4SiH2 w H and no significant substituent effect was found in
DH(Si w H), while DH(Si w X) in the same series for X ¼ Cl, F, Li showed such
effects [17].
Calculations on the enthalpy change for Reaction (2.1) were also reported.
DFT methods substantially underestimate the absolute bond dissociation energies,
whereas the relative ones are reliable enough and allow the rationalization
of substituent effects. Indeed, the substituent effect on the Si w H bond strength
was addressed at the BLYP/6–31G * level of theory and indicated that successive
Me substitutions strengthen the bonds, while successive SMe and SiH3 substitutions
weaken the bond in excellent accord with experimental data [18]. Good
absolute DH(XSiH2 w H) were obtained for relatively small molecules, using the
G3(MP2) method for calculating the enthalpy change of Reaction (2.1) [17]. The