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

Theoretical Approaches 45
studies in the gas phase and a number of theoretical investigations. For
attacking species such as H, Cl and Br atoms, the room temperature rate
constant corrected for reaction path degeneracy, k/n, is higher for Me3SiH
than H4Si by a factor of around 4 [36]. However, the interplay of activation
energy and preexponential factor is complex and is not fully understood. The
reaction of H: atoms is reported in Table 3.7 in detail, for a better understanding
of the reactivity trend in this case [36]. Indeed, the methyl substitution leads
to an increase in the Si w H bond reactivity when the statistical number of
abstracted hydrogens is taken into account. Theory reproduced very well the
experimental rate constants at 25 8C (values in parenthesis) [37]. However, the
experimental activation energies and A factors are difficult to rationalize because
the changes are small and may be also masked by experimental errors.
Theoretical calculations suggest that the activation energies for the three reactions
are nearly the same (i.e., unaffected by methylation), whereas the increase
of reactivity along the series reflects a parallel change in the preexponential
factors. The increase in A factor with methylation has been explained in terms
of an increase in the entropy of the transition state associated either with the
introduction of soft skeletal vibrations, or the freeing of the hindered internal
rotation of the methyl groups [38].
3.7 THEORETICAL APPROACHES
A number of theoretical methodologies, spanning from high-level ab initio to
empirical calculations have been applied to obtain information on the factors
that influence the reactivity of Si w H bond towards radicals and atoms.
High-level computational methods are limited, for obvious reasons, to very
simple systems. In the previous section we showed the contribution of the
theory for a better understanding of the entropic and enthalpic factors that
influence the reactions of hydrogen atom with the simplest series of silanes
Me4 nSiHn, where n ¼ 1–3. Calculated energy barriers for the forward and
reverse hydrogen atom abstraction reactions of Me:, Et:, i-Pr: and t-Bu:
radicals with Me4-nSiHn, where n ¼ 0–3, and (H3Si) 3SiH have been obtained at
Table 3.7 The substituent effect on the reactivity of silanes towards H: atoms a (in
parenthesis theoretical data) b
Silane n(Si w H bonds) kn 1 =10 7 M 1 s 1c An 1 =10 10 M 1 s 1 Ea=kJ mol 1
MeSiH3 3 7.8 (7.7) 1.7 (1.5) 13.2 (13.4)
Me2SiH2 2 11.9 (11.4) 2.4 (2.1) 13.2 (13.1)
Me3SiH 1 16.7 (15.4) 2.0 (3.0) 11.7 (13.2)
a
From [36].
b **
Using saddle-point structure at the BHLYP/6-311þG level and single-point ab initio at the PMP4/6-
311þG(3df,2p)//BHLYP level [37].
c
At 25 8C.