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

46 Hydrogen Donor Abilities of Silicon Hydrides
level of theory as high as QCISD/DZP//MP2/DZP [39]. Transition states in
these reactions were found to prefer a collinear arrangement of groups at the
hydrogen atom undergoing transfer. These barriers are calculated to decrease,
on moving from methyl to primary, secondary and tertiary alkyl radicals, in
agreement with the available experimental observations, but their absolute
values are overestimated.
A nonparametric method that is related to the original London equation has
been used to calculate energies of activation for hydrogen abstraction, which
agrees with experiment generally to within 4 kJ/mol [40,41]. Indeed, the activation
energies calculated for the hydrogen abstraction from Me3SiH and
(Me3Si) 3SiH by CH3CH2: and from Me3SiH by CH3O: are 36.8, 22.6 and
10.5 kJ/mol, respectively, and compare very nicely with the experimental values
of 33.5, 18.8 and 8.9 kJ/mol reported in this chapter [40]. In addition to the
reaction enthalpy, this method demonstrates the importance of triplet repulsion
between the terminal atoms of the reacting three-electron system.
An empirical algorithm that relates the activation energy to four molecules
involved in the reaction (an extended form of Evans–Polanyi equation) predicts
activation energies with a good accuracy [42]. The Ea for the reactions of
t-BuO: and Cl3C: radicals with Et3SiH are calculated to be 12.6 and 33.9 kJ/
mol, respectively, that match well with the values of 8.8 and 33.9 kJ/mol
reported in this chapter. Activation energies for the reaction of a large variety
of silanes with different type of radicals have been obtained by applying a semiempirical
method using intersecting parabolas [22,43].
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