Silanes and germanes as free-radical reducing agents: an ab initio study of hydrogen atom transfer from some trialkylsilanes and germanes to alkyl radicals

Abstract

Ab initio molecular orbital calculations using a (valence) double-ζ pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation predict that hydrogen atoms, methyl, ethyl, isopropyl and tert-butyl radicals abstract hydrogen atom from silane, methylsilane, dimethylsilane, trimethylsilane, trisilylsilane and the analogous germanes via transition states in which the attacking and leaving radicals adopt colinear (or nearly so) arrangements. Except for reactions involving trisilylsilane which are predicted at the MP2/DZP level to involve transition states with Si–C distances of about 3.19 Å, transition states which have (overall) Si–C and Ge–C separations of 3.12–3.15 and 3.24–3.26 Å respectively are calculated; these distances appear to be independent of the number of methyl substituents on the group(IV) element, but appear to be slightly sensitive to the nature of the attacking radical, with marginally earlier transition states calculated as the degree of alkyl substitution on the attacking radical is increased. At the highest level of theory (QCISD/DZP//MP2/DZP), energy barriers (ΔE₁^‡) of 27–57 (Si) or 26–44 (Ge) kJ mol^–1 are predicted for the forward reactions, while the reverse reactions (ΔE₂^‡) are calculated to require 85–134 (Si) or 102–138 (Ge) kJ mol^–1. These values are marginally affected by the inclusion of zero-point vibrational energy correction. Importantly, QCISD and MP2 calculations appear to predict correctly the relative ordering of activation energies for alkyl radical reduction by silanes: tertiary < secondary < primary; SCF/DZP, AM1 and AM1 (CI = 2) calculations perform somewhat more poorly in their prediction of relative radical reactivity.