SILANES AND GERMANES AS FREE-RADICAL REDUCING AGENTS - AN AB-INITIO STUDY OF HYDROGEN-ATOM TRANSFER FROM SOME TRIALKYLSILANES AND GERMANES TO ALKYL RADICALS

Ab initio molecular orbital calculations using a (valence) double-zeta pseudopotential (DZP) basis set, with (MP2, QCISD) and without (SCF) the inclusion of electron correlation predict that hydrogen atoms, met hyl, ethyl, isopropyl and tert-butyl radicals abstract hydrogen atom f rom silane, methylsilane, dimethylsilane, trimethylsilane, trisilylsil ane and the analogous germanes via transition states in which the atta cking 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 Angstrom, transition states which have (overall) Si-C and G e-C separations of 3.12-3.15 and 3.24-3.26 Angstrom respectively are c alculated; these distances appear to be independent of the number of m ethyl substituents on the group(IV) element, but appear to be slightly sensitive to the nature of the attacking radical, with marginally ear lier 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 (Delta E-1 double dagger) of 27- 57 (Si) or 26-44 (Ge) kJ mol(-1) are predicted for the forward reactio ns, while the reverse reactions (Delta E-2 double dagger) are calculat ed 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 radi cal reduction by silanes: tertiary < secondary < primary; SCF/DZP, AM1 and AM1 (CI = 2) calculations perform somewhat more poorly in their p rediction of relative radical reactivity.