Free Radical Homolytic Substitution by the Frontside Mechanism: Ab Initio Study of Homolytic Substitution Reactions at Silicon, Germanium, and Tin

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Ab initio calculations using the 6-311G**, cc-pVDZ, aug-cc-pVDZ, and (valence) double-ζ pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T)) and without (UHF) the inclusion of electron correlation, predict that degenerate homolytic substitution by silyl radical at the silicon atom in disilane can proceed by mechanisms which involve both backside and frontside attack at silicon. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/aug-cc-pVDZ), energy barriers (Δ E ⧧ ) of 52.7 and 58.2 kJ mol - 1 are calculated for the backside and frontside reactions, respectively. Similar results are obtained at the CCSD(T)/DZP//MP2/DZP level of theory for reactions involving germanium and tin with values of Δ E ⧧ of 65.2 kJ mol - 1 (backside) and 76.7 kJ mol - 1 (frontside) for reactions of germyl radical with digermane and 58.5 kJ mol - 1 (backside) and 59.1 kJ mol - 1 (frontside) for reactions of stannyl radical with distannane. CCSD(T)/DZP//MP2/DZP calculations involving the analogous nondegenerate reactions of disilane, digermane, and distannane, as well as reactions involving silylgermane, silylstannane, and germylstannane, predict that while homolytic substitution at silicon and germanium is expected to favor the backside mechanism, reactions involving free-radical attack at tin are predicted to be less discriminate; indeed, in many cases, the frontside mechanism is calculated to be preferred for reactions involving tin. CCSD(T)/DZP//MP2/DZP calculated energy barriers range from 39.4 kJ mol - 1 for the reaction of silyl radical with distannane by the frontside mechanism to 104.5 kJ mol - 1 for the analogous frontside reaction involving stannyl radical and disilane. Except for reactions involving attack at the tin atom in methylstannane, we were unable to locate transition states for frontside attack at correlated levels of theory for reactions involving methyl radical. The mechanistic implications of these computational data are discussed.