Free radical homolytic substitution by the frontside mechanism: Ab initio study of homolytic substitution reactions at silicon, germanium, and tin

Ab initio calculations using the 6-311G**, cc-pVDZ, aug-cc-pVDZ, and (valen ce) double-zeta pseudopotential (DZP) basis sets, with (MP2, QCISD, CCSD(T) ) and without (UHF) the inclusion of electron correlation, predict that deg enerate homolytic substitution by silyl radical at the silicon atom in disi lane can proceed by mechanisms which involve both backside and frontside at tack at silicon. At the highest level of theory (CCSD(T)/aug-cc-pVDZ//MP2/a ug-cc-pVDZ), energy barriers (Delta E-not equal) of 52.7 and 58.2 kJ mol(-1 ) are calculated for the backside and frontside reactions, respectively. Si milar results are obtained at the CCSD(T)/DZP// MP2/DZP level of theory for reactions involving germanium and tin with values of Delta E-not equal of 65.2 kJ mol(-1) (backside) and 76.7 kJ mo(-1) (frontside) for reactions of germyl radical with digermane and 58.5 kJ mol(-1) (backside) and 59.1 kJ mo l(-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 sily lgermane, silylstannane, and germylstannane, predict that while homolytic s ubstitution at silicon and germanium is expected to favor the backside mech anism, reactions involving free-radical attack at tin are predicted to be l ess discriminate; indeed, in many cases, the frontside mechanism is calcula ted to be preferred for reactions involving tin. CCSD(T)/DZP//MP2/DZP calcu lated 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. Ex cept for reactions involving attack at the tin atom in methylstannane, we w ere unable to locate transition states for frontside attack at correlated l evels of theory for reactions involving methyl radical. The mechanistic imp lications of these computational data are discussed.