Gas-phase SN2 reactions at silicon and carbon centers. An experimental appraisal of theory

Abstract

The gas-phase kinetics of the reactions of H-, D-, OH-, .C-, C2-, C4-, and C4H- with SiH4 and the reactions of D-, OH-, C-, C2-, and C2H- with CH4 have been investigated at 297 f 2 K using the flowing afterglow technique. The reactions with silane are characterized by a variety of channels and reaction efficiencies whereas methane was observed to be relatively unreactive. The rate constants measured for the reactions of H- and D- with SiH4 and D with CH4 are in accord with the energies of the reaction intermediates SiH5- and CHS- predicted by ab initio quantum mechanical calculations but do not corroborate the prediction of a reaction barrier for the attack of H- on silane. This latter reaction, for which the sN2 channel is observed to compete with proton transfer, proceeds rapidly at 297 K. The intrinsic energy surfaces of bimolecular nucleophilic substitution (SN2) reactions, which have been studied ex- tensively in a variety of solvents, have recently become the subject of a number of ab initio and semiempirical molecu- lar orbital calculations.2-'0 Although calculations have been reported which also explore the influence of solvent on the intrinsic energetics of such reactions,I1 computational dif- ficulties have generally prevented the incorporation of sol- vent effects into such calculations so that their direct com- parison with measurements made in solution have been of limited value. Fortunately, gas-phase techniques have now become available which allow the direct experimental inves- tigation of sN2 reactions proceeding in the absence of sol- vent molecules, viz., in vac~o.~~-l~ It has, therefore, become possible to conduct measurements on several specific sN2 reactions which have been scrutinized through molecular orbital calculations, and, consequently, to compare directly theoretical predictions with experiment. So far the experi- mental measurements have been restricted to specific rates of reaction at room temperature. The extent of the compari- sons with theory has therefore been limited but the compar- isons have nevertheless proven to be valuable. For example, quantum mechanical studies by Dedieu and Veillards and by Bader, Duke, and Messer6 had predicted energy barriers of 3.8 and 22 kcal mol-', respectively, for the sN2 reactions