The interplay between steric and electronic effects in S_N2 reactions

Quantum chemical calculations for S_N2 reactions of H ₃EX/X^- systems, in which E = C or Si and X = F or Cl, are reported. In the case of the carbon system we also report on bulkier species in which the hydrogen atoms are substituted by methyl groups. It is shown how the variation in the individual energy terms of the Morokuma/ Ziegler energy decomposition analysis (EDA) scheme along the reaction coor- . dinate from reactants to products provides valuable insight into the essential changes that occur in the bond-breaking/bond-forming process during S_N2 reactions. The EDA results for the prototypical S_N2 reaction of the systems [X⋯R₃E⋯X]^-, in which the interacting fragments are [X⋯X]^2- and [R₃E]⁺, have given rise to a new interpretation of the factors governing the reaction course. The EDA results for the carbon system (E = C) show that there is less steric repulsion and stronger electrostatic attraction in the transition structure than in the precursor complex and that the energy increase comes mainly from weaker orbital interactions. The larger barriers for systems in which R₃ is bulkier also do not arise from increased steric repulsion, which is actually released in the transition structure. It is rather the weakening of the electrostatic attraction, and in particular the loss of attractive orbital interactions, that are responsible for the activation barrier. The D _3h energy minima of the silicon homologues [XH₃SiX] ^- is driven by the large increase in the electrostatic attractions and also of stronger orbital interactions, while the steric interactions is destabilizing.