The Interplay between Steric and Electronic Effects in SN2 Reactions

Quantum chemical calculations for S(N)2 reactions of H3EX/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 coordinate from reactants to products provides valuable insight into the essential changes that occur in the bond-breaking/bond-forming process during S(N)2 reactions. The EDA results for the totypical S(N)2 reaction of the systems [X center dot center dot center dot R3E center dot center dot center dot X](-), in which the interacting fragments are [X center dot center dot center dot X](2-) and [R3E](+), have given rise to a new interprelation 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 R-3 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-3/l, 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.