Nucleophilic Substitution at C, Si and P: How Solvation Affects the Shape of Reaction Profiles

We have studied how solvation affects the shape of potential energy surfaces (PES) of archetypal nucleophilic substitution reactions at carbon (SN2@C), silicon (SN2@Si) and phosphorus (SN2@P), using the generalized gradient approximation (GGA) of density functional theory (DFT) at OLYP/TZ2P. Our model systems cover nucleophilic substitution, in water and in the gas phase, at carbon in X– + CH3Y (SN2@C), at silicon in X– + SiH3Y (SN2@Si), at tricoordinate phosphorus in X– + PR2Y (SN2@P3), and at tetracoordinate phosphorus in X– + POR2Y (SN2@P4) with substituents R = H, F, Cl, CH3, OCH3. In the gas phase, particular types of SN2 reactions are characterized by different shapes of reaction profiles, such assingle‐, double‐ and triple‐well PESs. The main effect of solvation is to turn the PESs of the SN2@C but also of SN2@Si and SN2@P into unimodal reaction profiles which lead from the reactants via one single barrier to the products. The results are discussed in terms of differential solvation of reactants and transition states. We also address the question how the relative heights of reaction barriers are affected by solvation. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)