Orbital Energy-Based Reaction Analysis of S N 2 Reactions

摘要

An orbital energy-based reaction analysis theory is presented as an extension of the orbital-based conceptual density functional theory. In the orbital energy-based theory, the orbitals contributing to reactions are interpreted to be valence orbitals giving the largest orbital energy variation from reactants to products. Reactions are taken to be electron transfer-driven when they provide small variations for the gaps between the contributing occupied and unoccupied orbital energies on the intrinsic reaction coordinates in the initial processes. The orbital energy-based theory is then applied to the calculations of several S N 2 reactions. Using a reaction path search method, the Cl ? + CH 3 I → ClCH 3 + I ? reaction, for which another reaction path called “roundabout path” is proposed, is found to have a precursor process similar to the roundabout path just before this S N 2 reaction process. The orbital energy-based theory indicates that this precursor process is obviously driven by structural change, while the successor S N 2 reaction proceeds through electron transfer between the contributing orbitals. Comparing the calculated results of the S N 2 reactions in gas phase and in aqueous solution shows that the contributing orbitals significantly depend on solvent effects and these orbitals can be correctly determined by this theory.