Abstract
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 N2 reactions. Using a reaction path search method, the Cl− + CH3I → ClCH3 + 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 SN2 reaction process. The orbital energy-based theory indicates that this precursor process is obviously driven by structural change, while the successor SN2 reaction proceeds through electron transfer between the contributing orbitals. Comparing the calculated results of the SN2 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.
Highlights
Reaction analyses based on molecular orbitals take on increasing importance
We applied the orbital energy-based reaction analysis theory to the Cl− + CH3 I → CH3 Cl + I− reaction, which involves the substitution of ions, to determine if this reaction is included in these exceptional SN 2 reactions
We have formed the foundation of the orbital energy-based reaction analysis theory [4] as the extension of the orbital-based reaction analysis theories. Based on this orbital energy-based theory, we have explored the SN 2 reactions, for which the minimum energy path is questioned, and the difference of the SN 2 reactions in gas-phase and in aqueous solution
Summary
Reaction analyses based on molecular orbitals take on increasing importance. This is because the experimental imaging of molecular orbitals has recently been made possible [1], and the recent progress in attosecond science enables us to perform the time-resolved analysis of electron dynamics. Electronic state variations from the initial to terminal states of reactions are explicitly taken into consideration in these theories, while it is usually neglected in the mainstream reaction analyses based on reaction energy diagrams using potential energy surfaces Since these theories reveal the highly reactive sites of molecules, they have been used as a guiding principle of reaction designs and molecular syntheses. Anti-activation-energy reactions, which have higher barrier heights in the forward processes than those in the reverse processes, are found to mostly proceed through electron transfer for the forward processes and run through structural change for the backward processes These results assure the high availability of the orbital energy-based reaction analysis method.
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