A Frontier Orbital Study with ab Initio Molecular Dynamics of the Effects of Solvation on Chemical Reactivity: Solvent-Induced Orbital Control in FeO-Activated Hydroxylation Reactions

摘要

Solvation effects on chemical reactivity are often rationalized using electrostatic considerations: the reduced stabilization of the transition state results in higher reaction barriers and lower reactivity in solution. We demonstrate that the effect of solvation on the relative energies of the frontier orbitals is equally important and may even reverse the trend expected from purely electrostatic arguments. We consider the H abstraction reaction from methane by quintet [EDTAH_n·FeO]~((n-2)+), (n = 0-4) complexes in the gas phase and in aqueous solution, which we examine using ab initio thermodynamic integration. The variation of the charge of the complex with the protonation of the EDTA ligand reveals that the free energy barrier in gas phase increases with the negative charge, varying from 16 kJ mol~(-1) for [EDTAH_4·FeO]~(2+) to 57 kJ mol~(-1) for [EDTAH_n·FeO]~(2-). In aqueous solution, the barrier for the +2 complex (38 kJ mol~(-1)) is higher than in gas phase, as predicted by purely electrostatic arguments. For the negative complexes, however, the barrier is lower than in gas phase (e.g., 45 kJ mol~(-1) for the -2 complex). We explain this increase in reactivity in terms of a stabilization of the virtual 3σ* orbital of FeO~(2+), which acts as the dominant electron acceptor in the H-atom transfer from CH_4. This stabilization originates from the dielectric screening caused by the reorientation of the water dipoles in the first solvation shell of the charged solute, which stabilizes the acceptor orbital energy for the -2 complex sufficiently to outweigh the unfavorable electrostatic destabilization of the transition-state relative to the reactants in solution.