期刊
JOURNAL OF CHEMICAL THEORY AND COMPUTATION
卷 8, 期 11, 页码 4347-4358出版社
AMER CHEMICAL SOC
DOI: 10.1021/ct3004595
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资金
- National Science Foundation [CHE09-57162]
- National Institutes of Health [GM46736]
- Direct For Mathematical & Physical Scien
- Division Of Chemistry [0957162] Funding Source: National Science Foundation
A:critical element in. theoretical characterization of the mechanism of proton coupled electron transfer (PCET) reactions, including hydrogen atom transfer (HAT), is the formulation of the electron and proton localized diabatic states, based on which a More O'Ferrall-Jencks diagram can be represented to determine the stepwise and concerted nature of the reaction. Although the More O'Ferrall-Jencks diabatic states have often been used empirically to develop theoretical models for PCET reactions, the potential energy surfaces for these states have never been determined directly based on first principles calculations using electronic structure theory. The difficulty is due to a lack of practical method to constrain. electron and proton localized diabatic states in wave function. or density functional theory. Employing a multistate density functional theory (MSDFT), in which the electron and proton localized diabatic configurations are constructed through block localization of Kohn-Sham orbitals, we show that distinction between concerted proton-electron. transfer (CPET) and HAT, which are not distinguishable experimentally from phenomenological kinetic data, can be made by examining,the,third dimension of More O'Ferrall-Jencks diagram that includes both the ground and excited state potential surfaces In addition, We formulate a pair of effective two state valence bond models to represent the CPET and HAT mechanisms. We found that the lower energy of the CPET and HAT effective diabatic states at the intersection point can be used as an energetic criterion to distinguish the two mechanisms. In the isoelectronic series of hydrogen exchange reaction in (PhX)(2)H-center dot, where X = O, NH, and CH2, there is a continuous transition from a CPET mechanism for the phenoxy radical-phenol pair to a HAT process for benzyl radical and toluene, while the reaction between PhNH2 and PhNH center dot has a mechanism intermediate of CPET and HAT. The electronically nonadiabatic nature of the CPET mechanism in the phenol system can be attributed to the overlap interactions between the ground and excited state surfaces, resulting in roughly orthogonal minimum energy paths on the adiabatic ground and excited state potential energy surfaces. On the other hand, the minimum energy path on the adiabatic ground state for the HAT mechanism coincides with that on the excited state, producing a large electronic coupling that separates the two surfaces by more than 120 kcal/mol.
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