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Formalisms for electron‐exchange kinetics in aqueous solution and the role of Ab initio techniques in their implementation
Author(s) -
Newton Marshall D.
Publication year - 2009
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.560180839
Subject(s) - semiclassical physics , chemistry , reaction rate constant , ab initio , quantum tunnelling , ab initio quantum chemistry methods , quantum , hamiltonian (control theory) , computational chemistry , quantum chemistry , thermodynamics , rotation formalisms in three dimensions , electronic structure , kinetics , molecule , quantum mechanics , physics , organic chemistry , mathematical optimization , geometry , mathematics , electrode , electrochemistry
Formalisms suitable for calculating the rate of electron exchange between transition metal complexes in aqueous solution are reviewed and implemented in conjunction with ab initio quantum chemical calculations which provide crucial off‐diagonal Hamiltonian matrix elements as well as other relevant electronic structural data. Rate constants and activation parameters are calculated for the hex‐aquo Fe 2+ ‐Fe 3+ system, using a simple activated complex theory, a nonadiabatic semiclassical model which includes nuclear tunneling, and a more detailed quantum mechanical method based on the Golden rule. Comparisons are made between calculated results and those obtained by extrapolating experimental data to zero ionic strength. All methods yield similar values for the overall rate constant (ca. 0.1 liter mole −1 sec −1 ). The experimental activation parameters (Δ H ‡ and Δ S ‡) are in somewhat better agreement with the semiclassical and quantum mechanical results than with those from the simple activated complex theory, thereby providing some indication that nonadiabaticity and nuclear tunneling may be important in the Fe 2+/3+ exchange reaction. It is concluded that a model based on direct metal‐metal overlap can account for the observed reaction kinetics provided the reactants are allowed to approach well within the traditional contact distance of 6.9 Å.