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Marcus theory is a theory originally developed by Rudolph A. Marcus, starting in 1956, to explain the rates of electron transfer reactions – the rate at which an electron can move or jump from one chemical species (called the electron donor) to another (called the electron acceptor). It was originally formulated to address outer sphere electron transfer reactions, in which the two chemical species only change in their charge with an electron jumping (e.g. the oxidation of an ion like Fe2+/Fe3+), but do not undergo large structural changes. It was extended to include inner sphere electron transfer contributions, in which a change of distances or geometry in the solvation or coordination shells of the two chemical species is taken into account (the Fe-O distances in Fe(H2O)2+ and Fe(H2O)3+ are different).〔Contrary to Marcus' approach the inner sphere electron transfer theory of Noel S. Hush refers to a ''continuous'' change of the electron density during transfer along a geometrical coordinate (adiabatic case), and takes also into account the solvent influence as did Marcus. Hush's formulation is known as Marcus-Hush theory.〕〔Hush, N.S. Trans. Faraday Soc. 1961, 57,557〕 For electron transfer reactions without making or breaking bonds Marcus theory takes the place of Eyring's transition state theory 〔P. W. Atkins: ''Physical Chemistry'', 6. Ed., Oxford University Press, Oxford 1998 p.830〕〔R.S. Berry, S. A. Rice, J. Ross: ''Physical Chemistry'', Wiley, New York 1980, S. 1147 ff,〕 which has been derived for reactions with structural changes. Both theories lead to rate equations of the same exponential form. However, whereas in Eyring theory the reaction partners become strongly coupled in the course of the reaction to form a structurally defined activated complex, in Marcus theory they are weakly coupled and retain their individuality. It is the thermally induced reorganization of the surroundings, the solvent (outer sphere) and the solvent sheath or the ligands (inner sphere) which create the geometrically favourable situation ''prior'' to and independent of the electron jump. The original classical Marcus theory for outer sphere electron transfer reactions demonstrates the importance of the solvent and leads the way to the calculation of the Gibbs free energy of activation, using the polarization properties of the solvent, the size of the reactants, the transfer distance and the Gibbs free energy G0 of the redox reaction. The most startling result of Marcus' theory was the "inverted region": whereas the reaction rates usually become higher with increasing exergonicity of the reaction, electron transfer should, according to Marcus theory, become slower in the very negative G0 domain. Scientists searched the inverted region for proof of a slower electron transfer rate for 30 years until it was unequivocally verified experimentally in 1984. R.A. Marcus received the Nobel Prize in Chemistry in 1992 for this theory. Marcus theory is used to describe a number of important processes in chemistry and biology, including photosynthesis, corrosion, certain types of chemiluminescence, charge separation in some types of solar cell and more. Besides the inner and outer sphere applications, Marcus theory has been extended to address heterogeneous electron transfer. == The one-electron redox reaction == Chemical reactions may lead to a substitution of a group in a molecule or a ligand in a complex, to the elimination of a group of the molecule or a ligand, or to a rearrangement of a molecule or complex. An electron-transfer reaction may, however, also cause simply an exchange of charges between the reactants, and these redox reactions without making or breaking a bond seem to be quite simple in inorganic chemistry for ions and complexes. These reactions often become manifest by a change of colour, e.g. for ions or complexes of transition metal ions, but organic molecules, too, may change their colour by accepting or giving away an electron (like the herbicide Paraquat (N,N-dimethyl-4,4'-bipyridinium dichloride) which becomes blue when accepting an electron, thence the alternative name of methyl viologen). For this type of electron-transfer reactions R.A. Marcus has developed his theory. Here the trace of argument and the results are presented. For the mathematical development and details the original papers〔Marcus, R.A. "On the Theory of Oxidation-Reduction Reactions Involving Electron Transfer I" ''J.Chem.Phys.''1956, 24, 966. or (Free Text )〕〔Marcus.R.A. "Electrostatic Free Energy and Other Properties of States Having Nonequilibrium Polarization I. ''J.Chem.Phys.''1956, 24, 979. or (Free Text )〕 should be consulted. In a redox reaction one partner acts as an electron donor D the other as an acceptor A. For a reaction to take place D and A must diffuse together. They form the precursor complex, usually a kinetic, unstable, solvated encounter complex, which by electron transfer is transformed to the successor complex, and finally this separates by diffusion. For a one electron transfer the reaction is : (D and A may already carry charges). Here k12, k21 and k30 are diffusion constants, k23 and k32 rate constants of activated reactions. The total reaction may be diffusion controlled (the electron transfer step is faster than diffusion, every encounter leads to reaction) or activation controlled (the "equilibrium of association" is reached, the electron transfer step is slow, the separation of the successor complex is fast). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Marcus theory」の詳細全文を読む スポンサード リンク
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