Marcus mechanism

Figure 2, which describes harpoon reactions in the gas phase, has strong relationships with Figure 1, which pictures the Marcus mechanism of an electron-... 

H.H. Strehblow, P. Marcus, Mechanisms of pitting corrosion, in P. Marcus (Ed.), Corrosion Mechanisms in Theory and Practice, Marcel Dekker Inc., New York, 2012, pp. 349-393. 

The mechanism of the process differs considerably from the Marcus mechanism of electron transfer. The particles on the way between electrode and electrolyte must overcome an activation energy. The activation energy depends on the electric field in the double layer. The activation energy and the additional electrical term are shown in Figure 6.14. 

Marcus R A 1966 On the analytical mechanics of chemical reactions. Quantum mechanics of linear collisions J. Chem. Phys. 45 4500... 

Clayton C R and Olefjord I 1995 Corrosion Mechanisms In Theory and Practice ed P Marcus and J Oudar (New York Dekker) p 175... 

This is the reverse of the first step in the SnI mechanism. As written here, this reaction is called cation-anion recombination, or an electrophile-nucleophile reaction. This type of reaction lacks the symmetry of a group transfer reaction, and we should therefore not expect Marcus theory to be applicable, as Ritchie et al. have emphasized. Nevertheless, the electrophile-nucleophile reaction possesses the simplifying feature that bond formation occurs in the absence of bond cleavage. 

The Marcus treatment applies to both inorganic and organic reactions, and has been particularly useful for ET reactions between metal complexes that adopt the outer-sphere mechanism. Because the coordination spheres of both participants remain intact in the transition state and products, the assumptions of the model are most often satisfied. To illustrate the treatment we shall consider a family of reactions involving partners with known EE rate constants. 

Figure 42. Scheme comparing expected potential-independent charge-transfer rates from Marcus-Gerischer theory of interfacia) electron transfer (left) with possible mechanisms for explaining the experimental observation of potential-dependent electron-transfer rates (right) a potential-dependent concentration of surface states, or a charge-transfer rate that depends on the thermodynamic force (electric potential difference) in the interface. 

Mandelbrot, on fractal surfaces, 52 Mao and Pickup, their work on the oxidation of polypyrrole, 587 Marcus model, inapplicability for interfacial electron transfer, 513 Mechanical breakdown model for passivity, 236... 

One type of process that can successfully be treated by the Marcus equation is the Sn2 mechanism (p. 390)... 

Excited state electron transfer also needs electronic interaction between the two partners and obeys the same rules as electron transfer between ground state molecules (Marcus equation and related quantum mechanical elaborations [ 14]), taking into account that the excited state energy can be used, to a first approximation, as an extra free energy contribution for the occurrence of both oxidation and reduction processes [8]. 

Wahl et al ° have suggested that the rate coefficient obtained by the nmr method" at cation concentrations 5x 10 M is that for the step k2 in the above mechanism. There is reasonable agreement between the data obtained by these two different procedures. A comparison has been made of the experimental results with those obtained from the Marcus theory °. 

The last two decades have seen a growing interest in the mechanism of inorganic reactions in solution. Nowhere is this activity more evident than in the topic covered by this review the oxidation-reduction processes of metal complexes. This subject has been reviewed a number of times previously, notably by Taube (1959), Halpern (1961), Sutin (1966), and Sykes (1967). Other articles and books concerned, wholly or partly, with the topic include those by Stranks, Fraser , Strehlow, Reynolds and Lumry , Basolo and Pearson, and Candlin et al ° Important recent articles on the theoretical aspects are those by Marcus and Ruff. Elementary accounts of redox reactions are included in the books by Edwards , Sykes and Benson . The object of the present review is to provide a more detailed survey of the experimental work than has hitherto been available. 

On this basis Cr(V), not Cr(IV), is the kinetically important intermediate such that k = 3 k4 and k = k Jk. The hydrogen-ion dependence of the reaction rate has been discussed. Furthermore, comparisons are drawn with the rate of the Cr(VI)+Fe(phen)3 reaction, and Sullivan has speculated on the intimate nature of both mechanisms in the light of Marcus theory... 

Ce(IV) + Fe(II) system, as calculated by Dulz and Sutin, on the grounds that the rate of the Fe(III)+Fe(II) exchange (/ci,i) and the corresponding oxidation potential relate to HCIO4 media, whereas the rate (A 2,2) and oxidation potentia of the Ce(IV)+Ce(III) system are for H2SO4 media. Adamson et arrive at a calculated value of 1.3 x 10 l.mole sec for the rate coefficient (A i,2) of the Ce(lV)+Fe(II) reaction in 0.5 M HCIO4 at 0 °C. Since this value is very much at variance with the observed value (700 l.mole sec" ), they conclude that this oxidation takes place by an atom-transfer mechanism, to which the theoretical treatment of Marcus is not appropriate. 

In conforming to an expected linear free energy relationship, the Ce(lV) oxidation of various 1,10-phenanthroline and bipyridyl complexes of Ru(II) in 0.5 M sulphuric acid are consistent with the requirements of the Marcus treatment . The results for the oxidation of the 3- and 5-sulphonic-substituted ferroin complexes by Ce(IV) suggest that the ligand does not function as an electron mediator, and that the mechanism is outer-sphere in type. Second-order rate coefficients for the oxidation of Ru(phen)j, Ru(bipy)3, and Ru(terpy)3 are 5.8x10, 8,8 X 10, and 7.0 x 10 l.mole . sec, respectively, in 0.5 M H2SO4 at 25 °C a rapid-mixing device was employed. 

Tang, J. and Marcus, R. A. (2005) Diffusion-controlled electron transfer processes and power-law statistics of fluorescence intermittency of nanoparticles. Phys. Rev. Lett, 95, 107401-1-107401-4 Tang, J. and Marcus, R. A. (2005) Mechanisms of fluorescence blinking in semiconductor nanocrystal quantum dots./. Chem. Phys., 123,054704-1-054704-12. 

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