Intramolecular electron transfer distance dependence

The charge photoseparation in porphyrin-quinone compounds with a rigid bicyclo[2.2.2]octyl bridge, ensuring a distance between the centres of P and Q of about 16 A, has been studied [57]. The rate constant of intramolecular electron transfer from P to Q was found to depend on the dielectric properties of the medium and reached 3.3 x 107s 1 for a solution of P-L-Q in propionitrile. 

The temperature dependence of the intramolecular electron transfer from Ru11 to Cu11 in Ru-modified stellacyanin from Rhus vernisifera was studied in Ref. [320] in the temperature interval 298-112.7 K. The activation enthalpy AH was found to be 19.1 3.1 kJ/mol. The entropy of activation found for the intramolecular electron transfer process AS = —201 + 40 J/mol K was in a good agreement with the calculated value AS = —193 J/mol K for intramolecular electron transfer in Ru-stellacyanin-Cu over the distance R = 16 A estimated from the tentative three-dimensional model of stellacyanin molecule [320]. 

The field of light-induced electron transfer remains in a state of rapid development in many of the areas on which this brief history has touched and in others that could not be included here. Exciting progress is being made in the delineation of the first picosecond of photosynthesis, in further characterizing photosynthetic reaction centers, in the area of artificial models. The dependence of intramolecular electron transfer upon distance, solvent, orientation is being delineated. Many of these developments are detailed in the following chapters. 

Electron-Transfer Reactions. It is well known that thermal and photochemical electron-transfer reactions exhibit characteristic pressure dependences and associated volumes of activation (see Sections II, III, and VI). It is therefore realistic to expect that photoinduced thermal electron-transfer reactions will also exhibit a characteristic pressure dependence that should reveal mechanistic information on the nature of the reaction. Recent interest in the mechanistic understanding of long-distance electron-transfer reactions prompted an investigation of the effect of pressure on intramolecular electron transfer in ruthenium-modified cytochrome c [151] (a typical example of a closely related intermolecular electron-transfer reaction was... 

Although electron transfer rates within myoglobin appear to follow an exponential dependence on distance, the derived rate expression is not directly transferable to other electron transfer proteins. A particularly striking comparison is between the c cytochromes and the copper proteins plastocyanin and azurin. Intramolecular electron transfer rates are at least 10-100 times slower in the copper proteins compared to the c cytochromes, even though the distances and driving forces for the reactions are comparable. The origin of this behavior is unclear, but it does suggest caution in the quantitative transfer of rate expressions between different systems. 

N. Koga, K. Sameshima, K. Morokuma, Ab initio MO calculations of electronic coupling matrix elements on model systems for intramolecular electron transfer, hole transfer, and triplet energy transfer distance dependence and pathway in electron transfer and relationship of triplet energy transfer with electron and hole transfer, J. Phys. Chem., 1993, 97, 13117-13125. 

The outer-sphere electron transfer reaction between [Fe(CN)6] and [Co(NH3)5H20] has been studied as a function of temperature and pressure in a variety of water-glycerol solvent mixtures. While AV remains essentially constant at 28 cm moP both A/f and A5 increase with an increase in the viscosity of the solvent. The intramolecular electron transfer processes in (NH3)5CoLFe(CN)5 complexes, where L is 3,3 -dimethyl-4,4 -bipyridine, 4,4 -bipyridylacetylene, 2,7-diazapyrene, and 3,8-phenathroline, have been studied. The activation free energies display an inverse dependence on the Fe-Co distance and, when corrected for solvent reorganization energies, are relatively constant at 14.0 0.5 kcal moP. ... 

Li, T. T. T. Weaver, M. J. Intramolecular electron transfer at metal surfaces. 4. Dependence of tunneling probability upon donor-acceptor separation distance. J Am Chem Soc 1984,106, 6107-6108. 

The possible mechanism of intramolecular electron transfer over this distance between the two centers within the molecule and the nature of the chemical groupings on which it depends is of considerable interest. However, we can do no more than speculate about the problems at present. A line of experimental work which might prove valuable would be to study further the phenomenon (Bray et al, 1975) of diminished rates of intramolecular electron transfer in chemically modified forms of xanthine oxidase, in comparison with the native form. 

Finally, attention should be drawn to the elegant studies of Meyer and his co-workers. Metal complexes have been designed which contain both an excitable (e. g. bpy) and a quencher (e. g. Af-Mebpy+) group. Following excitation, intramolecular electron and energy transfer occurs and the dependence of rate on distances, metal and so on, can be assessed. 

The feasibility of intramolecular electron- and energy-transfer depends on distance and is usually studied in covalently linked systems. However, donor-acceptor dyads can be also arranged by self-assembly what resembles the situation of electron transfer in biological systems. Artificial dyads tethered by a small number of hydrogen bonds immediately dissociate in methanol or water. To improve the binding while keeping the reversibility, a photoinducible electron donor-acceptor dyad linked by a kinetically labile bond was designed. [19]... 

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