Electron transfer energetics

The formation of 17-electron radicals as the result of thermal electron transfer from a carbonyl metallate to a cationic pyrylium acceptor (P+) is also observed with the charge-transfer salts of [TpM(CO)3 ] (Tp = hydrido-fn s-(3,5-dimethylpyrazolyl)-borate M = Mo, W, Cr) [127]. Owing to the stability of the TpM(CO)3 radical [182], electron-transfer equilibria (Eet) are established by UV-Vis and IR spectroscopy which generally favor the intimate ion-pair state (see Structure D in Figure 5) in accord with the electron-transfer energetics [127] (Eq. 57). 

The electron-transfer energetics from the neutralization of hydroxide ion by hydronium ion... 

Figure C1.5.12.(A) Fluorescence decay of a single molecule of cresyl violet on an indium tin oxide (ITO) surface measured by time-correlated single photon counting. The solid line is tire fitted decay, a single exponential of 480 5 ps convolved witli tire instmment response function of 160 ps fwiim. The decay, which is considerably faster tlian tire natural fluorescence lifetime of cresyl violet, is due to electron transfer from tire excited cresyl violet (D ) to tire conduction band or energetically accessible surface electronic states of ITO. (B) Distribution of lifetimes for 40 different single molecules showing a broad distribution of electron transfer rates. Reprinted witli pennission from Lu andXie [1381. Copyright 1997 American Chemical Society.

Computer simulations of electron transfer proteins often entail a variety of calculation techniques electronic structure calculations, molecular mechanics, and electrostatic calculations. In this section, general considerations for calculations of metalloproteins are outlined in subsequent sections, details for studying specific redox properties are given. Quantum chemistry electronic structure calculations of the redox site are important in the calculation of the energetics of the redox site and in obtaining parameters and are discussed in Sections III.A and III.B. Both molecular mechanics and electrostatic calculations of the protein are important in understanding the outer shell energetics and are discussed in Section III.C, with a focus on molecular mechanics. 

In principle, the oxidation of proceeds at an electrode potential that is more negative by about 0.7 V than the anodic decomposition paths in the above cases however, because of the adsorption shift, it is readily seen that practically there is no energetic advantage compared to CdX dissolution in competing for photogenerated holes. Similar effects are observed with Se and Te electrolytes. As a consequence of specific adsorption and the fact that the X /X couples involve a two-electron transfer, the overall redox process (adsorption/electron trans-fer/desorption) is also slow, which limits the degree of stabilization that can be attained in such systems. In addition, the type of interaction of the X ions with the electrode surface which produces the shifts in the decomposition potentials also favors anion substitution in the lattice and the concomitant degradation of the photoresponse. 

These spectroscopic and theoretical developments have stimulated the recent advances on electron-transfer dynamics at ITIES. In addition to the correlation between structure and dynamics of charge transfer, fundamental problems in connection with the energetics of ET reactions remain to be fully addressed. We shall consider these problems primarily before discussing kinetic aspects in full detail. 

In this chapter, a novel interpretation of the membrane transport process elucidated based on a voltammetric concept and method is presented, and the important role of charge transfer reactions at aqueous-membrane interfaces in the membrane transport is emphasized [10,17,18]. Then, three respiration mimetic charge (ion or electron) transfer reactions observed by the present authors at the interface between an aqueous solution and an organic solution in the absence of any enzymes or proteins are introduced, and selective ion transfer reactions coupled with the electron transfer reactions are discussed [19-23]. The reaction processes of the charge transfer reactions and the energetic relations... 

The observation of selective fluorescence quenching by G C base pairs is consistent with the energetics of electron transfer (charge separation) from the bases to the singlet excited state Sa, which can be estimated using Weller s equation [26] ... 

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