Electron tunneling, types

In equation (17), the rate constant for electron turmelling is ket, R the distance between donor and acceptor and P a distance dependent constant. This electron tunnelling type of mechanism indicates that the rate of electron transfer decreases by about one order of magnitude per base pair, since the typical value of p is in the range 1-1.5 A [123]. As shown in Table 4 typical values of P for... 

Figure 3.44. Dissociation of 02 adsorbed on Pt(lll) by inelastic tunneling of electrons from a STM tip. (a) Schematic ID PES for chemisorbed Of dissociation and illustrating different types of excitations that can lead to dissociation, (b) Schematic picture of inelastic electron tunneling to an adsorbate-induced resonance with density of states pa inducing vibrational excitation (1) competing with non-adiabatic vibrational de-excitation that creates e-h pairs in the substrate (2). (c) Dissociation rate Rd for 0 as a function of tunneling current I at the three tip bias voltages labeled in the figure. Solid lines are fits of Rd a IN to the experiments with N = 0.8, 1.8, and 3.2 for tip biases of 0.4, 0.3, and 0.2 V, respectively and correspond to the three excitation conditions in (a). Dashed lines are results of a theoretical model incorporating the physics in (a) and (b) and a single fit parameter. From Ref. [153].

Manifestations of nuclei tunneling in chemical reactions in gaseous, liquid, and solid phases are consecutively considered in Sects. 4.2-4.5. Also discussed in this chapter are (1) manifestations of nuclear tunneling in the vibrational spectra of ammonia-type molecules (Sect. 4.6), (2) electron tunneling in gas-phase chemical reactions of atom transfer (the so-called "harpoon reactions, Sect. 4.2), and (3) tunneling of hydrated electrons in the reactions of their recombination with some inorganic anions in aqueous solutions (Sect. 4.4). 

To summarise, the form of kinetic equations for electron tunneling reactions must strongly depend on two factors the type of dependence of the tunneling probability on the distance between the reagents and the form of the spatial distribution of the reagents. 

In the present chapter, (1) the macroscopic kinetics of the electron tunneling reaction is considered for various types of spatial distribution of the reagents and for situations when the reagents can be both immobile and mobile (2) the applicability of various kinetic models is analyzed under typical conditions of experimental studies on electron tunneling reactions and (3) methods are described of the determination, from the kinetic data, of various parameters which characterize the rates and distances of electron tunneling. 

Let us discuss what will be the influence on the kinetics of electron tunneling reactions of such factors as the more complicated, rather than the simple, exponential dependence of the tunneling probability W(R) on the mutual location of the reagents. To exercise such an analysis, it is necessary to consider in more detail the limits of applicability of the stepwise approximation of the function 9(R,t) = exp[ — W(/ )(], which was used in the previous section to derive the kinetic equations for electron tunneling reactions in the case of the exponential dependence of the eqn. (2) type for... 

It has been shown in Chap. 3 that the typical deviation of the dependence of the probability of tunneling, VF(i ), on the distance, from a simple exponent of the eqn. (2) type, is the presence of the multiplier (Rja)n [see eqn. (1)1. From the computations carried out in ref. 13, it follows that the kinetics of electron tunneling reactions for the dependence VF(i ) of the eqn. (1) type with 0 is described by the same equation... 

Most of the experiments on studying the influence of the electric field on the kinetics of electron tunneling reactions have been carried out under the conditions when the reagents are distributed in space in the form of isolated pairs. In this case, for W(R, tp) of the eqn. (38) type, the change of the concentration of the reagents with time, according to refs. 21 and 22, is described by the equation... 

Due to the extremely low translational mobility of the molecules in vitreous matrices, the kinetics of the chemical reactions in these matrices depends substantially on the form of the initial spatial distribution of the reagents. The study of the kinetics of electron tunneling reactions in vitreous matrices is often conducted in such a manner that one of the reagents is generated after vitrification of the solution by means of y- or / -radiolysis or photolysis, and the other is either generated in the similar manner or is introduced into the solution prior to freezing. In this connection, let us dwell upon the spatial distribution of both these types of reagent in vitreous matrices. 

Along with the temperature-independent electron tunneling reactions, there exist reactions of the type for which the rate depends on temperature. In the present section we shall discuss the methods of determining the activation energy for those processes from their kinetic curves. 

The possibility of long-range electron tunneling in reactions of etr was first suggested for recombination reactions of e with hole centres. Direct experimental proof of the reality of this phenomenon was also obtained in studying this type of reaction. 

One more type of luminescence phenomena associated with electron tunneling was suggested in ref. 72. In the scheme of Fig. 17, this transition is denoted by b. 

Thus the data obtained so far indicate that electron donor and electron acceptor centres on the surface of highly dispersed oxides, including adsorbed molecules, may undergo long-range electron tunneling reactions with centres of the opposite type located both on the surface and in the bulk of the oxides. 

The appearance, at T > 160 K, of a channel of decay with a relatively high activation energy, 2 ", can be accounted for, e.g. by a defreezing of some additional type of motion (rotation, vibration, or conformational transition) creating still more favourable conditions for electron tunneling than the channel with activation energy E A. 

A simple approach to understanding the factors which control the "conductivity of proteins towards electron tunneling is to develop "small molecule model systems to mimic intramolecular electron transfer in the protein systems. Appropriate models obviously require that the donor and acceptor be held at fixed distances and orientations which correspond to those in the protein-protein complexes. Models of this type have recently been obtained and investigated [103,104]. In these models the protein matrix is replaced by a simple synthetic spacer which separates two porphyrin molecules. By changing the chemical structure of the spacer, a series of molecules with different reaction distances and geometries has been synthesized. Typical examples of such molecules are presented in Fig. 21. 

The quenching of the fluorescence of donor particles D and the formation of reduced acceptor particles A" in photochemical reactions of the eqn. (1) type followed by a slow decay of particles A" have also been observed in a number of other donor-acceptor molecular layers of a similar structure [8, 9], The data obtained have also been explained by electron tunneling from the excited donor particles to the acceptor particles over distances exceeding 20 A. 

As a rule, however, the distance between the donor and the acceptor in such binuclear bridge metallocomplexes is not large. Only a few molecules of this type are known in which the electron transfer occurs over considerable distances, comparable with those for electron transfer between randomly arranged centres in vitreous matrices. Consider the results of research on electron tunneling over large distances in bridge systems. 

Abstract—Inelastic electron tunneling spectroscopy is used to investigate the adsorption of dimethyl-dimethoxysilane, dimethyldiethoxysilane, and dimethylvinylethoxy silane on alumina at the mono-layer level. Data obtained indicate that different adsorbed layers are produced when the silanes are introduced onto the oxide surface from solution or as a vapor. Silanes introduced in the same way onto different types of oxides suggest that alumina morphology also affects the adsorbed configuration. 

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