Tunneling, hydrated electron

The gas-phase lifetime of N20- is 10-3 s in alkaline solutions, it is still >10-8 s. Under suitable conditions, N20- may react with solutes, including N20. The hydrated electron reacts very quickly with NO (see Table 6.6). The rate is about three times that of diffusion control, suggesting some faster process such as tunneling. NO has an electron affinity in the gas phase enhanced upon solvation. The free energy change of the reaction NO + eh (NO-)aq is estimated to be --50 Kcal/mole. Both N02- and N03- react with eh at a nearly diffusion-controlled rate. The intermediate product in the first reaction, N02-, generates NO and... 

In the second or resonant tunneling model, the intermediate state is associated with the formation of a solvation cage for the hydrated electron [96]. This scheme has largely been discounted since organizing the state... 

FIG. 12. A schematic representation of the possible role of water on the potential profile within the tunnel junction. Fast electronic polarization of the solvent diminishes the barrier, while the possibility of forming an intermediate hydrated electron resonant state has also been suggested. (From Ref. 96.)... 

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). 

There are also certain data on electron tunneling in electron transfer reactions in liquids. The ideas about electron tunneling have been used by Anbar and Hart [75] to interpret the anomalously large rate constants for the diffusion controlled reactions of hydrated electrons with some inorganic anions in aqueous solution. Table 5 represents the data on the largest values of the rate constants, ke, observed for the reactions of eaq with various inorganic anions and cations. Theoretical diffusion rate constants, kA, for... 

The rate constants of reactions of hydrated electrons with some accep-tors-anions substantially exceed the diffusion rate constants calculated with the help of the Debye equation [Chap. 2, eqn. (45)l(see Chap. 2, Sect. 4). This excess is usually attributed to the capture of electrons by acceptors via tunneling at distances exceeding the sum of the reagents [28,89,111,1201- In this case, the tunneling distance can be estimated from experimental rate constants for reactions of eaq with acceptors [109] by means of the expression... 

E.L. Girina and B.G. Ershov, Izv. Akad. Nauk SSSR Ser. Khim., (1972) 278 (in Russian). V.M. Byakov, V.L. Grishutkin and A.A. Ovchinnikov, Effect of tunneling on chemical reactions of hydrated electrons. Preprint N 40 of Inst. Theor. Exp. Phys., Moscow, 1977 (in Russian). 

With the aid of electron tunneling it appears possible to regulate the selectivity of redox conversions. For practically important reactions this has not been realized so far, but that this approach may prove to be useful is demonstrated, e.g. by the data presented in Table 6. In this table, a comparison is made between the rate constants for reactions of three different acceptors with hydrated electrons in liquid water at 298 K and the characteristic times, t, for reactions of the same acceptors with trapped electrons in solid water-alkaline glasses at 77 K. The values of x have been calculated using the values of ve and ae from Ref. [21]. It can be seen in the liquid, when due to diffusion the reagents can approach to within short distances of each other (direct collisions), that the rate constants for all three... 

This process takes place within the very thin interfacial region at the electrode surface, and involves quantum-mechanical tunneling of electrons between the electrode and the electroactive species. The work required to displace the H20 molecules in the hydration spheres of the ions constitutes part of the activation energy of the process. 

Consider a hydrophobic sensitizer such as tetramethylbenzidine145) or phenothiazine146 incorporated into an anionic micelle. After excitation these molecules will eject electrons via a tunnelling mechanism147,148) from the lipid into the aqueous phase where formation of hydrated electrons (e q) occurs. The sensitizer cation will remain within the micellar aggregate. 

The primary cathodic step of hot electron-induced electrochemiluminescence (HECL) was postulated to be an injection of hot electrons into aqueous electrolyte solution by tunnel emission through an insulating barrier, followed by reduction reactions induced either by presolvated hot electrons or by fully hydrated electrons [37, 38]. The introduction and details of this kind of hot electron electrochemistry and HECL has been reviewed very recently [33] and only short descriptions of the basic features are given here. 

Because the path integral techniques can account for quantum effect directly in the simulations, the methodology has been used mostly in studies of the behavior of quantum solutes, including tunneling, charge transfer between solutes, and hydrated electrons. Simulations of pure water ° investigated quantum corrections to effective potentials. The Feynman-Hibbs effective potential is a computationally simple method for estimating quantum effects and has been used to examine the differences in the properties of H2O and 02 . ... 

Whereas a film formed in dry air consists essentially of an anhydrous oxide and may reach a thickness of 3 nm, in the presence of water (ranging from condensed films deposited from humid atmospheres to bulk aqueous phases) further thickening occurs as partial hydration increases the electron tunnelling conductivity. Other components in contaminated atmospheres may become incorporated (e.g. HjS, SO2, CO2, Cl ), as described in Sections 2.2 and3.1. 

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