Cavity hydrated electron

When an electron is injected into a polar solvent such as water or alcohols, the electron is solvated and forms so-called the solvated electron. This solvated electron is considered the most basic anionic species in solutions and it has been extensively studied by variety of experimental and theoretical methods. Especially, the solvated electron in water (the hydrated electron) has been attracting much interest in wide fields because of its fundamental importance. It is well-known that the solvated electron in water exhibits a very broad absorption band peaked around 720 nm. This broad absorption is mainly attributed to the s- p transition of the electron in a solvent cavity. Recently, we measured picosecond time-resolved Raman scattering from water under the resonance condition with the s- p transition of the solvated electron, and found that strong transient Raman bands appeared in accordance with the generation of the solvated electron [1]. It was concluded that the observed transient Raman scattering was due to the water molecules that directly interact with the electron in the first solvation shell. Similar results were also obtained by a nanosecond Raman study [2]. This finding implies that we are now able to study the solvated electron by using vibrational spectroscopy. In this paper, we describe new information about the ultrafast dynamics of the solvated electron in water, which are obtained by time-resolved resonance Raman spectroscopy. 

Of the three models that have been proposed to explain the properties of excess electrons in liquid helium, two are considered in detail (1) The electron is localized in a cavity in the liquid (2) The electron is a quasi-free particle. The pseudopotential method is helpful in studying both of these models. The most useful treatment of electron binding in polar solvents is based on a model with the solution as a continuous dielectric medium in which the additional electron induces a polarization field. This model can be used for studies with the hydrated electron. 

Calculations based on the continuum dielectric model have been performed by the hydrated electron in the limit of zero cavity size (19). The general treatment is based on a variational calculation using hydrogenic type wave functions for the ground and the first excited states. This treatment is based on a Hartree Fock scheme, where the Coulomb and exchange interaction of the excess electron with the medium are replaced by the polarization energy of a continuous dielectric. The results obtained are summarized in Table V. The fair agreement obtained with... 

After thermalization, the electron may recombine with a positive ion or be captured by a molecule forming a negative ion, or it may be locked in a trap the role of which may be played by fluctuation cavities or structural disturbances in the medium, or by polarization pits that the electron digs when it interacts with surrounding molecules. Such captured electrons are called solvated electrons (in water they are sometimes called hydrated electrons).31,32 According to the data obtained in picosecond pulsed-radiolysis sets,33 34 the solvation time of an electron is 2 x 10-12 s in water and —10 11 s in methanol. 

This mechanism is reasonable as a) reduction of benzene occurs at a cathode potential of -2,5 V vs. S.C.E., roughly corresponding to the standard potential of the hydrated electron 293 while the potential for the direct electron transfer to benzene is more negative ( -3,0 V) and b) in situ electrolysis in the ESR cavity produces at -100 °C the characteristic singlet of the solvated electron 293a>, which changes to the septett of the benzene radical anion, when benzene is added to the solution. 

Hydrated electron — When a free electron is injected into water, it localizes in a cavity (electrostatic potential well) between two water molecules within less than 1 ps ... 

There is clear evidence that the spectrum of the hydrated electron is best described as a charged particle in a cavity in solution, the simplest anion.The spectrum and the reactivity are very consistent with such an interpretation. However, there is also clear evidence that this is not the best description of the electron. There is no obvious way to reconcile the reaction... 

Fig. 2. (On the left) Isodensity map for s-Kke HOMO of hydrated electron given by MQC MD-DFT calculation a single snapshot is shown (only two solvation shells are shown, the embedding matrix of water molecules is removed for clarity). The central s-like orbital (grey) has the opposite sign to frontier O 2p orbitals in water molecules solvating the electron. Ca. 20% of the electron density is in these O 2p orbitals. Despite that, the ensemble average radial component of the HOMO (on the right, solid line) closely resembles hydrogenic wavefunction (broken line). On average, ca. 60% of the electron density is contained inside the cavity and 90-95% within the first solvation shell. See Ref. 51 for more detail.

Why is the lifetime of p-like states of hydrated electrons so short. What is the structure of these p-like states. Are there other cavity states in water.> Disjoint states. Multicavity states How to prove their (non)existence experimentally ... 

Figure 1. Schematic representation ofthe hydrated electron obtained by molecular simulations [10]. The delocalised negative charge is surrounded by water molecules creating a cavity with a radius of about 2.5 A.

The radius of charge distribution of the hydrated electron has been estimated by several methods. The values obtained from hydration energies, encounter radius, i.e. the radius required to account for experimental diffusion controlled rate coefficients, and Jortner s cavity-continuum model are all in the range 0.25—0.30 nm [33]. This clearly indicates that the electron is not associated with a single water molecule only, but rather that the charge of the electron is smeared out over 3—4 water molecules. 

Paik, D. H., I.-R. Lee, D.-S. Yang, J. S. Baskin and A. H. Zewail (2004)Electrons in finitesized water cavities Hydration dynamics observed in real time. Science 306, 672-675 Pakkanen, T. A. (1996) Study of formation of coarse particle nitrate aerosol. Atmospheric Environment 30, 2475-2482... 

Resonance Raman and Temperature-Dependent Electronic Absorption Spectra of Cavity and Noncavity Models of the Hydrated Electron. 

L. Turi and A. Madarasz, Science, 331, 1387 (2011). Comment on Does the Hydrated Electron Occupy a Cavity . 

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