Solvated electron spectral transitions

Ethylene glycol is a very viscous liquid and the molecule presents two close OH groups. It has to be noticed that, among all the different solvents studied by pulse radiolysis, the transition energy of the solvated electron absorption band is maximum in liquid ethylene glycol. For these reasons, the electron in EG seems to have a special behaviour and it is of great interest to study the dynamics of the formation of equilibrated solvated electron. Within this context, the present communication deals with the dynamics of solvation in EG of electrons produced by photoionisation of the solvent at 263 nm. The formation of solvated electrons is followed by pump-probe transient absorption spectroscopy in the visible spectral range from 425 to 725 nm and also in near IR. For the first time, the absorption spectrum of the precursor of the equilibrated electron is observed in EG. Our results are shortly compared by those obtained in water and methanol. 

The 77-SCF MO Cl method has also been used446 to interpret spectral transitions of a series of possible intermediates in the reaction of uracil and cytosine with the solvated electrons eaq, produced by radiolysis of water. Experimentally this reaction has been investigated by Hayon,447 who used the technique of flash radiolysis. Hayon measured the optical-absorption spectra of the transient species in the UV range to obtain information on the site of attack of eaq on the pyrimidine base. At pH 5.0 the solvated electrons react with the pyrimidine molecules mainly at the C-2 and C-4 carbonyls, and the intermediates are rapidly protonatcd to give the corresponding ketyl radicals. For uracil Hayon found two absorption maxima (at 305 and < 280 nm) at pH 5.1 and one peak at 310 nm at pH 11.7. In this last case, on ionization of one of the chromophores the ketyl radical anion of the other nondissociated carbonyl is formed. Several species, 44, 45, 46, have been suggested by... 

Goulet T, Pepin C, Houde D, Jay-Gerin J-P. (1999) On the relaxation kinetics following the absorption of light by solvated electrons in polar liquids Roles of the continuous spectral shifts and of the stepwise transition. Radiat Phys Chem 54 441 48. 

It is well known experimentally that the ability of a solvent to interact differently with the ground and excited states typically involves much more than just its dielectric constant it may also depend on the details of the solvent-solute interaction and the solvent structure. The solvent polarity scale is an empirically based approach to express quantitatively the differential solvation of the ground and excited states of a solute.It uses the electronic spectral shift as a convenient one-parameter characterization of the ability of the solvent to interact with the solute. Several solvent polarity scales have been developed on the basis of the spectral shift of several different dye molecules. For example, one of the most widely used scales, called the Ej-(30) scale, is equal to the spectral shift in kcal/mol for the Jt n transition in pyridinium N-phenolate betaine dye. The Et(30) values, as well as other polarity scales, have been tabulated for many solvents. ... 

It is worth noting that the so-obtained spectral shift, i.e. the difference in transition energy between solvated and gas phase molecules, can be seen as made up of two contributions 1) a polarization contribution AEp, reflecting the effects of the polarized solvent on the electronic structure of the solute, and 2) an... 

When a solvent molecule becomes attached to a solute (which may be a reactant or a transition state), the electrons in frontier orbitals are affected to some extent. Therefore, when the solvated species undergoes an electron transition, absorption occurs at wavelengths that vary somewhat with the nature of the solvent. It is possible to make use of the spectral shifts (the so-called solvatochromic ect) to give some indication of the strength of solvent-solute interactions. 

Electronic spectroscopy is also used in making qualitative analyses of systems and to gain insight into the state or nature of a solute or solvent. Except on the rare occasions where the theoretical model is well developed, such studies take the traditional form of observing the spectral changes produced by variations of some suitable parameter of the system. Variation in solute spectrum may result from changes in its shape, in its nature, or just some modification of the transitions by the solvent. The following arbitrary selection of examples illustrates occasions where these structural, chemical and solvation effects modify the observed spectra. 

Table 6 Computed Solvent Spectral Shifts and Changes in Solvation Energy for the n - >17 Electronic Transition of Acetone (cm ) ...

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