Solute-solvent hydrogen-bonded complexe

Analogous effects can, of course, be expected if one of the reactants participating in the hydrogen bond is the solvent itself. However, a solvent effect also arises in solutions containing hydrogen-bonded complexes, or other associates formed as a consequence of donor-acceptor interactions. Complexes of moderate polarity are particularly sensitive to the effect of the solvent [Ja 72, Pa 75]. 

It can therefore be noticed [68] that the complete tautomerization in this type of a solute-solvent complex may be blocked, since large solvent reorganization of solute-solvent hydrogen bonds is often accompanied by radiationless deactivation of an excited fluorophore. 

Similar measurements have given values for the fractionation factor of hydrogen-bonded complexes of the fluoride ion (Emsley et al., 1986c) and the acetate ion (Clark et al., 1988a) in acetic acid solution, [20] and [21]. For the chloride ion in acetic acid, the result (Emsley et al., 1986c) was cp = 1.26, which means that the exchangeable sites in acetic acid molecules in the solvation sphere of the chloride ion are favoured by deuterium compared to the sites in the bulk solvent. 

In other words, since for solvents with H-bonding ability (methanol and water) the aN of the nitroxide radical is shifted to higher values because of the influence of one or more hydrogen bonds between the solute and the solvent, it becomes necessary to build a model in which nonspecific effects are described in terms of continuum polarizable medium with a dielectric constant typical of the protic solvent under study, whereas specific effects are taken into account through an explicit hydrogen-bonded complex between the radical and some solvent molecules. Figure 2.6 reports the aN values for the complexes formed by TEMPO with phenol, methanol, and water measured experimentally at room temperature, and computed in the gas phase and in solution. The values computed in solution fit the experimental data quite well. 

Within the Hush formalism of electron transfer [129], the electronic coupling through hydrogen bond interfaces may be deduced from the intensity of a mixed-valence transition between juxtaposed donor-acceptor pairs. We highlight the only such detailed study in this section. Curtis and coworkers have studied a collection of hydrogen-bonded mixed-valence adducts formed between ruthenium(II) electron-donor and ruthenium(III) electron-acceptor complexes in solution [130]. Using acetonitrile or nitromethane as a solvent, hydrogen-bonded assemblies of the type [(tpy)(bpy)Ru (CN)]2, (en)2Ru (bpy) 5+ 45 (bpy = 2,2 -bipyridine, tpy = 2, 6", 2 -terpyridine, en = ethylenediamine) and [(bpy)2Ru (CN)2]2, (en)2Ru (bpy) +... 

It has been known for several years that the degree of association for hydrogen bonded complexes in solution is shifted in favor of the free species in Q, Br or I containing solvents with respect to hydrocarbon or other, more inert solvents ... 

Specific interactions between solvent and solute, such as hydrogen bonding, can cause quite large effects (as). It is not known whether the ASIS is caused by a time-averaged cluster of solvent molecules about a polar functional group or by a 1 1 solute-solvent charge transfer complex. In the latter case, the ASIS is more legitimately classified under as than under a. ... 

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