Aqueous solute-solvent interactions, vibrational

The solvent often exerts a profound influence on the quality and shape of the spectrum. For example, many aromatic chromophores display vibrational fine structure in non-polar solvents, whereas in more polar solvents this fine structure is absent due to solute-solvent interaction effects. A classic case is phenol and related compounds which have different spectra in cyclohexane and in neutral aqueous solution. In aqueous solutions, the pH exerts a profound effect on ionisable chromophores due to the differing extent of conjugation in the ionised and the non-ionised chromophore. In phenolic compounds, for example, addition of alkali to two pH units above the pKa leads to the classical red or bathochromic shift to longer wavelength, a loss of any fine structure, and an increase in molar absorptivity (hyper chromic... 

The efficiency of solvent-induced vibrational quenching is strongly enhanced as solute-solvent interactions increase in strength and specificity. Particularly important are the ubiquitous hydrogen bonding interactions of aqueous solutions. As a first step toward bridging the gap between gaseous... 

We applied the dual VFA approach to a neutral form (NF) glycine molecule in aqueous solution and compared the calculation results with those estimated by the conductor-like polarizable continuum model (CPCM) method in order to extract the explicit solvation effects. Table 8.1 shows a triple of typical vibrational frequencies (cugas, cocpcMi coee) of glycine molecule in the isolated state and in aqueous solution with their vibrational frequency shifts, (AcocpcM) Acofe). evaluated by two types of solute-solvent interactions, i.e., the CPCM and QM/MM method, scaled by the recommended factor of 0.9418 [43]. In addition, they were compared with the experimental values Acoexp obtained by the Fourier transform infrared (FT-IR)... 

The sensitivity of vibrational spectroscopy allows interactions of polyelectrolytes and surfactants to be monitored in aqueous and nonaqueous solutions. The solubilization and conformational properties of a comb-shaped copolymer of 1-octadecane-co-maleic anhydride in aqueous solution in the presence and absence of SDS depend on the degree of ionization of the copolymer (2i). The C-H stretching region of the Raman spectrum is sensitive to such interactions. Figure 11 illustrates how the C-H stretching band shifts as a function of solvent (in this case water and heptane). 

The reasons for micelle formation in organic solvents are somewhat different from those in aqueous solution. The main cause of micellization is the energy change due to dipole-dipole interactions between the polar head groups of the surfactant molecules. In certain cases hydrogen bond formation between head groups may also occur. Opposing micelle formation is the possible loss of translational, vibrational and rotational freedom of monomers when in the micelle. 

Investigation of electrode solution interfaces by in situ vibrational spectroscopy has two principal advantages firstly the species present and their structures are directly characterized by their spectra and, secondly, these spectra are sensitive to the environment and therefore can be used to probe complex interactions. Raman spectroscopy is particularly well suited to the investigation of aqueous systems and in certain cases the adsorption of neutral species, of anions in the double layer and of the solvent (as well as interactions between these species) can now be characterized[48]. Vibrational spectroscopy of systems of practical importance is illustrated by the Surface Enhanced Raman Spectra (SERS) of the corrosion inhibitor thiourea adsorbed at silver and copper electrodes[49] it should be noted that inhibitors such as thiourea are also used as plating additives. 

The ability of vibrational spectroscopy (infrared and Raman) to probe the different interactions which take place in a solution is well known. From the classic reviews by Irish and Brooker [1] and Gardiner [2], both published in 1977, which cover the Raman spectroscopy of ionic interactions in aqueous and nonaqueous solutions, a number of works have appeared reviewing vibrational spectroscopic studies [3-13]. However, many of these embrace only partial aspects or they are exclusively devoted to one specific type of solution (aqueous or nonaqueous) and do not include topics that will be discussed in the present chapter, such as solutions at high pressures and temperatures, electrolyte polymers, or solutions in the glassy state. The aims of this review are, as its title indicates, the ion-ion interactions whose theoretical aspects have been recently approached in a comprehensive monograph by Barthel and co-workers [14]. This means that aspects related to the Raman spectroscopic studies of the solvent s structure or the interactions between the solute and the solvent (ion hydration or, in general, solvation) will be treated briefly. 

When hv kT, the partition function ratio reduces to ftjH/< D and achieves its maximum value. Since for a vibration of normal isotopic sensitivity ojujoii, = 2, it is apparent that the contribution to the isotope effect from a thermally excited hydrogen vibration is the same as that from a free rotation. In practice weak hydrogen vibrations of a molecule normally correspond to hindered rotations arising from intermolecular interactions, and most commonly occur in solution. They can be important in proton-transfer reactions when, as is quite often the case, these are characterized by an acidic reactant in a hydrogen-bonding solvent. Extreme examples are provided by H2O and HaO", which in an aqueous lattice [26] achieve libration frequencies of about 700 cm" but in other instances lower frequencies occur. There is no difficulty in recognizing the possible intervention of such modes, since they correspond to free rotations of the... 

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