Aqueous solutions vibrational spectroscopic studies

The formation of complexes in aqueous solution may be studied by a number of methods. Physical methods (e.g. electronic and vibrational spectroscopic, solubility or conductivity measurements) usually provide reliable information and, in some cases, allow the determination of equilibrium constants for complex formation. It is also possible to test for modifications of chemical properties, but this has to be carried out with caution. All reactions are equilibria, and chemical tests are often only investigations of relative values of equilibrium constants. For example, in an aqueous solution of an Ag" " salt saturated with NH3, nearly aU the Ag" " is present as the complex [Ag(NH3)2] (eq. 7.62). 

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. 

Bickley Rl, Edwards HGM, Gustar R, Rose SJA(1991) Vibrational spectroscopic study of nickel(ll) citrate Ni3(C. H50,)2 and its aqueous solutions. J Mol Struct 246 217-228... 

In this report we describe novel pressure tuning vibrational spectroscopic techniques that can be used to study aqueous surfactant solutions and discuss in some detail two examples of such studies with micellar solutions of anionic surfactants, one using Fourier transform infrared (FT-IR) and another using Raman spectroscopy. 

Raman optical activity (RO A) Due to molecular chirality there is a difference in the intensity of Raman scattered right and left circularly polarized light. Raman optical activity (ROA) is a vibrational spectroscopic technique that is reliant on this difference and the spectrum of intensity differences recorded over a range of wavenumbers reveals information about chiral centers within a sample molecule. It is a useful probe to study biomolecular structures and their behavior in aqueous solution especially those of proteins, nucleic acids, carbohydrates, and viruses. The information obtained is in realistic conditions... 

Second, combined evidence from theoretical computer modeling studies of short peptides (too short to form any detectable a-helix or (3-sheet) in aqueous solution and a variety of spectroscopic studies, including ultraviolet CD (Rucker et al., 2002), nuclear magnetic resonance (NMR) (Poon et al., 2000), two-dimensional vibrational spectroscopy (Woutersen and Hamm, 2001), vibrational circular dichroism (VCD) (Keiderling et al., 1999), and vibrational Raman spectroscopy (Blanch et al., 2000), reveal that the PPII helix is the dominant conformation in a variety of these short peptides. 

Raman spectroscopy is a vibrational spectroscopic technique which can be a useful probe of protein structure, since both intensity and frequency of vibrational motions of the amino acid side chains or polypeptide backbone are sensitive to chemical changes and the microenvironment around the functional groups. Thus, it can monitor changes related to tertiary structure as well as secondary structure of proteins. An important advantage of this technique is its versatility in application to samples which may be in solution or solid, clear or turbid, in aqueous or organic solvent. Since the concentration of proteins typically found in food systems is high, the classical dispersive method based on visible laser Raman spectroscopy, as well as the newer technique known as Fourier-transform Raman spectroscopy which utilizes near-infrared excitation, are more suitable to study food proteins (Li-Chan et aL, 1994). In contrast the technique based on ultraviolet excitation, known as resonance Raman spectroscopy, is more commonly used to study dilute protein solutions. 

Raman spectroscopy has been successfully employed to follow the kinetics of reactions at HTR Kessler et al. [197] studied the decomposition of tertiary butyl peroxypivalate in solution, obtaining the same activation energy as those which have been obtained by other conventional methods. Brill et al. have recently published an interesting series of articles [198-200] on the vibrational spectroscopy [Fourier transform infrared (FTIR) and Raman] of hydrothermal reactions, covering the decomposition of species such as urea, ammonium carbonate, cyanamide, or dicyandiamide. All of these works demonstrate the ability of vibrational spectroscopic techniques to obtain detailed information about the composition and evolution of aqueous systems in extreme conditions,... 

Recent advances in the development of non-invasive, in situ spectroscopic scanned-probe and microscopy techniques have been applied successfully to study mineral particles in aqueous suspension (Hawthorne, 1988 Hochella and White, 1990). In situ spectroscopic methods often utilise molecular probes that have diagnostic properties sensitive to changes in short-range molecular environments. At the particle-solution interface, the molecular environment around a probe species is perturbed, and the diagnostic properties of the probe, which can be either optical or magnetic, then report back on surface molecular structure. Examples of in situ probe approaches that have been used fruitfully include electron spin resonance (ESR) and nuclear magnetic resonance (NMR) spin-probe studies perturbed vibrational probe (Raman and Fourier-transform IR) studies and X-ray absorption (Hawthorne, 1988 Hochella and White, 1990 Charletand Manceau, 1993 Johnston et al., 1993). 

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