Volume 82, Issue 10

The 31st International Conference on Solution Chemistry (ICSC-31) was held 21-25 August 2009 in Innsbruck, Austria. This conference series covers a wide range of topics related to solution chemistry, such as spectroscopy, thermodynamics, and kinetics to name just a few, addressing experimentalists and theoreticians alike. Seventeen outstanding plenary and invited lectures were given by renowned scientists. The topics of the plenaries were solution NMR stuctures of proteins (J. Wüthrich), porous coordination polymers (S. Kitagawa), ab initio-based water potentials for simulation studies (S. Xantheas), and the hydration structures of metal ions in solution (I. Persson). Additionally, 33 oral contributions were presented and two poster sessions were held. A total of 29 countries were represented in this conference. The best poster award was given to Matjaz Boncina for his poster entitled "Thermodynamics of the ion–lysozyme association". Pierre Turq presented details of the next ICSC meeting to be held in 2011 in La Grande Motte, France. Eight papers based on lectures presented at the ICSC-31 are included in this issue of Pure and Applied Chemistry . The manuscripts cover investigations using different spectroscopic approaches, molecular simulation studies as well as thermodynamic measurements. The systems treated range from pure water and aqueous solution to ionic liquids and solutions containing polyelectrolytes. These contributions feature the major themes of the conference, serve as a representative view of current activities in the field of solution chemistry, and demonstrate that solutions still prove to be challenging targets for contemporary physical and chemical research alike. Thomas Hofer Bernhard Randolf Conference Editors

Recently, there have been a number of claims about the nature of water as a liquid that seem to contradict traditional views. The present paper takes a close look at two of these claims—namely, that water is not the tetrahedral network that it is traditionally regarded as but a chain-like liquid, and that water is intrinsically heterogeneous by nature—and attempts to make sense of them.

Vibrational spectroscopy is an ideally suited tool for the study of solute hydration. Nevertheless, water is commonly considered by spectroscopists a difficult solvent to work with. However, by using the isotopic dilution technique, in which a small amount of D 2 O is introduced into H 2 O or vice versa with formation of semiheavy water (HDO), many technical and interpretative problems connected with measurement of infrared spectra of water may be circumvented. Particularly, the isotopic decoupling of stretching vibrational modes greatly simplifies interpretation of the spectra. Systematic studies conducted in several laboratories since the 1980s up to the present day have provided a vast amount of data, concerning mainly ionic hydration. Many of these experiments have been performed in our laboratory. The analysis method we applied is based on the quantitative version of the difference spectra technique and allows separation of the spectrum of solute-affected HDO from the bulk solvent. This review illustrates the development of vibrational spectroscopy of HDO and spectral analysis methods over the years, as well as summarizes the results obtained for ionic and nonionic solutes, including some general hydration models formulated on their basis.

Water is a highly structured liquid, as indicated by its stiffness (cohesive energy density) and being ordered (large entropy of vaporization), its three-dimensional hydrogen-bonded network being its most outstanding feature. The extent of this network depends on the definition of the hydrogen bond, and both computer simulation and thermodynamic data yield a consistent picture. The effects of ions on this network in dilute aqueous solutions have been studied from dynamic, thermodynamic, spectroscopic, and computer simulation aspects. A classification of ions from those that are highly water-structure-breaking ions through borderline cases to those that are highly water-structure-making ions results from such studies.

The hydration reaction is defined as the transfer of an ion or neutral chemical species from the gaseous phase into water, M n + (g) → M n + (aq). In this process, water molecules bind to metal ions through ion-dipole bonds of mainly electrostatic character. The hydration reaction is always strongly exothermic with increasing heat of hydration with increasing charge density of the ion. The structures of the hydrated metal ions in aqueous solution display a variety of configurations depending on the size and electronic properties of the metal ion. The basic configurations of hydrated metal ions in aqueous solution are tetrahedral, octahedral, square antiprismatic, and tricapped trigonal prismatic. This paper gives an overview of the structures of hydrated metal ions in aqueous solution with special emphasis on those with a non-regular coordination figure. Metal ions without d-electrons in the valance shell form regular aqua complexes with a coordination figure, allowing a maximum number of water molecules to be clustered around the metal ion. This number is dependent on the ratio of the metal ion radius to the atomic radius of oxygen in a coordinated water molecule (1.34 Å). The lighter lanthanoid(III) ions have a regular tricapped trigonal prismatic configuration with the M–O distance to the capping water molecules somewhat longer than to the prismatic ones. However, with increasing atomic number of the lanthanoid(III) ions, an increasing distortion of the capping water molecules is observed, resulting in a partial loss of water molecules in the capping positions for the heaviest lanthanoids. Metal ions with d 4 and d 9 valance shell electron configuration, as chromium(II) and copper(II), respectively, have Jahn–Teller distorted aqua complexes. Metal ions with low charge and ability to form strong covalent bonds, as silver(I), mercury(II), palladium(II), and platinum(II), often display distorted coordination figures due to the second-order Jahn–Teller effect. Metal ions with d 10 s 2 valence shell electron configuration may have a stereochemically active lone electron pair (hemi-directed complexes) or an inactive one (holo-directed). The hydrated tin(II), lead(II), and thallium(I) ions are hemi-directed in aqueous solution, while the hydrated bismuth(III) ion is holo-directed. The structures of the hydrated cationic oxo-metal ions are reported as well.

The electron photodetachment of the aqueous halides and hydroxide is studied after resonant excitation in the lowest charge-transfer-to-solvent (CTTS) state. The initially excited state is followed by an intermediate assigned to a donor-electron pair that displays a competition of recombination and separation. Using pump–repump–probe (PREP) spectroscopy, the pair species is verified via a secondary excitation with separation of the pairs so that the yield of released electrons is increased. The observed recombination process on the one hand and the similar absorptions of the intermediate and the hydrated electron on the other hand suggest that the donor-electron pairs incorporate only few if not just one water molecule. The geminate dynamics measured in the various CTTS systems reveal a strong influence of the parent radical. The electron survival probability decreases significantly from 0.77 to 0.29 going from F – to OH – . The extracted dissociation rates of the halogen-electron pairs seem to be proportional to the mutual diffusion coefficients of the geminate particles, while such a relation between the recombination rate and the diffusion coefficient is not found. Results for I – show that excitation of a higher-lying CTTS state opens a new relaxation channel, which directly leads to a fully hydrated electron, while the relaxation channel discussed above is not significantly affected.

Ionic liquids (ILs) are expected to have specific properties as solvents for chemical reactions in view of solution chemistry. Among physicochemical properties, liquid structure, acid–base, and electron-pair donating and accepting abilities of solvent play a crucial role in ion-solvation and acid–base, metal-ion complexation, and electrochemical reactions. Various types of ILs have been developed, and among others, the bis(trifluoromethanesulfonyl)amide (TFSA – )-based ILs are extensively used. TFSA – is a flexible molecule to give two stable conformers, cis (C1) and trans (C2), which are present in equilibrium in the liquid state. The conformational equilibrium shifts upon solvation to the metal ion. This is quantitatively studied to obtain thermodynamic parameters of conformational change from C2 to C1 in the bulk and in the solvation sphere of the lithium ion. On the other hand, with ethylammonium nitrate (EAN), a typical protic IL, it is revealed that the ammonium group is hydrogen-bonded with three nitrate ions to form a heterogeneous liquid structure. The solvent acid–base property of EAN and acid dissociation reaction in EAN have been quantitatively revealed, and the results will be discussed in comparison with those in normal molecular solvents.

An explicit water molecular dynamics (MD) simulation is presented of a solution modeling aliphatic 6,6-ionene oligocations mixed with low-molecular-weight electrolytes. In all cases, the co-ions were sodium cations and the counterions were fluoride, chloride, bromide, or iodide anions. The simple point charge/extended (SPC/E) model was used to describe water. The results of the simulation at T = 278 K (the data for 298 K were obtained earlier) and T = 318 K are presented in the form of pair distributions between various atoms and/or between ions in the system. We were interested in how temperature variation modifies the ion-specific effects, revealed by the various pair distribution functions (PDFs). The results were compared with previous calculations for the less hydrophobic 3,3-ionene solutions. Simulations of 6,6-ionene solutions containing mixtures of fluoride and iodide counter-ions at T = 298 K were also presented.

The light source of a photometer may induce chemical reactions in photosensitive reactive systems. Diode-array spectrophotometers are particularly suitable for producing such phenomena. This paper provides an overview on how this equipment can be used as a photoreactor. The principles of various techniques to control the intensity and spectral region of the illuminating light are discussed in detail. It will be shown that the quantum yields of various photochemically induced redox reactions can be determined by exploiting specific features of diode-array spectrophotometers. Kinetic coupling between primary photo-chemical and secondary thermally activated reaction steps are utilized to explore intimate details of composite redox reactions. Key aspects of the method applied are demonstrated via the photoreactions of 2,6-dichloro-1,4-benzoquinone (DCQ), the photoinduced autoxidation of S(IV) and a photochemically activated redox reaction of the chlorate ion.