Hydroxide ion hydration was studied in aqueous solutions of selected alkali metal hydroxides by means of Fourier transform infrared (FTIR) spectroscopy of HDO isotopically diluted in H2O. The quantitative difference spectra procedure was applied for the first time to investigate such systems. It allowed removal of bulk water contribution and separation of the spectra of solute-affected HDO. The obtained spectral data were confronted with ab initio calculated structures of small gas-phase and polarizable continuum solvation model (PCM) solvated aqueous clusters, OH-(H2O)n, n = 1-7, to establish the structural and energetic states of hydration spheres of the hydrated hydroxide anion. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. The energetic state of water in OH- hydration shells, as revealed by solute-affected HDO spectra, is similar to that of an isoelectronic F- anion. No evidence was found for the existence of stable hydroxide dimer, H3O2-, in an aqueous solution. Spectral data do confirm, however, existence of a weak interaction with a single water molecule at the hydrogen site of OH-.

Hydration of KPF 6 , NaPF 6 , and LiPF 6 has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H 2 O. The difference spectra procedure has been applied, which allowed removal of the contribution of bulk water and thus separation of the spectra of solute-affected HDO. In all cases, a high-wavenumber component band at 2667 cm -1 is present that corresponds to HDO affected by the PF 6anion, which is known as an extreme structure breaker. The band at ca. 2540 cm -1 corresponds to HDO affected by all studied cations, and another component band at ca. 2445 cm -1 is additionally visible in the case of the Li + cation. The probability distribution functions of interatomic O‚‚‚O distances for HDO affected by the cations clearly display a lower-distance structure for Li + , contrary to both other cations. The existence of a band situated at 2630 cm -1 for each of the salts is also significant. Its previous interpretation has been ambiguous. On the basis of our current results and known literature data, we ascribe this band to the outermost hydration sphere of cations, which is possible to detect with IR spectroscopy, which depends on the kind of anion.

This paper attempts to elucidate the number and nature of the hydration spheres around the proton in an aqueous solution. This phenomenon was studied in aqueous solutions of selected acids by means of Fourier transform infrared spectroscopy of semiheavy water (HDO), isotopically diluted in H(2)O. The quantitative version of difference spectrum procedure was applied for the first time to investigate such systems. It allowed removal of bulk water contribution and separation of the spectra of solute-affected HDO. The obtained spectral data were confronted with ab initio calculated structures of small gas-phase and polarizable continuum model (PCM) solvated aqueous clusters, H+(H2O)n, n=2-8, in order to help in establishing the structural and energetic states of the consecutive hydration spheres of the hydrated proton. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. The structure of proton hydration shells outside the first hydration sphere essentially follows the model structure of other hydrated cations, previously revealed by affected HDO spectra. The first hydration sphere complex in diluted aqueous solutions was identified as an asymmetric variant of the regular Zundel cation [The Hydrogen Bond: Recent Developments in Theory and Experiments, edited by P. Schuster, G. Zundel, and C. Sandorfy (North-Holland, Amsterdam, 1976), Vol. II, p. 683], intermediate between the ideal Zundel and Eigen structures [E. Wicke et al., Z. Phys. Chem. Neue Folge 1, 340 (1954)]. Evidence was found for the existence of strong and short hydrogen bonds, with oxygen-oxygen distance derived from the experimental affected spectra equal 2.435 A on average and in the PCM calculations about 2.41-2.44 A. It was also evidenced for the first time that the proton possesses four well-defined hydration spheres, which were characterized in terms of hydrogen bonds' lengths and arrangements. Additionally, an outer hydration layer, shared with the anion, as well as loosely bound water molecules interacting with free electron pairs of the central complex were detected in the affected spectra.

Hydration of carboxylate ions was studied in aqueous solutions of sodium salts by means of FTIR spectroscopy using the HDO molecule as a probe. The quantitative version of the difference spectra method has been applied to determine the solute-affected water spectra. They display two-component bands of affected HDO at ca. 2550 and 2420 cm(-1). These bands are attributed to the -COO(-) group of the R-COO(-) ion (R = H, CH(3), C(2)H(5)), because water molecules surrounding the substituent R behave roughly as molecules in the bulk phase. For the studied carboxylates the net water structure making effect is observed, which increases with electron-donor ability of R, by means of changing the relative intensity of solute-affected HDO component bands. The observed splitting of the carboxylate-ion-affected HDO band is unique for these anions. The experimental results were confronted with DFT-calculated structures of small gas-phase and polarizable continuum model (PCM) solvated aqueous clusters to establish the structural and energetic states of carboxylate ions hydrates. This was achieved by comparison of the calculated optimal geometries with the interatomic distances derived from HDO band positions. Different possibilities have been considered to explain the peculiar spectral results. The plausible explanation assumes symmetry breaking of the carboxylate ion induced by interaction with water solvent: C-O bond lengths of RCOO(-) and electric charge localization become unequal. It is demonstrated by nonequivalent interaction of oxygen atoms of the RCOO(-) anion with water molecules. Taking into account only the energetic effect, the phenomenon is explained by the anticooperative H-bond formation of the carboxylate group with water molecules, which increases with the electron-donor ability of the substituent R. In this interaction two water molecules play an important part, as appears from the calculated clusters. They interact with oxygen atoms of the RCOO(-) ion, forming a cooperative system, within which solvent molecules are nonequivalent with respect to H-bond formation with both proton-accepting sites of the solute. This additionally enhances solvent-induced symmetry breaking of carboxylate anion. Strongly hydrogen-bonded solvent is more effective in inducing symmetry breaking; thus, increasing the temperature decreases the splitting of the carboxylate-ion-affected water, as experimentally observed.

The hydration of formamide (F), N-methylformamide (NMF), N,N-dimethylformamide (DMF), acetamide (A), N-methylacetamide (NMA), and N,N-dimethylacetamide (DMA) has been studied in aqueous solutions by means of FTIR spectra of HDO isotopically diluted in H2O. The difference spectra procedure has been applied to remove the contribution of bulk water and thus to separate the spectra of solute-affected HDO. To facilitate the interpretation of obtained spectral results, DFT calculations of aqueous amide clusters were performed. Molecular dynamics (MD) simulation for the cis and trans forms of NMA was also carried out for the SPC model of water. Infrared spectra reveal that only two to three water molecules from the surrounding of the amides are statistically affected, from among ca. 30 molecules present in the first hydration sphere. The structural-energetic characteristic of these solute-affected water molecules differs only slightly from that in the bulk and corresponds to the clathrate-like hydrogen-bonded cage typical for hydrophobic hydration, with the possible exception of F. MD simulations confirm such organization of water molecules in the first hydration sphere of NMA and indicate a practical lack of orientation and energetic effects beyond this sphere. The geometry of hydrogen-bonded water molecules in the first hydration sphere is very similar to that in the bulk phase, but MD simulations have affirmed subtle differences recognized by the spectral method and enabled their understanding. The spectral data and simulations results are highly compatible. In the case of F, NMF, and A, there is a visible spectral effect of water interactions with N-H groups, which have destabilizing influence on the amides hydration shell. There is no spectral sign of such interaction for NMA as the solute. The energetic stability of water H-bonds in the amide hydration sphere and in the bulk fulfills the order: NMA > DMA > A > NMF > bulk > DMF > F. Microscopic parameters of water organization around the amides obtained from the spectra, which have been used in the hydration model based on volumetric data, confirm the more hydrophobic character of the first three amides in this sequence. The increased stability of the hydration sphere of NMA relative to DMA and of NMF relative to DMF seems to have its origin in different geometries, and so the stability, of water cages containing the amides.