Proton hydration in aqueous solution: Fourier transform infrared studies of HDO spectra

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 simple amides. FTIR spectra of HDO and theoretical studies

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.     

Hydration of aprotic donor solvents studied by means of FTIR spectroscopy

The paper attempts to explain the mutual influence of nonpolar and electron-donor groups on solute hydration, the problem of big importance for biological aqueous systems. Aprotic organic solvents have been used as model solutes, differing in electron-donating power. Hydration of acetonitrile, acetone, 2-butanone, and triethylamine has been studied by HDO and (partially) H2O spectra. The quantitative version of difference spectra method has been applied to determine solute-affected water spectra. Analysis of the data suggests that solvent-water interaction via the donor center of the solute is averaged between water-water interactions around the solute. Such behavior can be simply explained by the model of solute rotating in a cavity of water structure, which is formed by clathratelike hydrogen-bonded water network. On the basis of the band shape of solute-affected HDO spectra and the corresponding distribution of intermolecular distances, the criterion for hydrophobic type hydration has been proposed. From that point of view, all the studied solutes could be treated as hydrophobic ones. The limiting band position and the corresponding intermolecular distance of affected water, gained with increasing electron-donating power of solutes, has been inferred from the data obtained. These observations are important for interpretation of vibrational spectra of water as well as for volumetric measurements of solutions. The simple model of hydration, proposed to better justify the results, connects the values obtained from the methods providing microscopic and macroscopic characteristics of the system studied.     

Raman spectra from very concentrated aqueous NaOH and from wet and dry, solid, and anhydrous molten, LiOH, NaOH, and KOH

High-temperature, high-pressure Raman spectra were obtained from aqueous NaOH solutions up to 2NaOHH2O, with X(NaOH)=0.667 at 480 K. The spectra corresponding to the highest compositions, X(NaOH)> or =0.5, are dominated by H3O2-. An IR xi-function dispersion curve for aqueous NaOH, at 473 K and 1 kbar, calculated from the data of Franck and Charuel indicates that the OH- ion forms H3O2- by preferential H bonding with nonhydrogen-bonded OH groups. Raman spectra from wet to anhydrous, solid LiOH, NaOH, and KOH yield sharp, symmetric OH- stretching peaks at 3664, 3633, and 3596 cm(-1), respectively, plus water-related, i.e., H3O2-, peaks near LiOH, 3562 cm(-1), NaOH, 3596 cm(-1), and, KOH, 3500 cm(-1). Absence of H3O2- peaks from the solid assures that the corresponding melt is anhydrous. Raman spectra from the anhydrous melts yield OH- stretching peak frequencies: LiOH, 3614+/-4 cm(-1), 873 K; NaOH, 3610+/-2 cm(-1), 975 K; and, KOH, 3607+/-2 cm(-1), 773 K, but low-frequency asymmetry due to ion-pair interactions is present which is centered near 3550 cm(-1). The ion-pair-related asymmetry corresponds to the sole IR maximum near 3550 cm(-1) from anhydrous molten NaOH, at 623 K. Bose-Einstein correction of published low-frequency Raman data from molten LiOH revealed an acoustic phonon, near 205 cm(-1), related to restricted translation of OH- versus Li+, and an optical phonon, at 625 cm(-1) and tau approximately 0.05 ps, due to protonic precession and/or pendular motion. Strong H bonding between water and the O atom of OH- forms H3O2-, but the proton of OH- does not bond with H significantly. Large Raman bandwidths (aqueous solutions) are explained in terms of inhomogeneous broadening due to proton transfer in a double well. Vibrational assignments are presented for H3O2-.     

Fourier transform infrared spectroscopic and theoretical study of water interactions with glycine and its N-methylated derivatives

In this study we attempt to explain the molecular aspects of amino acids' hydration. Glycine and its N-methylated derivatives: N-methylglycine, N,N-dimethylglycine, and N,N,N-trimethylglycine were used as model solutes in aqueous solution, applying FT-IR spectroscopy as the experimental method. The quantitative version of the difference spectra method enabled us to obtain the solute-affected HDO spectra as probes of influenced water. The spectral results were confronted with density functional theory calculated structures of small hydration complexes of the solutes using the polarizable continuum model. It appears that the hydration of amino acids in the zwitterionic form can be understood allowing a synchronized fluctuation of hydrogen bonding between the solute and the water molecules. This effect is caused by a noncooperative interaction of water molecules with electrophilic groups of amino acid and by intramolecular hydrogen bond, allowing proton transfer from the carboxylic to the amine group, accomplishing by the chain of two to four water molecules. As a result, an instantaneous water-induced asymmetry of the carboxylate and the amino group of amino acid molecule is observed and recorded as HDO band splitting. Water molecules interacting with the carboxylate group give component bands at 2543 ± 11 and 2467 ± 15 cm(-1), whereas water molecules interacting with protons of the amine group give rise to the bands at 2611 ± 15 and 2413 ± 12 cm(-1). These hydration effects have not been recognized before and there are reasons to expect their validity for other amino acids.     

Theoretical studies on photoelectron and IR spectral properties of Br2.-(H2O)n clusters

We report vertical detachment energy (VDE) and IR spectra of Br2.-.(H2O)n clusters (n=1-8) based on first principles electronic structure calculations. Cluster structures and IR spectra are calculated at Becke's half-and-half hybrid exchange-correlation functional (BHHLYP) with a triple split valence basis function, 6-311++G(d,p). VDE for the hydrated clusters is calculated based on second order Moller-Plesset perturbation (MP2) theory with the same set of basis function. On full geometry optimization, it is observed that conformers having interwater hydrogen bonding among solvent water molecules are more stable than the structures having double or single hydrogen bonded structures between the anionic solute, Br2.-, and solvent water molecules. Moreover, a conformer having cyclic interwater hydrogen bonded network is predicted to be more stable for each size hydrated cluster. It is also noticed that up to four solvent H2O units can reside around the solute in a cyclic interwater hydrogen bonded network. The excess electron in these hydrated clusters is localized over the solute atoms. Weighted average VDE is calculated for each size (n) cluster based on statistical population of the conformers at 150 K. A linear relationship is obtained for VDE versus (n+3)(-1/3) and bulk VDE of Br2.- aqueous solution is calculated as 10.01 eV at MP2 level of theory. BHHLYP density functional is seen to make a systematic overestimation in VDE values by approximately 0.5 eV compared to MP2 data in all the hydrated clusters. It is observed that hydration increases VDE of bromine dimer anion system by approximately 6.4 eV. Calculated IR spectra show that the formation of Br2.--water clusters induces large shifts from the normal O-H stretching bands of isolated water keeping bending modes rather insensitive. Hydrated clusters, Br2.-.(H2O)n, show characteristic sharp features of O-H stretching bands of water in the small size clusters.     

Nature of the aqueous hydroxide ion probed by X-ray absorption spectroscopy

X-ray absorption spectra of aqueous 4 and 6 M potassium hydroxide solutions have been measured near the oxygen K edge. Upon addition of KOH to water, a new spectral feature (532.5 eV) emerges at energies well below the liquid water pre-edge feature (535 eV) and is attributed to OH- ions. In addition to spectral changes explicitly due to absorption by solvated OH- ions, calculated XA spectra indicate that first-solvation-shell water molecules exhibit an absorption spectrum that is unique from that of bulk liquid water. It is suggested that this spectral change results primarily from direct electronic perturbation of the unoccupied molecular orbitals of first-shell water molecules and only secondarily from geometric distortion of the local hydrogen bond network within the first hydration shell. Both the experimental and the calculated XA spectra indicate that the nature of the interaction between the OH- ion and the solvating water molecules is fundamentally different than the corresponding interactions of aqueous halide anions with respect to this direct orbital distortion. Analysis of the Mulliken charge populations suggests that the origin of this difference is a disparity in the charge asymmetry between the hydrogen atoms of the solvating water molecules. The charge asymmetry is induced both by electric field effects due to the presence of the anion and by charge transfer from the respective ions. The computational results also indicate that the OH- ion exists with a predominately "hyper-coordinated" solvation shell and that the OH- ion does not readily donate hydrogen bonds to the surrounding water molecules.     

Microhydration of the guanine-cytosine (GC) base pair in the neutral and anionic radical states: a density functional study

A density functional study of the effects of microhydration on the guanine-cytosine (GC) base pair and its anion radical is presented. Geometries of the GC base pair in the presence of 6 and 11 water molecules were fully optimized in the neutral (GC-nH2O) and anion radical [(GC-nH2O)*-] (n = 6 and 11) states using the B3LYP method and the 6-31+G** basis set. Further, vibrational frequency analysis at the same level of theory (B3LYP/6-31+G**) was also performed to ensure the existence of local minima in these hydrated structures. It was found that water molecules surrounding the GC base pair have significant effects on the geometry of the GC base pair and promote nonplanarity in the GC base pair. The calculated structures were found to be in good agreement with those observed experimentally and obtained in molecular dynamics (MD) simulation studies. The water molecules in neutral GC-nH2O complexes lie near the ring plane of the GC base pair where they undergo hydrogen bonding with both GC and each other. However, in the GC anion radical complexes (GC-nH2O, n = 6, 11), the water molecules are displaced substantially from the GC ring plane. For GC-11H2O*-, a water molecule is hydrogen-bonded with the C6 atom of the cytosine base. We found that the hydration shell initially destabilizes the GC base pair toward electron capture as a transient anion. Energetically unstable diffuse states in the hydration shell are suggested to provide an intermediate state for the excess electron before molecular reorganization of the water molecules and the base pair results in a stable anion formation. The singly occupied molecular orbital (SOMO) in the anion radical complexes clearly shows that an excess electron localizes into a pi orbital of cytosine. The zero-point-energy (ZPE-) corrected adiabatic electron affinities (AEAs) of the GC-6H2O and GC-11H2O complexes, at the B3LYP/6-31+G** level of theory, were found to be 0.74 and 0.95 eV, respectively. However, the incorporation of bulk water as a solvent using the polarized continuum model (PCM) increases the EAs of these complexes to 1.77 eV.     

Electron scattering on OH-(H2O)n clusters (n = 0-4)

The cross sections for electron scattering on OH-(H2O)n for n = 0-4 were measured from threshold to approximately 50 eV. All detachment cross sections were found to follow the classical prediction given earlier [Phys. Rev. Lett. 74, 892 (1995)] with a threshold energy for electron-impact detachment that increased upon sequential hydration, yielding values in the range from 4.5 eV +/- 0.2 eV for OH- to 12.10 eV +/- 0.5 eV for OH-(H2O)4. For n > or = 1, we found that approximately 80% of the total reaction events lead to electron detachment plus total dissociation of the clusters into the constituent molecules of OH and H2O. Finally, we observed resonances in the cross sections for OH-(H2O)3 and for OH-(H2O)4. The resonances were located at approximately 15 eV and were ascribed to the formation of dianions in excited states.     

Spectroscopic characterization of hydroxide and aqua complexes of Fe(II)-protoheme, structural models for the axial coordination of the atypical heme of membrane cytochrome b6f complexes

Electronic absorption and resonance Raman (RR) spectra are reported for hydroxide and aqua complexes of iron(II)-protoporphyrin IX (Fe(II)PP) respectively formed in alkaline and neutral aqueous solutions. These compounds with weak axial ligand(s) represent a biomimetic approach of the unusual coordination of the atypical heme c(i) of membrane cytochrome b6f complexes. Absorption spectra and spectrophotometric titrations show that Fe(II)PP in alkaline aqueous cetyltrimethylammonium bromide (CTABr) binds one hydroxide ion, forming a five-coordinated high-spin (HS) complex. In alkaline aqueous ethanol, we confirm the formation of a dihydroxy complex of Fe(II)PP. In the RR spectra of Fe(II)PP dissolved in neutral aqueous CTABr, a mixture of a four-coordinated intermediate spin form with an HS monoaqua complex (Fe(II)PP(H2O)) was observed. The spectroscopic information obtained for Fe(II)PP(OH-), Fe(II)PP(H2O), and Fe(II)PP(OH-)2 was compared with that previously reported for the 2-methylimidazole and 2-methylimidazolate complexes of Fe(II)PP, representative of the most common axial ligation in HS heme proteins. This investigation reveals a very remarkable analogy in the spectral properties of, in one hand, the Fe(II)PP(H2O) and mono-2-methylimidazole complexes and, in the other hand, the Fe(II)PP(OH-) and mono-2-methylimidazolate complexes. The comparisons of the absorption and RR spectra of Fe(II)PP(OH-) and Fe(II)PP(OH-)2 clearly establish that both a redshift of the pi-pi electronic transitions and an upshift of the v8 RR frequency are spectral parameters indicative of porphyrin doming in HS ferrous complexes. Based upon isotopic substitutions (16OH-,16OD-, and 18OH-), stretching modes of the Fe-OH bond(s) of a ferrous porphyrin were assigned for the first time, i.e., at 435 cm(-1) for Fe(II)PP(OH-) (nu(Fe(II)-OH-)) and at 421 cm(-1) for Fe(II)PP(OH-)2 (nu(s)(Fe(II)-(OH-)2). The spectroscopic and redox properties of Fe(II)PP(H2O), Fe(II)PP(OH-), and heme c(i) were discussed and favor a water coordination for the heme c(i) iron.     

Photoelectron spectroscopic study of the hydrated nucleoside anions: Uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1)

The hydrated nucleoside anions, uridine(-)(H(2)O)(n=0-2), cytidine(-)(H(2)O)(n=0-2), and thymidine(-)(H(2)O)(n=0,1), have been prepared in beams and studied by anion photoelectron spectroscopy in order to investigate the effects of a microhydrated environment on parent nucleoside anions. Vertical detachment energies (VDEs) were measured for all eight anions, and from these, estimates were made for five sequential anion hydration energies. Excellent agreement was found between our measured VDE value for thymidine(-)(H(2)O)(1) and its calculated value in the companion article by S. Kim and H. F. Schaefer III.     

Microhydration shell structure in Cl2*-.nH2O clusters: A theoretical study

We present the results of a detailed study on structure and electronic properties of hydrated cluster Cl2*-.nH2O (n = 1-7) based on a nonlocal density functional, namely, Becke's [J. Chem. Phys. 98, 1372 (1993)] half and half hybrid exchange-correlation functional with a split valence 6-311++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with various possible initial guess structures without any symmetry restriction. Several minimum energy structures (conformers) are predicted with a small difference in total energy. There is a competition between the binding of solvent H2O units with Cl2*- dimer radical anion directly through ion-molecule interaction and forming interwater hydrogen-bonding network in Cl2*-.nH2O (n > or = 2) hydrated cluster. Structure having interwater H-bonded network is more stable over the structure where H2O units are connected to the solute dimer radical anion Cl2*- rather independently either by single or double H bonding in a particular size (n) of hydrated cluster Cl2*-.nH2O. At the maximum four solvent H2O units reside in interwater H-bonding network present in these hydrated clusters. It is observed that up to six H2O units are independently linked to the anion having four double H bondings and two single H bondings suggesting the primary hydration number of Cl2*- to be 6. In all these clusters, the odd electron is found to be mostly localized over the two Cl atoms and these two atoms are bound by a three-electron hemibond. Calculated interaction (between solute and different water clusters) and vertical detachment energy profiles show saturation at n = 6 in the hydrated cluster Cl2*-.nH2O (n = 1-7). However, calculated solvation energy increases with the increase in number of solvent H2O molecules in the cluster. Interaction energy varies linearly with vertical detachment energy for the hydrated clusters Cl2*-.nH2O (n < or = 6). Calculation of the vibration frequencies show that the formation of Cl2*(-)-water clusters induces significant shifts from the normal stretching modes of isolated water. A clear difference in the pattern of IR spectra is observed in the O-H stretching region of water from hexa- to heptahydrated cluster.     

Vibrational and orientational dynamics of water in aqueous hydroxide solutions

We report the vibrational and orientational dynamics of water molecules in isotopically diluted NaOH and NaOD solutions using polarization-resolved femtosecond vibrational spectroscopy and terahertz time-domain dielectric relaxation measurements. We observe a speed-up of the vibrational relaxation of the O-D stretching vibration of HDO molecules outside the first hydration shell of OH(-) from 1.7 ± 0.2 ps for neat water to 1.0 ± 0.2 ps for a solution of 5 M NaOH in HDO:H(2)O. For the O-H vibration of HDO molecules outside the first hydration shell of OD(-), we observe a similar speed-up from 750 ± 50 fs to 600 ± 50 fs for a solution of 6 M NaOD in HDO:D(2)O. The acceleration of the decay is assigned to fluctuations in the energy levels of the HDO molecules due to charge transfer events and charge fluctuations. The reorientation dynamics of water molecules outside the first hydration shell are observed to show the same time constant of 2.5 ± 0.2 ps as in bulk liquid water, indicating that there is no long range effect of the hydroxide ion on the hydrogen-bond structure of liquid water. The terahertz dielectric relaxation experiments show that the transfer of the hydroxide ion through liquid water involves the simultaneous motion of ~7 surrounding water molecules, considerably less than previously reported for the proton.     

Microsolvation of the dicyanamide anion: [N(CN)(2)(-)](H(2)O)n (n = 0-12)

Photoelectron spectroscopy is combined with ab initio calculations to study the microsolvation of the dicyanamide anion, N(CN)(2)(-). Photoelectron spectra of [N(CN)(2)(-)](H2O)n (n = 0-12) have been measured at room temperature and also at low temperature for n = 0-4. Vibrationally resolved photoelectron spectra are obtained for N(CN)(2)(-), allowing the electron affinity of the N(CN)2 radical to be determined accurately as 4.135 +/- 0.010 eV. The electron binding energies and the spectral width of the hydrated clusters are observed to increase with the number of water molecules. The first five waters are observed to provide significant stabilization to the solute, whereas the stabilization becomes weaker for n > 5. The spectral width, which carries information about the solvent reorganization upon electron detachment in [N(CN)(2)(-)](H2O)n, levels off for n > 6. Theoretical calculations reveal several close-lying isomers for n = 1 and 2 due to the fact that the N(CN)(2)(-) anion possesses three almost equivalent hydration sites. In all the hydrated clusters, the most stable structures consist of a water cluster solvating one end of the N(CN)(2)(-) anion.     

Vibrational spectra and structure of CH3Cl:H2O, CH3Cl:HDO, and CH3Cl:D2O complexes. IR matrix isolation and ab initio calculations

The infrared spectra of CH3Cl + H2O isolated in solid neon at low temperatures have been investigated. The CH3Cl + H2O system is remarkable because of its propensity to form CH3Cl:H2O and CH3Cl:(H2O)n (n > or = 2) complexes. We focus here on the CH3Cl:H2O species. Low concentration studies (0.01-0.5%) and subsequent annealing lead to formation of the 1:1 CH3Cl:H2O complex with O-H. . .Cl-C or O. . .H-C intermolecular hydrogen bonds. Vibrational modes of this complex have been detected. In addition, spectra of D2O + CH3Cl and HDO + CH3Cl have also been recorded. A detailed vibrational analysis of partially deuterated species shows that HDO is exclusively D bonded to CH3Cl. This is a consequence of the preference for HDO to form a deuterium bonding complex rather than a hydrogen bonding one.     

On the structure of the matrix isolated water trimer

Infrared spectra of partially deuterated water trimers have been investigated. It is found that HDO(H(2)O)(2) has a single, bound OD stretching fundamental, (HDO)(2)H(2)O two bound OD stretches. (HDO)(3) has a single, bound OD stretch and (H(2)O)(3) has a pair of bound OH stretches. Ab initio and discrete Fourier transform (DFT) calculations predict that the water trimer has C(1) symmetry with six different, isoenergetic minima. These calculations consequently give three numerically different OD stretches for HDO(H(2)O)(2), six for (HDO)(2)H(2)O, three for (HDO)(3), and three bound OH stretches for (H(2)O)(3). The connection between the observations and the pseudorotation of the trimer is discussed with the help of Wales' pseudorotation model. It is found that pseudorotation is sufficiently fast to average the effective symmetry of the A(3) trimer to C(3h) and to eliminate the difference between the different ab initio minima for A(2)B. The only exception is (H(2)O)(3) where the splitting between the different bound OH stretches is largest. Here a doublet is observed due to incomplete averaging. DFT calculations indicate that the D-bonded form of HDO(H(2)O)(2) is between 50 and 60 cm(-1) more stable than the H-bonded form. The energy difference is determined by differences in zero point vibration energy of intermolecular librations of the two forms. Attempts to measure the energy difference indicate that the energy difference is larger, of the order of 100 cm(-1).     

Products of the addition of water molecules to Al3O3- clusters: structure, bonding, and electron binding energies in Al3O4H2-, Al3O5H4-, Al3O4H2, and Al3O5H4

Two stable products of reactions of water molecules with the Al3O3- cluster, Al3O4H2- and Al3O5H4-, are studied with electronic structure calculations. There are several minima with similar energies for both anions and the corresponding molecules. Dissociative absorption of a water molecule to produce an anionic cluster with hydroxide ions is thermodynamically favored over the formation of Al3O3-(H2O)n complexes. Vertical electron detachment energies of Al3O4H2- and Al3O5H4- calculated with ab initio electron propagator methods provide a quantitative interpretation of recent anion photoelectron spectra. Contrasts and similarities in these spectra may be explained in terms of the Dyson orbitals associated with each transition energy.     

Thermodynamics and kinetics of hydroxide ion formation in 12 CaO x 7 Al2O3

We have examined the thermodynamics and kinetics of hydroxide (OH-) ions that formed in cages of 12 CaO x 7 Al2O3 (C12A7) with nanoporous structures. It is confirmed using thermogravimetric-evolved gas analyses (TG-EGA) that hydration in C12A7 is mediated by a reaction between an oxide (O2-) ion in the cage and an H2O molecule in the atmosphere to form two OH- ions in the cages. To simply and exactly quantify the OH- content from infrared absorption measurements of OH-stretching band, we propose a method combined with a thermodynamic analysis, allowing the simultaneous determination of the molar extinction coefficient of the OH-band, enthalpy, and entropy for the hydration. Hydration enthalpy in C12A7 is extremely high compared with other oxides and was enhanced by the marked instability of O2- ion in the cage. Consequently, high solubility of OH- ion is retained up to unusually high temperatures. Furthermore, we determined diffusion coefficients of species relevant to the hydration process and demonstrated that inward diffusion of OH- ions is the rate-determining process.     

From Molecule to Cluster to Bulk: Water OH Vibrations in Different Surroundings

Vibrational spectra for the O—H stretching motion of HDO molecules in different surroundings have been calculated by quantum mechanical ab initio methods and compared with experimental spectra. The free water molecule, water chains, and ion–water clusters are discussed. Solvent effects on OH vibrations in liquid water have been calculated as well as “in-crystal” OH frequencies in some ice and ionic crystalline hydrate structures. The importance of nonadditivity effects, electron correlation (at the mp 2 level), and long-range interactions for the total frequency downshift is demonstrated. It is shown that the inclusion of these effects, in conjunction with a variational quantum mechanical treatment of the anharmonic vibrational stretching motion (force constants up to the fourth order), yields vibrational frequencies in quantitative agreement with experiment for a wide range of aqueous systems.     

Photoelectron spectroscopy of hydrated hexafluorobenzene anions

We present a synergetic experimental/theoretical study of hydrated hexafluorobenzene anions. Experimentally, we measured the anion photoelectron spectra of the anions, C6F6(-)(H2O)n (n=0-2). The spectra show broad peaks, which shift to successively higher electron binding energies with the addition of each water molecule to the hexafluorobenzene anion. Complementing these results, we also conducted density functional calculations which link adiabatic electron affinities to the optimized geometric structures of the negatively charged species and their neutral counterparts. Neutral hexafluorobenzene-water complexes are not thought to be hydrogen bonded. In the case of C6F6(-)(H2O)1, however, its water molecule was found to lie in the plane of the hexafluorobenzene anion, bound by two O-H...F ionic hydrogen bonds. Whereas in the case of C6F6(-)(H2O)2, both water molecules also lie in the plane of and are hydrogen bonded to the hexafluorobenzene anion but on opposite ends. This study and that of Schneider et al. [J. Chem. Phys. 127, 114311 (2007), preceding paper] are in agreement regarding the geometry of C6F6(-)(H2O)1.