HOH Bending Modes Research Articles

Modeling of the Hydrogen Bond-Induced Frequency Shifts of the HOH and HOD Bending Modes of Water.

The intramolecular bending mode of water is a possible useful probe of the hydrogen-bond situations in aqueous systems, but the behavior of its frequency and intensity should be further elucidated for better understanding on its nature and, hence, for its better utilization as a probe. Here, an analysis toward this goal is conducted by doing theoretical calculations on molecular clusters of normal isotopic and deuterated species of water and examining the correlations among the vibrational, structural, and electrostatic properties. It is shown that electrostatic interactions, particularly both of the in-plane components of the electric field along the OH bond and perpendicular to it, play a major role in controlling the hydrogen bond-induced shifts of the force constant, but additional factors, including the intermolecular structural and/or charge-transfer properties, are also important. Models of the hydrogen bond-induced shifts of the force constant are presented in a form that may be combined with classical molecular dynamics. With regard to the infrared intensity changes, it is shown on the basis of the electron density analysis that the intermolecular charge flux and polarization effect play an important role, depending on the angular characteristics of the hydrogen bond.

Interactions between adsorbed hydrogen and water molecules on a polycrystalline Pt electrode studied by in-situ ATR-FTIR spectroscopy

In order to gain further insight into the interactions between hydrogen adsorbed on platinum and water in an acid electrolyte, in situ infrared spectra were examined in detail. A polycrystalline Pt electrode/sulfuric acid solution interphase was observed by in-situ attenuated total reflectance (ATR) FTIR spectroscopy in the potential region between the double layer and hydrogen evolution by use of the potential difference method. The background potential in the measurements was chosen first at 0.7 V vs. RHE to examine the effect of underpotentially deposited hydrogen (HUPD) adsorption on the interfacial water spectra. Analysis of the spectra revealed two well-defined HOH bending bands, a sharp one at 1620 cm−1 and a broad one at 1700 cm−1. The intensity of the 1620 cm−1 band was found to increase linearly with increasing coverage of HUPD determined by the cyclic voltammogram (CV) corrected for the HER current. This led to an assignment to water molecules co-adsorbed with HUPD with only weak interactions with the latter and between each other. The broad band at 1700 cm−1 was assigned to the HOH bending mode of water associated with hydronium ions. Its intensity was found to increase at lower potentials due to its electrostatic attraction to the increasingly negative surface charge. An intense, broad, negative-going band around 2970 cm−1 was found to develop below 0.4 V, which is assigned to water dimers adsorbed at on-top Pt sites in the double layer potential region and desorbed upon adsorption of HUPD. A new choice of the reference potential at 0.2 V was successful in detecting a distinct OH stretching band around 3580 cm−1, which developed in close correlation with the band assigned to overpotentially deposited hydrogen (HOPD) at ca. 2080 cm−1. The band was assigned to the asymmetric OH stretching mode of water molecules hydrogen-bonded with HOPD. Its band intensity was found to increase linearly with that of HOPD with a slope close to 1.0, which suggests a highly polar nature of the Pt-HOPD bond.

Character of the OH Bend-Stretch Combination Band in the Vibrational Spectra of the "Magic" Number H3O+(H2O)20 and D3O+(D2O)20 Cluster Ions.

The fundamental transitions that contribute to the diffuse OH stretching spectrum of water are known to increase in width and intensity with increasing red shift from the free OH frequency. In contrast, the profile of the higher-energy combination band involving the OH stretching and the intramolecular HOH bending modes displays a qualitatively different spectral shape with a much faster falloff on the lower-energy side. We elucidate the molecular origin of this difference by analyzing the shapes of the combination bands in the IR spectra of cryogenically cooled H3O+(H2O)20 and D3O+(D2O)20 clusters. The difference in the shapes of the bands is traced to differences in the dependence of their transition dipole matrix elements on the hydrogen-bonding environment. The fact that individual transitions across the combination band envelope have similar intensities makes it a useful way to determine the participation of various sites in extended H-bonding networks.

Solute-induced changes in the water H-bond network of different alcohol-aqueous systems

In this work, alcohol-water solutions for eight monohydric and two polyhydric alcohols at different concentrations and temperatures were analyzed by infrared spectroscopy to investigate the perturbation of the structure of water surrounding the amphiphilic alcohol molecule. The solute-induced structural changes of water were studied based on the solute-correlated spectral component, which reflects the vibrational properties of the solute and the water molecules perturbed by the solute. This component was obtained from the spectral decomposition using the vibrational multivariate curve resolution method. We used the HOH bending mode as the main probe to investigate the water hydrogen bonding network and estimate the number of water molecules perturbed by the solute molecule. In addition, the CH and CO stretching modes were analyzed to gain a more comprehensive insight into the hydration of these amphiphilic molecules and to provide a reasonable explanation for the observed phenomena.The blue shifts we identified in the HOH bending frequency from the solute-correlated spectra can be attributed to the water molecules with strengthened hydrogen bonds (i.e., the water molecules perturbed by the solute). The perturbation of the water H-bond network is found to depend on the size and shape of the hydrophobic moiety of alcohol, although the differences are small, i.e., the number of perturbed water molecules per alcohol molecule varies depending on the type of alcohol, but is generally between 1 and 3. It seems that this small perturbation of the solvent–water structure does not occur at the immediate water contacts, but rather at the distances beyond the immediate neighbor contact distance. A strong indication of the latter is the relatively large fraction of the bulk-like water (compared to the fraction of perturbed water) still present even in the highly concentrated alcohol solutions. In summary, the number of perturbed water molecules testifies to the fact that it is unlikely that hydration of alcohols at atmospheric pressure and at room temperature and above will result in larger distorted structural units of water different from those found in bulk water.

Vibrational spectroscopy of uranium tetrafluoride hydrates

Uranium tetrafluoride (UF4) is an important intermediate in the production of UF6 and nuclear fuel. Historical characterization of UF4 with Raman spectroscopy was plagued with ambiguity until the first accurate Raman spectrum was published by our group in 2016. Although generally considered to be relatively stable, UF4 can hydrolyze to form numerous UF4 hydrates that may play a role in future uranium waste forms. In contrast to anhydrous UF4, the hydrates, with their OH stretch and HOH bending modes, can be spectroscopically characterized by the type and degree of water bonding in the crystal lattice, which can yield additional information about their crystal structure. Herein, vibrational spectroscopy (Raman and infrared) was used to characterize three different UF4 hydrates: UF4(H2O)0.33, U3F12(H2O), and UF4(H2O)2.5. Spectra show the different hydrates vary in the number of observed bands, full-width half-maximum of the bands, and band intensity. These differences are due to varying interactions between the OH stretch and HOH bending modes with UF4 and the polymeric UF4 structure in the crystal lattice. These vibrational data, in combination with spectral fitting and crystallographic structures measured with powder X-ray diffraction and single crystal X-ray diffraction, provide unique details on the location of water molecules in the crystal lattice of hydrated UF4, and provide an interesting contrast to the vibrational spectra of anhydrous UF4.

FTIR-ATR analysis of the H-bond network of water in branched polyethyleneimine/TEMPO-oxidized cellulose nano-fiber xerogels

The present paper reports a detailed experimental vibrational analysis, performed by Fourier transform infrared spectroscopy in attenuated total reflectance geometry (FTIR-ATR), of water confined in the pores of cellulose nano-sponges (CNSs), prepared using TEMPO oxidized and ultra-sonicated cellulose nano-fibers (TOUS-CNFs) as three-dimensional scaffolds, and branched polyethyleneimine (bPEI) as the cross-linking agent. The analysis was carried out by varying hydration and cross-linker amount, with the aim of achieving a deep understanding of how the hydrogen bond (H-bond) scheme developed by engaged water molecules can play a role in the water adsorption process already observed at macroscopic level, furnishing at the same time evidence of a nano-porous network for CNSs. In particular, the combined investigation of the FTIR-ATR spectra of CNSs hydrated with H2O and D2O allowed for the selective analysis of vibrational modes of entrapped water molecules, namely O–H stretching and HOH bending modes. As main result, a destructuring effect of hydration on the H-bond pattern of interfacial water molecules is revealed, associated to structural modifications of the bPEI/TOUS-CNFs network previously detected by small angle neutron scattering (SANS) technique. It turned out to be more relevant for low bPEI amounts. In addition, a supercooled behavior of entrapped water molecules is detected, supporting the idea of a nano-confinement for water in these systems. The obtained information can be very helpful in view of all the possible applications of bPEI-TOCNF sponges as efficient adsorbent materials, especially for water remediation.

Simulating two-dimensional infrared-Raman and Raman spectroscopies for intermolecular and intramolecular modes of liquid water.

Full classical molecular dynamics (MD) simulations of two-dimensional (2D) infrared-Raman and 2D Raman spectroscopies of liquid water were carried out to elucidate a mode-mode coupling mechanism using a polarizable water model for intermolecular and intramolecular vibrational spectroscopy (POLI2VS). This model is capable of describing both infrared and Raman spectra. Second-order response functions, which consist of one molecular polarizability and two molecular dipole moments for 2D IR-Raman and three molecular polarizabilities for 2D Raman spectroscopies, were calculated using an equilibrium-non-equilibrium hybrid MD approach. The obtained signals were analyzed using a multi-mode Brownian oscillator (BO) model with nonlinear system-bath interactions representing the intramolecular OH stretching, intramolecular HOH bending, hydrogen bonded (HB)-intermolecular librational motion and HB-intermolecular vibrational (translational) motion of liquid water. This model was applied through use of hierarchal Fokker-Planck equations. The qualitative features of the peak profiles in the 2D spectra obtained from the MD simulations are accurately reproduced with the BO model. This indicates that this model captures the essential features of the intermolecular and intramolecular motion. We elucidate the mechanisms governing the 2D signal profiles involving anharmonic mode-mode coupling, the nonlinearities of the polarizability and dipole moment, and the vibrational dephasing processes of liquid water even in the case that the 2D spectral peaks obtained from the MD simulation overlap or are unclear. The mode coupling peaks caused by electrical anharmonic coupling (EAHC) and mechanical anharmonic coupling (MAHC) are observed in all of the 2D spectra. We find that the strength of the MAHC between the OH-stretching and HB-intermolecular vibrational modes is comparable to that between the OH-stretching and HOH bending modes. Moreover, we find that this OH-stretching and HB-intermolecular vibrational coupling should be observed as off-diagonal cross peaks in the 2D spectra.

In Situ ATR-FTIR Analysis of the Structure of Nafion-Pt/C Interface By Use of MEA-Type Cell

To improve the performance of polymer electrolyte fuel cell (PEFC), fundamental understanding of the catalyst/electrolyte interface under various operating conditions is essential. In particular, the investigation of the adsorption behavior of Nafion electrolyte (or binder) and water on the Pt cathode catalyst surfaces during the cell operation is one of the most important topics for the increase in the mass activity for the oxygen reduction reaction (ORR) of the catalysts and optimizing the operating conditions. Recently, a new MEA-type attenuated total reflectance-Fourier transform infrared (ATR-FTIR) cell was applied to observe a structure of Nafion/polycrystalline Pt, Pt/C or Pt3Co/C interface at room temperature in our laboratory.1,2 We succeeded in analyzing the adsorption of Nafion and water on Pt, as well as an intermediate in the ORR, as a function of potential. In this study, we have examined interactions between Nafion, water, and Pt/C catalysts in MEA-type cell by subtractively normalized interfacial Fourier transform infrared reflection spectroscopy (SNIFTIRS) under a humidified N2 and O2atmosphere at 60 °C. The experimental setup was described in our previous report in detail.2 The MEA-type ATR-FTIR cell consisted of a Nafion-coated Pt/C (TEC10E50E, Tanaka Kikinzoku Kogyo) working electrode prepared on Au thin film/Si plate/ZnSe-ATR prism, a gas diffusion electrode (GDE) as a counter electrode placed on a carbon separator with a gas flow field, and a Nafion NRE211 electrolyte membrane. The Pt loading of working and counter electrodes were 10 μg cm− 2 and 0.5 ± 0.1 mg cm− 2, respectively. A GDE-type reversible hydrogen electrode (RHE) was used as the reference electrode. The SNIFTIRS measurements were conducted with the combination of a potentiostat (HZ-5000, Hokuto Denko), a trigger interface (Varian), and an FTIR spectrometer (FTS6000, Bio-Rad) equipped with a MCT detector. The spectral resolution and angle of incidence were set at 4 cm− 1 and 70°, respectively. A set of SNIFTIR spectra of the Nafion–Pt/C interface was obtained at 60 °C and 100% RH under N2 and O2atmosphere. The single beam spectrum at 0.1 V was chosen as the reference state. The SNIFTIR spectra obtained on the Nafion coated Pt/C catalysts from 0.2 to 1.0 V in fully humidified N2 atmosphere were shown in Figure 1. Two downward bands around 3500 and 1600 cm− 1 were assigned to the OH stretching mode, ν(OH), and HOH bending mode, δ(HOH), of water molecules, respectively. The δ(HOH) band consists of the broad band of hydrated protons, H+(H2O)n, centered around 1710 cm− 1 and adsorbed water on the catalyst surface, H2Oad, around 1610 cm− 1. Several bands observed between 1500 and 900 cm− 1 were assigned to vibration modes of Nafion. Three upward bands around 1050, 990, and 960 cm− 1 were assigned to the SO3 − symmetric stretching mode, ν(SO3 −), and two C−O−C stretching modes, ether moiety close to the polymer backbone, ν(COC)1, and ether stretching band mechanically coupled to -SO3 −, ν(COC)2, of the Nafion side chain, respectively.3 The bands around 1250 and 1100 cm− 1 were assigned to the CF2 assymmetric and symmetric stretching modes, ν as(CF) and ν s(CF), respectively, related to the perfluorocarbon backbone in the main chain and side chain. Figure 2 shows changes in the band intensities of these species as a function of potential. The intensities of both δ(HOH) and ν(OH) decreased with increasing potential. In contrast, the band intensities for ν(SO3 −) and ν(COC) as well as ν s(CF) increased with potential. This suggests that the sulfonate group in the Nafion side chain adsorbed specifically on the Pt catalyst surface while expelling H+(H2O)n or H2Oadoriginally adsorbed at less positive potentials. We have also examined changes in the band intensities for water and Nafion under O2 atmosphere. The intensities for bands of the water molecules during the ORR were larger than those measured in N2. It was found that the ν(SO3 −) band in O2 commenced to increase at less positive potential (ca. 0.3 V) than that observed under N2atmosphere. Our results reveal that the ORR affects the adsorption of Nafion on the Pt catalysts surface. This work was supported by funds for the HiPer-FC project from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References 1) K. Kunimatsu, T. Yoda, D. A. Tryk, H. Uchida, and M. Watanabe, Phys. Chem. Chem. Phys., 12, 621 (2010). 2) H. Hanawa, K. Kunimatsu, M. Watanabe, and H. Uchida, J. Phys. Chem. C, 116, 21401 (2012). 3) A. M. Gomez-Marin, A. Berna, and J. M. Feliu, J. Phys. Chem. C, 114, 20130 (2010). Figure 1

Benchmark Structures and Harmonic Vibrational Frequencies Near the CCSD(T) Complete Basis Set Limit for Small Water Clusters: (H2O)n=2, 3, 4, 5, 6.

A series of (H2O)n clusters ranging from the dimer to the hexamer have been characterized with the CCSD(T) and the 2-body:Many-body CCSD(T):MP2 methods near the complete basis set (CBS) limit to generate benchmark-quality optimized structures and harmonic vibrational frequencies for these important systems. Quadruple-ζ correlation-consistent basis sets that augment the O atoms with diffuse functions have been employed in the analytic computation of harmonic vibrational frequencies for the global minima of the dimer, trimer, tetramer, and pentamer as well as the ring, book, cage, and prism isomers of the hexamer. Prior calibration [J. Chem. Phys. 2013, 139, 184113 and J. Chem. Theory Comput. 2014, 10, 5426] suggests that harmonic frequencies computed with this approach will lie within a few cm(-1) of the canonical CCSD(T) CBS limit. These data are used as reference values to gauge the performance of harmonic frequencies obtained with other ab initio methods (e.g., LCCSD(T) and MP2) and water potentials (e.g., TTM3-F and WHBB). This comparison reveals that it is far more challenging to converge harmonic vibrational frequencies for the bound OH stretching modes in these (H2O)n clusters to the CCSD(T) CBS limit than the free OH stretches, the n intramonomer HOH bending modes and even the 6n - 6 intermonomer modes. Deviations associated with the bound OH stretching harmonic frequencies increase rapidly with the size of the cluster for all methods and potentials examined, as do the corresponding frequency shifts relative to the monomer OH stretches.

Probing the molecular connectivity of water confined in polymer hydrogels.

The molecular connectivity and the extent of hydrogen-bond patterns of water molecules confined in the polymer hydrogels, namely, cyclodextrin nanosponge hydrogels, are here investigated by using vibrational spectroscopy experiments. The proposed spectroscopic method exploits the combined analysis of the vibrational spectra of polymers hydrated with water and deuterated water, which allows us to separate and selectively investigate the temperature-evolution of the HOH bending mode of engaged water molecules and of the vibrational modes assigned to specific chemical groups of the polymer matrix involved in the physical interactions with water. As main results, we find a strong experimental evidence of a liquid-like behaviour of water molecules confined in the nano-cavities of hydrogel and we observe a characteristic destructuring effect on the hydrogen-bonds network of confined water induced by thermal motion. More interestingly, the extent of this temperature-disruptive effect is found to be selectively triggered by the cross-linking degree of the hydrogel matrix. These results give a more clear picture of the molecular mechanism of water confinement in the pores of nanosponge hydrogel and open the possibility to exploit the spectroscopic method here proposed as investigating tools for water-retaining soft materials.

SEM, EDX and vibrational spectroscopic study of the phosphate mineral ushkovite MgFe23+(PO4)2(OH)2·8H2O – Implications of the molecular structure

The mineral ushkovite has been analyzed using a combination of electron microscopy with EDX and vibrational spectroscopy. Chemical analysis shows the mineral contains P, Mg with very minor Fe. Thus, the formula of the studied ushkovite is Mg32+(PO4)2·8H2O. The Raman spectrum shows an intense band at 953cm−1 assigned to the ν1 symmetric stretching mode. In the infrared spectra complexity exists with multiple antisymmetric stretching vibrations observed, due to the reduced tetrahedral symmetry. This loss of degeneracy is also reflected in the bending modes. Strong infrared bands around 827cm−1 are attributed to water librational modes. The Raman spectra of the hydroxyl-stretching region are complex with overlapping broad bands. Hydroxyl stretching vibrations are identified at 2881, 2998, 3107, 3203, 3284 and 3457cm−1. The wavenumber band at 3457cm−1 is attributed to the presence of FeOH groups. This complexity is reflected in the water HOH bending modes where a strong infrared band centered around 1653cm−1 is found. Such a band reflects the strong hydrogen bonding of the water molecules to the phosphate anions in adjacent layers. Spectra show three distinct OH bending bands from strongly hydrogen-bonded, weakly hydrogen bonded water and non-hydrogen bonded water. Vibrational spectroscopy enhances our knowledge of the molecular structure of ushkovite.

Water Bending Mode at the Water-Vapor Interface Probed by Sum-Frequency Generation Spectroscopy: A Combined Molecular Dynamics Simulation and Experimental Study.

We present a combined molecular dynamics simulation and experimental study on the water bending mode at the water-vapor interface using sum-frequency generation (SFG) spectroscopy. The SFG spectrum simulated using an ab initio-based water model shows good agreement with the experimental data. The imaginary part of the SFG response shows a negative peak at ∼1650 cm(-1) and a positive peak at ∼1730 cm(-1). Our results reveal that these widely (∼80 cm(-1)) separated peaks result from the interference of two closely spaced (∼29 cm(-1)) peaks of opposite sign. The positive peak at ∼1689 cm(-1) originates from water with two donor hydrogen atoms with the HOH angular bisector pointing down toward the bulk, and the negative peak at ∼1660 cm(-1) from water with free O-H groups, pointing up. The small frequency difference of 29 cm(-1) indicates that the HOH bending mode frequency of interfacial water is relatively insensitive to the number of hydrogen bonds.

Quantum fluctuation and vibrational dynamics of aqueous Cu+ and Ag+ clusters

The structures and infrared spectra of Ag+(H2O)n (n=1–4) and Cu+(H2O) are studied by “on-the-fly” ab initio MD, ab initio PIMD, and ab initio RPMD simulations. It is found that the anharmonicity due to nuclear quantum and thermal effects acts differently depending on the system as well as the type of vibrational mode. In the low-frequency region, the spectra become a broad band as the cluster size increases due to the complex mode couplings, which is qualitatively different from the harmonic spectra. In contrast, the HOH bending modes are systematically red-shifted by a small amount due to the anharmonicity. The OH stretching modes are found to be also red-shifted, but the amount of shift is more dependent on the system. Consequently, these effects cannot be described by unique scaling of harmonic frequencies.

The crystal structure of galgenbergite-(Ce), CaCe2(CO3)4∙H2O

Galgenbergite-(Ce) from the type locality, the railroad tunnel Galgenberg between Leoben and St. Michael, Styria, Austria, was investigated. There it occurs in small fissures of an albite-chlorite schist as very thin tabular crystals building rosette-shaped aggregates associated with siderite, ancylite-(Ce), pyrite and calcite. Electron microprobe analyses gave CaO 9.49, Ce2O3 28.95, La2O3 11.70, Nd2O3 11.86, Pr2O3 3.48, CO2 30.00, H2O 3.07, total 98.55 wt.%. CO2 and H2O calculated by stoichiometry. The empirical formula (based on Ca + REE ∑3.0) is \( \mathrm{C}{{\mathrm{a}}_{1.00 }}{{\left( {\mathrm{C}{{\mathrm{e}}_{1.04 }}\mathrm{L}{{\mathrm{a}}_{0.42 }}\mathrm{N}{{\mathrm{d}}_{0.42 }}\mathrm{P}{{\mathrm{r}}_{0.12 }}} \right)}_{2.00 }}{{\left( {\mathrm{C}{{\mathrm{O}}_3}} \right)}_4}\cdot {{\mathrm{H}}_2}\mathrm{O} \), and the simplified formula is \( \mathrm{CaC}{{\mathrm{e}}_2}{{\left( {\mathrm{C}{{\mathrm{O}}_3}} \right)}_4}\cdot {{\mathrm{H}}_2}\mathrm{O} \). According to X-ray single crystal diffraction galgenbergite-(Ce) is triclinic, space group \( P\overline{1},a=6.3916(5) \), b = 6.4005(4), c = 12.3898(9) A, α = 100.884(4), β = 96.525(4), γ = 100.492(4)°, V = 483.64(6) A3, Z = 2. The eight strongest lines in the powder X-ray diffraction pattern are [dcalc in A/(I)/hkl]: 5.052/(100)/011; 3.011/(70)/0-22; 3.006/(66)/004; 5.899/(59)/-101; 3.900/(51)/1-12; 3.125/(46)/-201; 2.526/(42)/022; 4.694/(38)/-102. The infrared absorption spectrum reveals H2O (OH-stretching mode at 3,489 cm−1, HOH bending mode at 1,607 cm−1) and indicates the presence of distinctly non-equivalent CO3-groups by double and quadruple peaks of their ν1, ν2, ν3 and ν4 modes. The crystal structure of galgenbergite-(Ce) was refined with X-ray single crystal data to R1 = 0.019 for 2,448 unique reflections (I > 2σ(I)) and 193 parameters. The three cation sites of the structure Ca(1), Ce(2) and Ce(3) have a modest mixed site occupation by Ca and small amount of REE (Ce, La, Pr, Nd) and vice versa. The structure is based on double layers parallel to (001), which are composed of Ca(1)Ce(2)(CO3)2 single layers with an ordered chessboard like arrangement of Ca and Ce, and with a roof tile-like stacking of the CO3 groups. Perpendicular to (001) the double layers are connected to a triclinic framework structure with good cleavage parallel to (001) by a differently organized and more open part of the structure formed by Ce(3)(CO3)2(H2O). Based on the topology of the CaCe(CO3)2 single layer in galgenbergite-(Ce), structural relationships to rutherfordine, to aragonite and ancylite type minerals, and to lanthanite are outlined.

Local vibrational modes of the water dimer – Comparison of theory and experiment

Local and normal vibrational modes of the water dimer are calculated at the CCSD(T)/CBS level of theory. The local H-bond stretching frequency is 528cm−1 compared to a normal mode stretching frequency of just 143cm−1. The adiabatic connection scheme between local and normal vibrational modes reveals that the lowering is due to mass coupling, a change in the anharmonicity, and coupling with the local HOH bending modes. The local mode stretching force constant is related to the strength of the H-bond whereas the normal mode stretching force constant and frequency lead to an erroneous underestimation of the H-bond strength.

Ion Association in Aqueous Solutions Probed through Vibrational Energy Transfers among Cation, Anion, and Water Molecules

KSCN and NH4SCN aqueous solutions were investigated with intermolecular vibrational energy transfer methods. In a KSCN/H2O (1/10 molar ratio) solution, 90% of the initial excitation of the CN stretch (~2066 cm(-1)) of the SCN(-) anion is transferred to the HOH bending mode (~1636 cm(-1)) of water molecules with an energy transfer time constant 3.1 ps. In a NH4SCN/H2O (1/10 molar ratio) solution, only 49% of the CN excitation flows to the water HOH bending mode with a time constant 6.3 ps. Most of the remaining CN excitation goes to the NH bending mode (~1460 cm(-1)) of the NH(+) cation with a time constant of 7.0 ps. The results indicate that about 50% of the energy transfer channel from the CN stretch to the HOH bending observed in the KSCN solution is overpowered by the NH4(+) cations in the NH4SCN/H2O solution. Ion concentration dependent measurements support this argument. According to the dipole/dipole approximation, the CN/OH energy transfer occurs most efficiently between SCN(-) anions and the water molecules closest to them. The experimental results therefore suggest that more than 50% of the water molecules closest to the SCN(-) anions are replaced by the NH4(+) cations in the NH4SCN/H2O (1/10 molar ratio) solution. The percentage is much larger than the NH4(+)/water ratio of 10%, indicating that the ion association between NH4(+) and SCN(-) is caused by the chemical nature of the solution rather than the statistical "forced contact" because of the high ion concentration.

Water Adsorption on the Stoichiometric (001) and (010) Surfaces of Hydroxyapatite: A Periodic B3LYP Study

H2O adsorption on hexagonal hydroxyapatite (001) and (010) stoichiometric surfaces has been studied at B3LYP level with a localized Gaussian basis set of polarized double-zeta quality using the periodic CRYSTAL06 code. Because four Ca2+ cations are available at both surfaces, the considered H2O coverages span the 1/4<or=theta<or=5/4 range. The affinity of both HA surfaces for H2O is large: on the (001) surface, H2O adsorbs molecularly (binding energies BE approximately 80 kJ mol(-1) per adsorbed molecule), whereas it dissociates on the (010) surface, giving rise to new surface terminations (CaOwHw and POHw). The highly negative reaction energy for H2O dissociation (between -250 and -320 kJ mol(-1) per adsorbed H2O molecule) strongly suggests that the pristine (010) surface "as cut" from the hydroxyapatite bulk cannot survive in aqueous environment. Conversely, on the reacted surface, H2O adsorbs molecularly with BE similar to those computed for the (001) surface. The B3LYP BEs have been contrasted to the experimental water adsorption enthalpies measured by microcalorimetry on polycrystalline hydroxyapatite samples, showing a fairly good agreement and supporting the suggestion that H2O vapor adsorbs on the already reacted (010) crystalline faces. Harmonic B3LYP vibrational features of adsorbed H2O show, when compared to modes of the gas-phase H2O, a hypsochromic shift of the HOH bending mode (Deltadelta(HOH)=49 cm(-1)) and a bathochromic shift of the OH stretching modes larger than 1700 cm(-1) (Deltanu(OH)=427 cm(-1)), which are both in good agreement with literature experimental data.

Infrared Spectroscopy of Water Adsorbed on M(111) (M = Pt, Pd, Rh, Au, Cu) Electrodes in Sulfuric Acid Solution

Spectroscopic characterization of adsorbed water has been carried out on (111) surfaces of Pt, Pd, Rh, Au, and Cu in sulfuric acid solution using infrared reflection−absorption spectroscopy (IRAS). The absorption band of the HOH bending mode (δHOH) is measured below the onset potential of (bi)sulfate adsorption. The band frequency of δHOH of adsorbed water is lower than that of bulk water. The δHOH band shifts to higher frequencies with the increase of the positive potentials on Pt, Pd, and Rh, whereas the band intensity is unchanged between the potentials of hydrogen evolution and (bi)sulfate adsorption. This suggests that water is adsorbed through an oxygen lone pair, with its molecular plane slightly tilted from the surface. On Au(111), the δHOH band intensity decreases with the increase of the electrode potential. This result indicates that the orientation of water depends on the applied potential. No absorption band of water is found on Cu(111), which indicates different behavior of adsorbed water co...

Crystal structure of a vivianite-type mineral: Mg-rich erythrite, (Co2.16Ni0.24Mg0.60)(AsO4)2 · 8 H2O

The crystal structure of erythrite from Bouzaer, south Morocco, was refined through single crystal X-ray diffraction data, within monoclinic space group C2/m, with the following unit cell constants: a = 10.187(1) Å, b = 13.470(1) Å, c = 4.755(1) Å, β = 104.971(2)°, V = 630.3(2) Å3. Transition metal and magnesium cations are randomly distributed over two special positions Me(1) and Me(2), making up single and dimerical octahedral groups, connected by (AsO4) tetrahedra groups, in order to form infinite sheets down b. Such sheets are held together within the three-dimensional framework owing to the presence of an intricate hydrogen bond network. An infrared spectrum was collected in order to achieve information on the HOH bending modes of the erythrite.

Evidence of the existence of the low-density liquid phase in supercooled, confined water

By confining water in a nanoporous structure so narrow that the liquid could not freeze, it is possible to study properties of this previously undescribed system well below its homogeneous nucleation temperature TH = 231 K. Using this trick, we were able to study, by means of a Fourier transform infrared spectroscopy, vibrational spectra (HOH bending and OH-stretching modes) of deeply supercooled water in the temperature range 183 < T < 273 K. We observed, upon decreasing temperature, the building up of a new population of hydrogen-bonded oscillators centered around 3,120 cm(-1), the contribution of which progressively dominates the spectra as one enters into the deeply supercooled regime. We determined that the fractional weight of this spectral component reaches 50% just at the temperature, TL approximately 225 K, where the confined water shows a fragile-to-strong dynamic cross-over phenomenon [Ito, K., Moynihan, C. T., Angell, C. A. (1999) Nature 398:492-494]. Furthermore, the fact that the corresponding OH stretching spectral peak position of the low-density-amorphous solid water occurs exactly at 3,120 cm(-1) [Sivakumar, T. C., Rice, S. A., Sceats, M. G. (1978) J. Chem. Phys. 69:3468-3476.] strongly suggests that these oscillators originate from existence of the low-density-liquid phase derived from the occurrence of the first-order liquid-liquid (LL) phase transition and the associated LL critical point in supercooled water proposed earlier by a computer molecular dynamics simulation [Poole, P. H., Sciortino, F., Essmann, U., Stanley, H. E. (1992) Nature 360:324-328].