Polarization and experimental configuration analyses of sum frequency generation vibrational spectra, structure, and orientational motion of the air/water interface

Here we report a detailed study on spectroscopy, structure, and orientational distribution, as well as orientational motion, of water molecules at the air/water interface, investigated with sum frequency generation vibrational spectroscopy (SFG-VS). Quantitative polarization and experimental configuration analyses of the SFG data in different polarizations with four sets of experimental configurations can shed new light on our present understanding of the air/water interface. Firstly, we concluded that the orientational motion of the interfacial water molecules can only be in a limited angular range, instead of rapidly varying over a broad angular range in the vibrational relaxation time as suggested previously. Secondly, because different vibrational modes of different molecular species at the interface has different symmetry properties, polarization and symmetry analyses of the SFG-VS spectral features can help the assignment of the SFG-VS spectra peaks to different interfacial species. These analyses concluded that the narrow 3693 cm(-1) and broad 3550 cm(-1) peaks belong to C(infinityv) symmetry, while the broad 3250 and 3450 cm(-1) peaks belong to the symmetric stretching modes with C2v symmetry. Thus, the 3693 cm(-1) peak is assigned to the free OH, the 3550 cm(-1) peak is assigned to the singly hydrogen-bonded OH stretching mode, and the 3250 and 3450 cm(-1) peaks are assigned to interfacial water molecules as two hydrogen donors for hydrogen bonding (with C2v symmetry), respectively. Thirdly, analysis of the SFG-VS spectra concluded that the singly hydrogen-bonded water molecules at the air/water interface have their dipole vector directed almost parallel to the interface and is with a very narrow orientational distribution. The doubly hydrogen-bonded donor water molecules have their dipole vector pointing away from the liquid phase.     

Intensities of IR stretching bands as a criterion of polarization and initial chemical activation of adsorbed molecules in acid catalysis. Ethane adsorption and dehydrogenation by zinc ions in ZnZSM-5 zeolite

A DRIFT study of ethane adsorbed by zinc cations in ZnZSM-5 prepared by chemical reaction of the hydrogen form of the zeolite with zinc vapor at 770 K, or by wet ion exchange, reveals unusual spectra of adsorbed C2H6 species. In addition to the weakly perturbed narrow bands in the region of C-H stretching vibrations, these spectra exhibit a very intense broad IR band with a frequency that is more than 200 cm(-1) lower than those of the C-H stretching vibrations of gaseous or physically adsorbed ethane. The very high relative intensity of this band indicates a very strong polarizability of the corresponding vibrational mode. It is concluded that these strongly polarized vibrations are closely connected with the subsequent heterolytic dissociation of ethane at moderately elevated temperatures, resulting in the formation of acidic hydroxyl groups and zinc ethyl fragments. At higher temperatures, the zinc ethyl fragments decomposed, resulting in the formation of zinc hydrides and ethylene. The unusual DRIFT spectrum of the molecular form of ethane adsorption by zinc cations represents a first example of anisotropy of polarizability of adsorbed molecules, which may provide a new explanation for selectivity of the acid-catalyzed reactions. In this connection, it is suggested to use the relative intensities of IR bands of adsorbed molecules as a reactivity index that is directly connected with chemical activation of adsorbed molecules via their polarization by the active sites.     

A new structure of water layer on Cu(111) electrode surface during hydrogen evolution

A layered structure of water molecules formed on a Cu(111) electrode surface during hydrogen evolution in sulfuric acid solution was studied by surface X-ray diffraction and infrared reflection absorption methods. Water molecules in the surface layers take a closest pack-like stacking structure with nearest-neighbor oxygen-oxygen distances in intra-(0.322(5) nm) and inter-(0.275(15) nm) layers of multi-domains; the infrared spectra of the layered water on the Cu electrode surface showed the existence of free OH(OD) and hydrogen-bonded OH(OD) of water molecule.     

Solvation dynamics in Ni+ (H2O)n clusters probed with infrared spectroscopy

Infrared photodissociation spectroscopy is reported for mass-selected Ni+ (H2O)n complexes in the O-H stretching region up to cluster sizes of n = 25. These clusters fragment by the loss of one or more intact water molecules, and their excitation spectra show distinct bands in the region of the symmetric and asymmetric stretches of water. The first evidence for hydrogen bonding, indicated by a broad band strongly red-shifted from the free OH region, appears at the cluster size of n = 4. At larger cluster sizes, additional red-shifted structure evolves over a broader wavelength range in the hydrogen-bonding region. In the free OH region, the symmetric stretch gradually diminishes in intensity, while the asymmetric stretch develops into a closely spaced doublet near 3700 cm(-1). The data indicate that essentially all of the water molecules are in a hydrogen-bonded network by the size of n = 10. However, there is no evidence for the formation of clathrate structures seen recently via IR spectroscopy of protonated water clusters.     

Pressure-induced hydration and order-disorder transition in a synthetic potassium gallosilicate zeolite with gismondine topology

Two high-pressure phases of a potassium gallosilicate with a gismondine framework (K-GaSi-GIS) were characterized using Rietveld refinements of in-situ high-pressure, high-resolution synchrotron X-ray powder diffraction data. The observed response of the K-GaSi-GIS framework under hydrostatic pressure is a gradual flattening of the so-called "double crankshaft" structural chain units. At pressures below 1.0(1) GPa, additional water molecules from the hydrostatic pressure-transmitting medium are inserted into the potassium-water guest network ("pressure-induced hydration") resulting in a "super-hydrated" high-pressure phase I. As the flattening of the double crankshaft structural units in the GIS framework continues above 1.6 GPa, the ellipticity of the cross-linking 8-ring windows is reduced below a certain threshold, and a disordering of the potassium-water guest structure along the 8-ring channel, characteristic of a disordered high-pressure phase II, is observed. The concerted framework distortion and guest network disordering accommodates the increased hydration level while maintaining the seven-fold coordination environment of the potassium cations to framework oxygen atoms and water molecules. We have thus established the atomistic details of a guest-host order-disorder transition under pressure-induced hydration conditions in a zeolite with GIS framework and compared it to other zeolites during pressure-induced hydration. We find that the structural changes mediated by the extra-framework cations and their coordination environment under PIH conditions are at the core of these different mechanisms and are driving the changes in the ellipticity of pore openings, order-disorder and disorder-order transitions, and framework distortions.     

Are recent water models obtained by fitting diffraction data consistent with infrared/Raman and x-ray absorption spectra?

X-ray absorption (XA) spectra have been computed based on water structures obtained from a recent fit to x-ray and neutron diffraction data using models ranging from symmetrical to asymmetrical local coordination of the water molecules [A. K. Soper, J. Phys.: Condens. Matter 17, S3273 (2005)]. It is found that both the obtained symmetric and asymmetric structural models of water give similar looking XA spectra, which do not match the experiment. The fitted models both contain unphysical structures that are allowed by the diffraction data, where, e.g., hydrogen-hydrogen interactions may occur. A modification to the asymmetric model, in which the non-hydrogen-bonded OH intramolecular distance is allowed to become shorter while the bonded OH distance becomes longer, improves the situation somewhat, but the overall agreement is still unsatisfactory. The electric field (E-field) distributions and infrared (IR) spectra are also calculated using two established theoretical approaches, which, however, show significant discrepancies in their predictions for the asymmetric structural models. Both approaches predict the Raman spectrum of the symmetric model fitted to the diffraction data to be significantly blueshifted compared to experiment. At the moment no water model exists that can equally well describe IR/Raman, x-ray absorption spectroscopy, and diffraction data.     

Infrared spectroscopy of hydrogen-bonded 2-fluoropyridine-water clusters in supersonic jets

The hydrogen-bonded clusters of 2-fluoropyridine with water were studied experimentally in a supersonic free jet and analyzed with molecular orbital calculations. The IR spectra of 2-fluoropyridine-(H2O)(n) (n = 1 to 3) clusters were observed with a fluorescence detected infrared depletion (FDIR) technique in the OH and CH stretching vibrational regions. The frequencies of OH stretching vibrations show that water molecules bond to the nitrogen atom of 2-fluoropyridine in the clusters. The hydrogen-bond formation between aromatic CH and O was evidenced in the 1:2 and 1:3 clusters from the experimental and calculated results. The overtone vibrations of the OH bending mode in hydrogen-bonded water molecules appear in the IR spectra, and these frequencies become higher with the increase of the number of water molecules in the clusters. The band structure of the IR spectra in the CH stretching region changes depending on the number of coordinating water molecules.     

Infrared photodissociation spectroscopy of Al(+)(CH(3)OH)(n) (n = 1-4)

Infrared photodissociation spectra of Al(+)(CH(3)OH)(n) (n = 1-4) and Al(+)(CH(3)OH)(n)-Ar (n = 1-3) were measured in the OH stretching region, 3000-3800 cm(-1). For n = 1 and 2, sharp absorption bands were observed in the free OH stretching region, all of which were well reproduced by the spectra calculated for the solvated-type geometry with no hydrogen bond. For n = 3 and 4, there were broad vibrational bands in the energy region of hydrogen-bonded OH stretching vibrations, 3000-3500 cm(-1). Energies of possible isomers for the Al(+)(CH(3)OH)(3),4 ions with hydrogen bonds were calculated in order to assign these bands. It was found that the third and fourth methanol molecules form hydrogen bonds with methanol molecules in the first solvation shell, rather than a direct bonding with the Al(+) ion. For the Al(+)(CH(3)OH)(n) clusters with n = 1-4, we obtained no evidence of the insertion reaction, which occurs in Al(+)(H(2)O)(n). One possible explanation of the difference between these two systems is that the potential energy barriers between the solvated and inserted isomers in the Al(+)(CH(3)OH)(n) system is too high to form the inserted-type isomers.     

23Na NMR study of the mechanism for the dehydration of zeolite NaA

Conclusions Water molecules in partially dehydrated zeolite directly interact with Na⁺ cations. The final three or four water molecules per unit cell removed upon the dehydration of zeolite are apparently bound to Na⁺ cations of the eight-membered windows, while the previous 10 molecules are bound to Na⁺ cations of six-membered windows.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1160–1162, May, 1988.     

Iron-substituted Beta molecular sieve: Synthesis and characterization

A synthesis of crystalline ferrisilicate having zeolite BEA topology (Fe-Beta) and containing significant quantities of iron in the framework (22 Fe atoms per unit cell based on 192 T-atoms per unit cell) has been reported. The synthesis of Fe-Beta was carried out using tetraethylammonium hydroxide (TEAOH), 25% methanolic instead of an aqueous solution, as a source of organic template. X-Ray powder diffraction (XRD) and scanning electron micrography (SEM) were used to check the structural identity and phase purity of the Fe-Beta samples. A thermogravimetric analysis (TGA) study showed that the amount of TEAOH ions interacting with Fe-Beta framework increases with increase in the iron content of the sample. The Fe ions in Fe-Beta were in the trivalent oxidation state which was confirmed by an X-ray photoelectron spectroscopy (XPS) study. A comparison of Si/Fe ratios determined by chemical analysis and XPS method allowed us to conclude that Fe³⁺ species are distributed homogeneously from the bulk to the surface of Fe-Beta crystallites. However, the O_1s XP spectra indicated the presence of small amounts of extraframework iron oxidic impurities in as-synthesized as well as calcined samples. The infrared (IR) spectra showed three types of hydroxyl groups in Fe-Beta located at 3740, 3670 and a very broad band at ∼ 3540 cm ⁻¹. The band at 3680 cm⁻¹ was found to be affected by pyridine adsorption and therefore was assigned to structural bridging hydroxyl groups (SiOHFe) formed by substitution of iron in the zeolite framework. The shift in the IR OH stretching frequency towards higher wave number and the desorption of NH₃ at lower temperature for Fe-Beta samples as compared to Al-Beta zeolite allowed us to conclude that the strength of Brönsted acid sites in Fe-Beta materials is lower than those Brönsted acids sites in Al-Beta zeolites.     

Infrared spectroscopy of acetone-water liquid mixtures. II. Molecular model

In aqueous acetone solutions, the strong bathochromic shifts observed on the OH and CO stretch infrared (IR) bands are due to hydrogen bonds between these groups. These shifts were evaluated by factor analysis (FA) that separated the band components from which five water and five acetone principal factors were retrieved [J. Chem. Phys. 119, 5632 (2003)]. However, these factors were abstract making them difficult to interpret. To render them real an organization model of molecules is here developed whose abundances are compared to the experimental ones. The model considers that the molecules are randomly organized limited by the hydrogen bond network formed between the water hydrogen atoms and the acetone or water oxygen atoms, indifferently. Because the oxygen of water has two covalent hydrogen atoms which are hydrogen-bonded and may receive up to two hydrogen atoms from neighbor molecules hydrogen-bonded to it, three types of water molecules are found: OH2, OH3, and OH4 (covalent and hydrogen bonds). In the OH stretch region these molecules generate three absorption regimes composed of nu3, nu1, and their satellites. The strength of the H-bond given increases with the number of H-bonds accepted by the oxygen atom of the water H-bond donor, producing nine water situations. Since FA cannot separate those species that evolve concomitantly the nine water situations are regrouped into five factors, the abundance of which compared exactly to that retrieved by FA. From the factors' real spectra the OH stretch absorption are simulated to, respectively, give for the nu3 and nu1 components the mean values for OH2, 3608, 3508; OH3, 3473, 3282 and OH4, 3391, 3223 cm(-1). The mean separations from the gas-phase position which are respectively about 150, 330, and 400 cm(-1) are related to the vacancy of the oxygen electron doublets: two, one, and zero, respectively. No acetone hydrate that sequesters water molecules is formed. Similarly, acetone produces ten species, two of which evolve concomitantly. Spectral similarities further reduce these to five principal IR factors, the abundance of which compared adequately to the experimental results obtained from FA. The band assignment of the five-acetone spectra is given.     

An FT-IR demonstration of the occurrence of satellite bands in the spectra of hydrogen bonds in crystals

OH stretching fundamentals in the spectra of hydrogen-bonded crystals are accompanied by satellite bands due to sum and difference transitions involving one quantum of the OH stretching vibration and one or more quanta of the low-frequency vibration of the hydrogen bond. Such satellites were detected in the FT-IR spectra of two hydrogen-bonded crystals, ice and methanol.NRCC No. 29006     

An infrared spectroscopic study of hydrogen bonding in ethyl phenol: A model system for polymer phenolics

A detailed analysis of the bands appearing in the OH stretching region of the infrared spectrum of ethyl phenol solutions is presented. In cyclohexane solutions, the band due to “free” (non-hydrogen-bonded groups) contains overlapping contributions from both monomeric and end-group species. Other assignments are made on the basis of whether the proton and oxygen in a particular OH group are both involved in hydrogen bonds (as “donors” and “acceptors”, respectively), or if only the proton is acting as a donor. The strongest band in the spectra obtained at the highest concentration of ethyl phenol is due to OH groups present in linear chains of hydrogen-bonded OH groups (as recognized in numerous other studies), but a band due to cyclic trimers has also been identified. The assignment of other modes is more uncertain and various possibilities are discussed. In toluene solutions, assignments are more complicated, because bands due to OH–π hydrogen bonds are observed instead of free groups. Finally, the data from cyclohexane solutions was used to calculate equilibrium constants capable of describing the distribution of species present. A new methodology for determining the equilibrium constant describing association in the form of dimers is described.     

Specific Sorption Sites for Nitrogen in Zeolites NaLSX and LiLSX

Specific sorption sites for nitrogen, N₂, in NaLSX and LiLSX zeolites were investigated using a DRIFT spectroscopic method. Sorption of molecular hydrogen, H₂, by NaLSX or LiLSX zeolite at 77 K with DRIFT control of perturbation of sorbed molecules allowed to discriminate two or three different types of specific sorption sites in the respective zeolites. Their H–H stretching frequencies are 4077 and 4081 cm^–1 for NaLSX, and 4061, 4084 and 4129 cm^–1 for LiLSX. With reference to an independent investigation by methods of both sorption thermodynamics and molecular modeling for N₂ sorption on LiLSX, the first two of the corresponding bands were ascribed to H₂ sorption on lithium cations, Li⁺, localized in supercages of the faujasite, FAU, zeolite framework at sites SIII and SIII, while the latter band most likely belongs to H₂ sorption on Li⁺ cations at sites SII, and on hydroxyl groups, OH. Sorption of N₂ by Li⁺ cations at sites SIII and SIII is the strongest, resulting in a decrease of intensity of the corresponding DRIFT bands that stem from subsequent H₂ sorption. Nitrogen sorption by Li⁺ cations at sites SII is much weaker. Sorption of N₂ on Na⁺ cations at sites SIII in NaLSX zeolite is also stronger than by Na⁺ cations at sites SII.     

Unconventional, highly selective CO2 adsorption in zeolite SSZ-13

Low-pressure adsorption of carbon dioxide and nitrogen was studied in both acidic and copper-exchanged forms of SSZ-13, a zeolite containing an 8-ring window. Under ideal conditions for industrial separations of CO(2) from N(2), the ideal adsorbed solution theory selectivity is >70 in each compound. For low gas coverage, the isosteric heat of adsorption for CO(2) was found to be 33.1 and 34.0 kJ/mol for Cu- and H-SSZ-13, respectively. From in situ neutron powder diffraction measurements, we ascribe the CO(2) over N(2) selectivity to differences in binding sites for the two gases, where the primary CO(2) binding site is located in the center of the 8-membered-ring pore window. This CO(2) binding mode, which has important implications for use of zeolites in separations, has not been observed before and is rationalized and discussed relative to the high selectivity for CO(2) over N(2) in SSZ-13 and other zeolites containing 8-ring windows.     

Effect of water on the formamide-intercalation of kaolinite

The molecular structures of low defect kaolinite completely intercalated with formamide and formamide-water mixtures have been determined using a combination of X-ray diffraction, thermoanalytical techniques, DRIFT and Raman spectroscopy. Expansion of the kaolinite to 10.09 A was observed with subtle differences whether the kaolinite was expanded with formamide or formamide-water mixtures. Thermal analysis showed that greater amounts of formamide could be intercalated into the kaolinite in the presence of water. New infrared bands were observed for the formamide intercalated kaolinites at 3648, 3630 and 3606 cm(-1). These bands are attributed to the hydroxyl stretching frequencies of the inner surface hydroxyls hydrogen bonded to formamide with water, formamide and interlamellar water. Bands were observed at similar positions in the Raman spectrum. At liquid nitrogen temperature, the 3630 cm(-1) Raman band separated into two bands at 3633 and 3625 cm(-1). DRIFT spectra showed the hydroxyl deformation mode at 905 cm(-1). Changes in the molecular structure of the formamide are observed through both the NH stretching vibrations and the amide 1 and 2 bands. Upon intercalation of kaolinite with formamide, bands are observed at 3460, 3344, 3248 and 3167 cm(-1) attributed to the NH stretching vibration of the NH involved with hydrogen bonded to the oxygens of the kaolinite siloxane surface. In the DRIFT spectra of the formamide intercalated kaolinites bands are observed at 1700 and 1671 cm(-1) and are attributed to the amide 1 and amide 2 vibrations.     

Compatibility between methanol and water in the three-dimensional cage formation of large-sized protonated methanol-water mixed clusters

Infrared spectra of large-sized protonated methanol-water mixed clusters, H(+)(MeOH)(m)(H(2)O)(n) (m=1-4, n=4-22), were measured in the OH stretch region. The free OH stretch bands of the water moiety converged to a single peak due to the three-coordinated sites at the sizes of m+n=21, which is the magic number of the protonated water cluster. This is a spectroscopic signature for the formation of the three-dimensional cage structure in the mixed cluster, and it demonstrates the compatibility of a small number of methanol molecules with water in the hydrogen-bonded cage formation. Density functional theory calculations were carried out to examine the relative stability and structures of selected isomers of the mixed clusters. The calculation results supported the microscopic compatibility of methanol and water in the hydrogen-bonded cage development. The authors also found that in the magic number clusters, the surface protonated sites are energetically favored over their internal counterparts and the excess proton prefers to take the form of H(3)O(+) despite the fact that the proton affinity of methanol is greater than that of water.     

Structure and vibrational study of the trimethylammonium hexafluorosilicate [(CH3)3NH]2SiF6 compound

The X-ray powder diffraction pattern of [(CH(3))(3)NH](2)SiF(6) was obtained and indexed on the basis of a centred cubic unit cell with the P4(1)32 as the likely space group. The Infrared and Raman spectra of this compound have been recorded at room temperature and discussed in relation to the crystal structure. In this salt, the bands corresponding to the cation vibrational modes show that the symmetry of these cations is distorted from the free C(3v) one and that they are strongly hydrogen-bonded to the respective anions. However, the spectra of the anions can be interpreted in term of ordered groups as indicated by the splitting of the bands corresponding to some degenerate vibrational modes. The harmonic frequencies, corresponding to the (CH(3))(3)NH-SiF(6)-NH(CH(3))(3) optimised geometry, were calculated using the SCF semi-empirical MNDO-PM3 method.     

Pressure response of Raman spectra of water and its implication to the change in hydrogen bond interaction

In situ Raman spectroscopic measurements of water in the region of OH vibration were conducted up to 0.4 GPa at 23 and 52 degrees C. The frequencies of the decomposed OH stretching bands initially decreased with increasing pressure, reached a minimum at 0.15 GPa and increased up to 0.3 GPa and then decreased, which corresponds to the variations of the strength of hydrogen bonding. This variation was observed at 23 degrees C, but not at 52 degrees C, which suggests a change in pressure dependence on the hydrogen bond interaction between these two temperatures. Based on the equilibration model between hydrogen-bonded and nonhydrogen-bonded molecules, the present experimental results indicate that the pressure variation of the viscosity depends on the ratio of hydrogen-bonded molecules, rather than the strength of hydrogen bonding between molecules.     

Minerals from Macedonia XXIII. Spectroscopic and structural characterization of schorl and beryl cyclosilicates

IR and Raman spectroscopy study on two collected cyclosilicate species: schorl (from tourmaline group), Na(Fe,Mg)(3)Al(6)(BO(3))(3)Si(6)O(18)(OH,F)(4) and beryl (Be,Mg,Fe)(3)Al(2)Si(6)O(18) were carried out. Although beryl is nominally anhydrous mineral, vibrational results strongly indicate that H(2)O molecules exist in the structural channels. The number of vibrational bands and their frequencies revealed the presence of H(2)O type II, in which C(2) symmetry axis of the water molecule is parallel to the structural channel (and to the c-axis of beryl). On the other hand, it was found that observed bands in the IR and Raman OH stretching region of the other tourmaline varieties appear as a result of the cation combinations involving dominant presence of Mg and Fe cations in the Y structural sites. The strong indication derived from the vibrational spectroscopic results that the studied mineral represents a schorl variety, coincide very well with the results obtained by powder X-ray diffraction and X-ray microprobe analysis. Both minerals show IR spectral similarities in the region below 1500 cm(-1), whereas the resemblance between the Raman spectra (1500-100 cm(-1)) is less expressed confirming that these spectra are more sensitive to compositional changes and to structural disorder. The identification of both minerals was additionally supported by studying the powder X-ray diffraction diagrams.