Additional H2O Molecules Research Articles

Interlayer structure model of tri-hydrated low-charge smectite by X-ray diffraction and Monte Carlo modeling in the Grand Canonical ensemble

The present study aims primarily at refining a structure model for interlayer cations and H2O molecules in tri-hydrated (3W) smectite (d001 = 18-19 Å). The <2 mm fraction of the SWy-2 source clay (low-charge montmorillonite) was saturated by Mg2+, Ca2+, Ba2+, or Na+ cations, before collection of X-ray diffraction (XRD) patterns at 98% relative humidity. Experimental d001 values derived for the essentially homogeneous 3W hydrates provided volume constraints for Grand Canonical Monte Carlo (GCMC) simulations. Computed atomic density distribution of interlayer species were used in turn to calculate XRD intensities of 00l reflections. The agreement between calculated and experimental 00l intensities allowed validating the GCMC results of both interlayer H2O content and distribution of interlayer species (cations and H2O molecules). Computed atomic density profiles do not correspond to the usual model of three discrete planes of H2O molecules but rather exhibit two sharp planes of H2O molecules wetting the clay surfaces (at ~2.7 Å from the clay layer surface). Additional H2O molecules belong to cation hydration shells or define a poorly organized ensemble filling internal voids. This alternative model suggests that the stability of the 3W hydrate results from the dual interaction of some H2O molecules with interlayer cation, through their second hydration shell, and with the 2:1 clay surface. Computed atomic density profiles were approximated to propose an interlayer structure model for 3W smectite. This simplified model includes two sets of two planes (symmetrical relative to the interlayer mid-plane) for H2O molecules and one set for interlayer cations. This model allows reproducing experimental XRD patterns for the different samples investigated and thus represents a valid set of parameters for routine quantitative analysis of XRD profiles in an effort to determine smectite reactivity close to water-saturated conditions. Implications of such studies are crucial to provide experimental constraints on the behavior of the main vector of element transfer under conditions common in surficial environments and prevailing in waste repositories. In addition, the present study provides an experimental validation of structure models derived from the widely used ClayFF model, and thus allows its use to predict the fate of water in clayey systems close to water-saturated conditions.

Effect of H2O on the hydrolysis of UF6 in the gas phase

In our previous works (Hu et al., 2008, 2009), we found the theoretical evidence that indicates UF5OH is an intermediate produced in the first step of UF6 hydrolysis, UF6+H2O. When UF6/H2O ratio is high, UF5OH may react with UF6 or another UF5OH, forming species containing UOU bond. In this work, we considered another situation – the hydrolysis of UF6 in excess of H2O, and explored the probable initial reactions of UF6+2H2O and UF6+3H2O systems using the same relativistic density functional theory calculations. Water molecules may form dimer or trimer and then coordinate with UF6, forming complexes UF6·2H2O or UF6·3H2O, which are more stable than UF6·H2O. Compared to the conversion UF6·H2O→UF5 OH·HF, the energy barriers of reactions UF6·2H2O→UF5OH·H2O·HF and UF6·3H2O→UF5OH·2H2O·HF are 4 and 6kcal/mol lower respectively. The additional H2O molecules catalyze the reactions by bridging the hydrogen which transfers from ligand H2O to F and stabilizes the transition states. In addition, as water content increases, the reaction step UF6+3H2O becomes exothermic and the products-HF, UF5 OH and two H2O molecules-tend to bond tightly into a stable complex.

Smectite fluorination and its impact on interlayer water content and structure: A way to fine tune the hydrophilicity of clay surfaces?

In addition to isomorphic cation substitutions, smectite layers may present anionic substitutions with fluorine replacing the structural hydroxyl groups and inducing a reduced number of H2O molecules hydrating interlayer cations. The resulting additional versatility of smectite layers could be used to fine-tune the hydrophilicity of pure or intercalated smectite pending a detailed understanding of interlayer water organization. The present article thus reports on the hydration of (fluoro-)hectorite samples exhibiting similar charge density (structural formulae: [Na0.8·nH2O]inter[Mg5.2Li0.8]oct[Si8.0]tetO20(OH,F)4). Water sorption isotherms and PIGE/PIXE analyses allowed constraining the water content over the probed range of relative humidity and the bulk chemistry, respectively. Modeling of X-ray diffraction (XRD) profiles obtained along water vapor desorption isotherms allowed gaining additional insights in the distribution of interlayer H2O molecules. Compared to hydroxylated smectites of similar charge, fluorinated hectorites contain ∼30% less interlayer H2O molecules although transitions between discrete hydration states are observed for similar values of relative humidity. These molecules (3–4 and 6–8 H2O molecules per cation in the mono- and bi-hydrated states, respectively) predominantly belong to the hydration sphere of interlayer cations, although additional H2O molecules are present in the bi-hydrated state. As a consequence, the positional disorder of interlayer H2O molecules is much reduced in fluorinated samples, thus increasing the minimum distance from an interlayer H2O molecule to the smectite layer, most likely owing to the hydrophobicity of fluorinated layers. Finally, the distribution of layers with a given hydration state is more heterogeneous within smectite crystals for fluorinated smectites, compared to hydroxylated ones, possibly as the result of the improved crystallinity.

Proton transfer potentials in hydrogen-bonded systems: (H5O2)+

Ab initio molecular orbital calculations at the split-valence shell level are used to examine a theory of proton transport across biomembranes involving the transfer of protons along hydrogen-bonded chains. Potentials for central proton motion in (H2OHOH2)+ are calculated using the 4-31G basis set. This procedure is shown to provide results in close agreement with more extended calculations including electron correlation. A potential with a shallow single minimum corresponding to a centrally located proton is obtained for small interoxygen separations. However, for separations greater than about 2.4 A, double-well potentials are calculated. The potentials are relatively insensitive to the inclusion of additional H2O molecules or geometry optimizations. The effects of angular distortions of the hydrogen bond on the barriers separating the two minima are investigated. Small distortions of less than 20d or so increase the barriers slightly, but a sharp rise is noted for greater distortions. In addition, rotations of the two water molecules in opposite directions produce the largest increase in proton transfer barriers.

Halide anion solvation and recognition by a macrotricyclic tetraammonium host in an ionic liquid: a molecular dynamics study

We report a molecular dynamics study of halide anions X− and their inclusion complexes X− ⊂ L4+ with a macrotricyclic tetrahedral host L4+ built from four quaternary ammonium sites, in an ionic liquid (IL) based on the 1-butyl-3-methylimidazolium (BMI+) cation and the PF6− anion. The “dry” and “humid” forms of the [BMI][PF6] IL are compared, showing the importance of IL ions in the “dry” IL and, in some cases, of water molecules in the “humid” IL. In the “dry” IL the F−, Cl−, Br− and I− uncomplexed halides are surrounded by 4–5 BMI+ cations whose binding mode evolves from hydrogen bonding to facial coordination along this series. Solvent humidity has the largest impact on the solvation of F− whose first shell BMI+ cations are all displaced by H2O molecules, while the first solvation shell of Cl−, Br− and I− comprises 3–4 BMI+ cations plus ca. 4 H2O molecules. The solvation of the L4+ host and of its X− ⊂ L4+ complex mainly involves PF6− anions in the “dry” IL, and additional H2O molecules in the “humid” IL. The question of anion binding selectivity is addressed by free energy perturbation calculations which predict that, in the “dry” liquid, F− is preferred over Cl−, Br− and I−, which contrasts with the aqueous solution where L4+ is selective for Cl−. In the “humid” liquid however, there is no F−/Cl− discrimination, showing the importance of “small amounts” of water on the complexation selectivity.

Synthesis and Crystal Structures of M2 [(UO2)3(SeO4)5] (H2O)16 (M: Co, Zn).

AbstractFor Abstract see ChemInform Abstract in Full Text.

Excited state hydrogen atom transfer in ammonia-wire and water-wire clusters

We review experimental and theoretical investigations of excited-state hydrogen atom transfer (ESHAT) reactions along unidirectionally hydrogen bonded solvent ‘wire’ clusters. The solvent wire is attached to the aromatic ‘scaffold’ molecule 7-hydroxyquinoline (7HQ), which offers an O–H and an N hydrogen bonding site, spaced far enough apart to form two- to four-membered wires. S 1 ← S 0 photoexcitation renders the O–H group more acidic and the quinolinic N more basic. This provides a driving force for the enol → keto tautomerization, probed by the characteristic fluorescence of the 7-ketoquinoline in the molecular beam experiments. For 7-hydroxyquinoline·(NH3)3, excitation of ammonia-wire vibrations induces the tautomerization at ∼200 cm−1. Different reaction pathways have been explored by excited-state ab initio calculations. These show that the reaction proceeds by H-atom transfer along the wire as a series of Grotthus-type translocation steps. There is no competition with a mechanism involving successive proton translocations. The rate-controlling S 1 state barriers arise from crossings of a π π* with a Rydberg-type πσ* state and the proton and electron movements along the wire are closely coupled. The excited state reactant, H-transferred intermediates and product structures are characterized. The reaction proceeds by tunnelling, as shown by deuteration of the solvent molecules (ND3) in the wire. The first step of the reaction exhibits intra/intermolecular vibrational mode selectivity. Substitution of NH3 by one, two or three H2O molecules in the wire leads to increasing threshold with each additional H2O molecule, up to >2000 cm−1 for the 7-hydroxyquinoline·(H2O)3 water-wire cluster. No 7-ketoquinoline fluorescence is observed upon insertion of even a single H2O molecule. The calculations show that insertion of each H2O molecule into the solvent wire introduces a high barrier, which blocks any further H-atom transfer. Contents PAGE 1. Introduction 458 2. 7-Hydroxyquinoline 460 2.1. Ground and excited state acid/base properties 460 2.2. O–H bond breaking in the ππ* and πσ* states 461 3. The 7-hydroxyquinoline · (NH3)n ammonia-wire clusters: an overview 463 3.1. Structures and electronic origins 463 3.2. Resonant two-photon ionization spectra 466 4. Excited-state hydrogen atom transfer in 7-hydroxyquinoline · (NH3)3 468 4.1. UV–UV depletion and fluorescence action spectra 468 4.2. Two reaction paths: ESHAT vs. ESPT 470 4.3. The enol → HT1 reaction coordinate 474 4.4. Vibrational mode selectivity 474 5. Solvent e.ects on excited state H-atom transfer: mixed ammonia/water clusters 479 5.1. Comparison of 7HQ· (H2O)3 and 7HQ · (NH3)3 479 5.2. The mixed 7HQ · (NH3)2 · H2O and 7HQ · NH3 · (H2O)2 clusters 482 5.3. Discussion 484 6. Conclusion 485 References 487

Structural Diversity of Sheets in Rubidium Uranyl Oxoselenates: Synthesis and Crystal Structures of Rb2 [(UO2)(SeO4)2(H2O)] (H2O), Rb2 [(UO2)2(SeO4)3 (H2O)2](H2O)4, and Rb4 [(UO2)3(SeO4)5(H2O)

AbstractFor Abstract see ChemInform Abstract in Full Text.

Structural Diversity of Sheets in Rubidium Uranyl Oxoselenates: Synthesis and Crystal Structures of Rb2[(UO2)(SeO4)2(H2O)](H2O), Rb2[(UO2)2(SeO4)3(H2O)2](H2O)4, and Rb4[(UO2)3(SeO4)5(H2O)

AbstractSingle crystals of three rubidium uranyl selenates, Rb2[(UO2)(SeO4)2(H2O)](H2O) (1), Rb2[(UO2)2(SeO4)3(H2O)2](H2O)4 (2), and Rb4[(UO2)3(SeO4)5(H2O)] (3), have been prepared by evaporation from aqueous solutions made out of mixtures of uranyl nitrate, selenic acid and Rb2CO3. The structures of all compounds have been solved by direct methods on the basis of X‐ray diffraction data sets. The crystallographic data are as follows: (1): orthorhombic, Pna21, a = 13.677(2), b = 11.8707(13), c = 7.6397(9) Å, V = 1240.4(3) Å3, R1 = 0.045 for 2396 independent observed reflections; (2): triclinic, P1¯, a = 8.4261(12), b = 11.8636(15), c = 13.3279(18) Å, α = 102.612(10), β = 107.250(10), γ = 102.510(10)°, V = 1183.7(3) Å3, R1 = 0.067 for 4762 independent observed reflections; (3): orthorhombic, Pbnm, a = 11.3761(14), b = 15.069(2), c = 19.2089(17) Å, V = 3292.9(7) Å3, R1 = 0.075 for 3808 independent observed reflections. The structures of the phases 1, 2, and 3 are based upon uranyl selenate hydrate sheets composed from corner‐sharing pentagonal [UO7]8— bipyramids and [SeO4]2— tetrahedra. In the crystal structure of 1, the sheets have composition [(UO2)(SeO4)2(H2O)]2— and run parallel to (001). The interlayer contains Rb+ cations and additional H2O molecules. In structure of 2, the [(UO2)2(SeO4)3(H2O)2]2— sheets are oriented parallel to (101). Highly disordered Rb+ cations and H2O molecules are located between the sheets. The structure of 3 is based upon [(UO2)3(SeO4)5(H2O)]4— sheets stacked parallel to (010) and contains Rb+ cations in the interlayers. The topologies of the uranyl oxoselenate sheets observed in the structures of 1, 2, and 3 are related to the same simple and highly‐symmetric graph consisting of 3‐connected white and 6‐connected black vertices.

The Crystal Structure of Tobermorite 14 Å (Plombierite), a C–S–H Phase

The crystal structure of tobermorite 14 Å (plombierite) was solved by means of the application of the order–disorder (OD) theory and was refined through synchrotron radiation diffraction data. Two polytypes were detected within one very small crystal from Crestmore, together with possibly disordered sequences of layers, giving diffuse streaks along c*. Only one of the two polytypes could be refined: it has B11b space group symmetry and cell parameters a=6.735(2) Å, b=7.425(2) Å, c=27.987(5) Å, γ=123.25(1)°. The refinement converged to R=0.152 for 1291 reflections with Fo&gt;4σ(Fo). The characteristic reflections of the other polytype, F2dd space group, a≈11.2 Å, b≈7.3 Å, c≈56 Å, were recognized but they were too weak and diffuse to be used in a structure refinement. The structure of tobermorite 14 Å is built up of complex layers, formed by sheets of sevenfold coordinated calcium cations, flanked on both sides by wollastonite‐like chains. The space between two complex layers contains additional calcium cations and H2O molecules; their distribution, as well as the system of hydrogen bonds, are presented and discussed. The crystal chemical formula indicated by the structural results is Ca5Si6O16(OH)2·7H2O.

Cation extraction by 18-crown-6 to a room-temperature ionic liquid: The effect of solvent humidity investigated by molecular dynamics simulations

We report a molecular dynamics study of the solvation of 18-crown-6 (“18C6”) and of its K+, Cs+ and Sr2+ complexes in a room-temperature ionic liquid (IL) based on 1-butyl-3-methyl-imidazolium+, PF6−. “Dry” [BMI][PF6] versus “humid” [BMI][PF6][H2O] models of the IL are compared, demonstrating the importance of solvent humidity on the solvation properties. Upon “dissolution” of a piece of crystal, 18C6 is found to undergo a conformational change from Ci to D3d, mainly due to enhanced interactions with the BMI+ solvent cations and H2O molecules, when present. The complexes were first studied with dissociated counterions. In the dry IL, the complexed K+ and Sr2+ cations are locked at the center of the crown by 1 + 1 (K+), 1 + 2 or 1 + 3 (Sr2+) PF6− anions in facial positions, respectively. The Cs+ cation is perched over the crown, solvated by 3 PF6− anions. In the humid IL, the complexed K+ also binds to 1 + 1 PF6− facial anions only (no water), whereas Sr2+ is asymmetrically coordinated to at least 3 H2O molecules. When co-complexed with Cl− or NO3− counterions, Sr2+ is shielded from the dry IL, but coordinates up to 3 additional H2O molecules in the humid IL, while K+ is not hydrated. The solvation of the “naked” K+, Cs+ and Sr2+ ions also markedly depends on the solvent humidity. K+ is coordinated to 4 PF6− anions in the dry IL and by 2 PF6− plus 3–5 H2O in the humid IL. The most spectacular difference concerns Sr2+, whose first shell is purely anionic (5 PF6−) in the dry IL, but all neutral (8 H2O) in the humid IL. According to an energy component analysis, the 18C6 crown, the cations and their complexes are better solvated by the humid than by the dry IL. Finally, we report simulations of 18C6 and on its Sr ⊂ 18C6(NO3)2 complex at the aqueous interface with the ionic liquid, showing enhanced solvent mixing, compared to the interface with classical organic liquids. The microscopic views obtained by these simulations show the active role of the ionic and aqueous components of the liquid on the solvation of the free crown, the free cations and their complexes.

Solvation of uranyl(II) and europium(III) cations and their chloro complexes in a room-temperature ionic liquid. A theoretical study of the effect of solvent "humidity".

We report a molecular dynamics study of the solvation of the UO2(2+) and Eu3+ cations and their chloro complexes in the [BMI][PF6][H2O] "humid" room-temperature ionic liquid (IL) composed of 1-butyl-3-methylimidazolium+ and PF6- ions and H2O in a 1:1:1 ratio. When compared to the results obtained in dry [BMI][PF6], the present results reveal the importance of water. The "naked" cations form UO2(H2O)5(2+) and Eu(H2O)9(3+) complexes, embedded in a shell of 7 and 8 PF6- anions, respectively. All studied UO2Cln(2-n) and EuCln(3-n) chloro complexes remain stable during the dynamics and coordinate additional H2O molecules in their first shell. UO2Cl4(2-) and EuCl6(3-) are surrounded by an "unsaturated" water shell, followed by a shell of BMI+ cations. According to an energy component analysis, the UO2Cl4(2-) and EuCl6(3-) species, intrinsically unstable toward dissociation, are more stable than their less halogenated analogues in the IL solution, due to the solvation forces. The different chloro species also interact better with the humid than with the dry IL, which hints at the importance of solvent humidity to improve their solubility. Humidity markedly modifies the local ion environment, with major consequences as far as their spectroscopic properties are concerned. We finally compare the aqueous interface of [BMI][PF6] and [OMI][PF6] ionic liquids, demonstrating the importance of imidazolium substituents (N-butyl versus N-octyl) to the nature of the interface and miscibility with water.

Wickenburgite, a double-layer silicate Pb3Al[6][Ca[6]Al[4]Si[4] 10O27(H2O)3]·H2O: Crystal chemistry and thermal behaviour

Abstract To clarify discrepancies in the literature with regard to the water content and crystal structure of wickenburgite, chemical and thermal analyses and IR spectroscopic measurements were made and the structure was redetermined with single-crystal X-ray diffraction data. It was found that the idealised composition of the mineral is Pb3Al2CaSi10O27·4 H2O rather than Pb3Al2CaSi10O27·3 H2O as reported by Lam et al. in 1994. Based on chemical analysis and crystal structure determination, there is considerable isomorphous replacement on the cation positions, leading to the crystal-chemical formula Pb3(Al0.79Pb0.22)Σ1.01 [6]{ uB , 4, 2∞ 2} [Ca1.02 [6](Al0.81Fe0.06Si0.14)Σ1.01[4]Si10.00 [4]O27(H2O)3]·H2O. With regard to crystal structure, that described by Lam et al. is found to be essentially correct, except for an additional H2O molecule per formula unit, whereas the structure suggested by Penna &amp; Giese in 1968 and by Hesse &amp; Liebau in 1992 and 1993 is incorrect. The wickenburgite structure contains silicate double-layers in which one out of twelve [(Al, Si)O4] tetrahedra is replaced by an [O3Ca(H2O)3] octahedron. Adjacent double layers are held together by Al3+ and Pb2+ cations that are octahedrally and irregularly eight-fold coordinated, respectively. The fourth H2O molecule per formula unit occupies a small cavity which is formed by six oxygen atoms from two adjacent double layers and three interlayer lead ions, thus shielding the repulsive forces between the lone-pair electrons of the latter. Minor but varying amounts of adhering, non-crystalline water is reversibly lost upon heating below 100°C. The cavity water is steadily and irreversibly lost between approximately 100 and 550°C (at a heating rate of 10°C per minute). Before all cavity water has been driven out, water coordinating the Ca2+ ions begins to escape. This dehydration process is accompanied by an approximately 4.4 percent reduction of the unit-cell volume without substantial disruption of the structure. At 740 °C the dehydration is not yet complete. The structure of wickenburgite is related to that of jagoite which contains both silicate single- and double-layers. In the double-layers of wickenburgite one out of twelve [(Al, Si)O4] tetrahedra is replaced by an [O3Ca(H2O)3] octahedron whereas in jagoite within the double-layers two out of twelve [(Fe, Si)O4] tetrahedra are replaced by [PbO3] pyramids.

Density Functional Studies of the Formation of Nitrous Acid from the Reaction of Nitrogen Dioxide and Water Vapor

Reaction mechanisms for the production of nitrous acid (HONO) from the homogeneous gas-phase hydrolysis of nitrogen dioxide (NO2 ) are examined by density functional theory calculations. The molecular structures and energies of the NO2−(H2O)n (n = 1, 2, 3) and N2O4−(H2 O)n (n = 1, 2) systems corresponding to the stationary points on the potential energy surface along the reaction pathways are calculated using the B3LYP method with the 6-311+G(2d,p) basis set. These reaction pathways represent the homogeneous hydrolysis of NO2 or N2O4 with a varying number of water (H2O) molecules. The reactions of NO2 with water produce HONO, along with the OH radical which was postulated to combine in the next step with a second NO2 to form nitric acid (HNO3). The simple NO2 + H2O bimolecular reaction leads to the highly unstable OH radical which reacts reversibly with HONO without an energy barrier. The introduction of single solvating H2O molecule appears to stabilize the transition state as well as an intermediate that contains the OH radical. However, the energy barrier is found to be near 30 kcal mol-1 and is not affected by multiple additional H2O molecules. On the other hand, the reaction of N2O4 with water leads directly to HONO and HNO3. The energy barrier for the N2O4 reaction is above 30 kcal mol-1 and is also unaffected by additional H2O molecules. The study demonstrates that the gas-phase hydrolysis of NO2 or N2O4 is insignificant regardless of water vapor pressure. The physical origin responsible for the unusual hydrolysis reaction of NO2 is explored with the contrasting examples of N2O5 and SO3 hydrolysis reactions.

Clinotobermorite, Ca5[Si3O8(OH)]2 · 4 H2O \p=n-\ Ca5[Si6O17] · 5 H2O, a natural C\p=n-\S\p=n-\H(I) type cement mineral: determination of the substructure

Abstract The substructure of natural clinotobermorite Ca5[Si3O8(OH)]2 · 4 H2O \p=n-\ Ca5[Si6Ol7] · 5 H2O from the Wessels mine, Kalahari manganese field, South Africa, was determined from single-crystal X-ray data. The diffraction pattern (monoclinic: a = 11.19 Å, b = 7.29 Å, c = 22.46 Å, β = 96.97°) is characterised by rather diffuse reflections of the type hkl with h, k = 2n + 1. The substructure (space group I2/m, Z = 1, a/2 = 5.593(6) Å, b/2 = 3.645(4) Å, c = 22.46(3) Å, β = 96.97(2)°) was solved from the sharp Bragg reflections (hkl, with h, k = 2n) and refined on the basis of 1877 unique reflections to R(F) = 6.7%. The structure of clinotobermorite is closely related to the one of so called 11 Å tobermorite Ca5Si6O16(OH)2 · 2 H2O (pseudo-orthorhombic, a = 11.17 Å, b = 7.38 Å, c = 22.6 Å). In both minerals layers parallel (001), formed by seven-fold co-ordinated Ca, are flanked on either side by wollastonite-like [Si3O9] single chains (ca. 7.3 Å periodicity along b) creating a sandwich-like assemblage. The short b/2 axis (3.645 Å) and the short a/2 axis (5.593 Å) of the subcell are caused by an overlay of two half-occupied single chains of tetrahedra with one shifted by b/2 against the other. In clinotobermorite the sandwich-like layers are stacked along c yielding an interlayer distance of 4.5 Å with a free aperture of 3.2 Å where additional Ca ions and H2O molecules connect the sandwich units. Clinotobermorite with 4–5 H2O represents a much denser structure than 11 Å tobermorite with only 2 H2O or 14 Å tobermorite with ca. 6 H2O (pseudo-orthorhombic, a = 11.17 Å, b = 7.38 Å, c = 27.94 Å). The substructure of clinotobermorite can be interpreted as either composed of ‘Dreiereinfachketten’ leading to the formula Ca5[Si3O8(OH)]2 · 4 H2O, or of ‘Dreierdoppelketten’ leading to the formula Ca5[Si6O17] · 5 H2O, or of a mixture of both variants. The surface structure of clinotobermorite was studied by atomic force microscopy (AFM) where low magnifications reveal a complex layering or twinning parallel (001). High resolutions scans parallel (100) establish enhanced contrast fringes with a periodicity of ca. 10 Å interpreted as the repeat distance of the Ca polyhedral layers parallel to c.

Eight‐Coordinate [Bis(oxyiminodiactate titanium(IV)]2− and Nine‐Coordinate [Bis(oxyiminodiacetate)aquazirconium(IV)]2−; Variation in Coordination Mode in Amavadin‐Like Complexes

N-hydroxyiminodiacetic acid (H3(hida) reacts with [TiO(acac)2] or [Zr)acac)4] to form [Ti(hida)2]2− or These complexes crystallize from water with Ca2+ counter ions in the form of extended networks. The metal center in the TiIV complex is eight-coordinate, whereas the larger ZrIV center is nine-coordinated with ligation of an additional H2O molecule (see picture on the right).

Infrared Spectroscopic Analyses on the Nature of Water in Montmorillonite

AbstractInterlayer cations and moisture content greatly influence the molecular vibrations of H2O in montmorillonite as shown through reflectance spectroscopy in the infrared. The absorptions due to H2O have been studied in montmorillonites exchanged with H, Na, Ca, Mg and Fe3+ interlayer cations under variable moisture environments. Band assignments have been made for absorptions in the 3 µm region due to structural OH vibrations, symmetric and asymmetric H2O stretching vibrations and the H2O bending overtone. Changes in the energies of the absorptions due to H2O stretching vibrations were observed as the samples were dehydrated by reducing the atmospheric pressure. Absorptions near 3620 cm−1 and 3550 cm−1 have been assigned to water bound directly to cations (inner sphere) and surface-bonded H2O and absorptions near 3450 cm−1 and 3350 cm−1 have been assigned to additional adsorbed water molecules. Band assignments have been made for combination bands in the near-infrared as well. Absorptions near 1.41 μm and 1.91 μm are assigned to bound H2O combination bands, while the shoulders near 1.46μm and 1.97 μm are assigned to combinations of additional H2O molecules adsorbed in the interlayer regions and along grain surfaces.

Structural transformations in natrolite and edingtonite

Results of structural transformation studies in the natural fibrous zeolites natrolite and edingtonite are presented. The minerals were studied in situ in a wide range of temperatures, pressures and compositions by differential scanning microcalorimetry, thermogravimetry, dilatometry, X-ray diffractometry, nuclear magnetic resonance (NMR)and Raman-spectroscopy. The high-pressure experiments were done in diamond anvil cell and Be-bronze bomb for NMR using liquids with various dimensions of molecules as pressure-transmitting media. A number of structural transformations in natrolites and edingtonite have been found with constant or changing water content during transformation. Under high water pressures, some additional H2O molecules entered the framework channels causing a framework deformation and an anisotropic “swelling” of the crystal. Under compression in nonpenetrating liquids no transformations were detected and above 70 kbar amorphization of the minerals was observed. The same displacive-tilt transformations were observed in zeolites at elevated temperatures as a result of the dehydration, e.g. natrolite ⇌ α-metanatrolite at 280° C. Fully reversible phase transitions at constant H2O content were observed in natrolites and edingtonite at low temperatures (down to -120° C). These are connected with a variation in the mobility and position of exchange cations and water molecules within the framework channels and are followed by significant volume and thermal effects. In dehydrated zeolites, the transformations were found to be similar to the α ⇌ β transition in quartz (α-metanatrolite ⇌ β-metanatrolite). Heating of fibrous zeolites above 500 ÷ 700° C causes their amorphization and formation of porous quasiglass. The principal difference in structural behaviour of microporous crystals under compression in penetrating and nonpenetrating media has essential geochemical implications. Structural transformations of zeolites in P-T-X space demonstrate crystal chemical analogy of these parameters. Some deviations from this analogy depend on complex interactions between channel “filling”, H2O and cations, and the [(Al,Si)-O4/2] framework.

Conformation of a 16-residue zervamicin IIA analog peptide containing three different structural features: 3(10)-helix, alpha-helix, and beta-bend ribbon.

Boc-Trp-Ile-Ala-Aib-Ile-Val-Aib-Leu-Aib-Pro-Ala-Aib-Pro-Aib-Pro-Phe-OMe (where Boc is t-butoxycarbonyl and Aib is alpha-aminoisobutyric acid), a synthetic apolar analog of the membrane-active fungal peptide antibiotic zervamycin IIA, crystallizes in space group P1 with Z = 1 and cell parameters a = 9.086 +/- 0.002 A, b = 10.410 +/- 0.002 A, c = 28.188 +/- 0.004 A, alpha = 86.13 +/- 0.01 degrees, beta = 87.90 +/- 0.01 degrees, and gamma = 89.27 +/- 0.01 degrees; overall agreement factor R = 7.3% for 7180 data (F0 greater than 3 sigma) and 0.91-A resolution. The peptide backbone makes a continuous spiral that begins as a 3(10)-helix at the N-terminus, changes to an alpha-helix for two turns, and ends in a spiral of three beta-bends in a ribbon. Each of the beta-bends contains a proline residue at one of the corners. The torsion angles phi i range from -51 degrees to -91 degrees (average value -64 degrees), and the torsion angles psi i range from -1 degree to -46 degrees (average value -31 degrees). There are 10 intramolecular NH...OC hydrogen bonds in the helix and two direct head-to-tail hydrogen bonds between successive molecules. Two H2O and two CH3OH solvent molecules fill additional space with appropriate hydrogen bonding in the head-to-tail region, and two additional H2O molecules form hydrogen bonds with carbonyl oxygens near the curve in the helix at Pro-10. Since there is only one peptide molecule per cell in space group P1, the molecules repeat only by translation, and consequently the helices pack parallel to each other.