Stepwise Dehydration Research Articles

Spectral and thermal studies of MgI2·8H2O

In the present contribution special attention is paid to the spectroscopic and thermal characterization of MgI2·8H2O which is the stable hydrated form at room temperature. The infrared spectra of MgI2·8H2O and its deuterated analogues recorded at room and liquid nitrogen temperature are presented and interpreted. In the low-temperature diference infrared spectrum of the slightly deuterated analogue (≈5% D) at least four bands are found out of the expected five (at 2595, 2550, 2538 and 2495 cm−1) as a result of the uncoupled O–D oscillators in the isotopically isolated HOD molecules. Multiple bands are observed in the water bending region and only two bands of the HOH librational modes are found. For more precise and deep description of the processes occurring upon heating of MgI2·8H2O we have applied simultaneous TG/DTA/Mass spectrometry technique identifying the gases evolved during the thermal transformations. We have established that the thermal decomposition of MgI2·8H2O is a complex process that takes place in two main stages. In the first stage (between 120 and 275 °C) the salt undergoes a partial stepwise dehydration to MgI2·2H2O followed by a hydrolytic decomposition with formation of magnesium hydroxyiodide Mg(OH)1.44I0.56 accompanied with simultaneous release of H2O and HI. In the second stage Mg(OH)1.44I0.56 is completely decomposed to MgO with elimination of gaseous H2O, HI, I2 and H2. Infrared spectra of the annealed samples heated between 190 and 270 °C confirmed the formation of magnesium hydroxyiodide.

Amorphous Nb2O5 as a Selective and Reusable Catalyst for Furfural Production from Xylose in Biphasic Water and Toluene

Aqueous-phase dehydration of xylose into furfural was studied in the presence of amorphous Nb2O5 with water-compatible Bronsted and Lewis acid sites. Nb2O5 was determined as a more active and selective catalyst for xylose dehydration than typical homogeneous Bronsted and Lewis acid catalysts including HCl and Sc(OTf)3, and Nb2O5 converted 93% of xylose with 48% selectivity toward furfural in water at 393 K. No significant loss of the original catalytic activity was observed after Na+-exchange treatment, which indicates that the reaction proceeded only on Lewis acid sites. Isotope-labeling experiments using D2O and xylose suggested that furfural is formed through stepwise dehydration via a highly reactive dicarbonyl intermediate on Nb2O5, whereas typical Lewis acids such as CrCl3 and Sc(OTf)3 convert xylose to furfural in water through hydride transfer and subsequent dehydration via xylulose as a ketose-type intermediate. The difference in the reaction mechanism accounts for the lower activation energy (83...

Cation Migration and Structural Deformations upon Dehydration of Nickel-Exchanged NaY Zeolite: A Combined Neutron Diffraction and Monte Carlo Study

Combining neutron diffraction and classical molecular simulations, we describe the cation migration and associated structural changes taking place in a Ni-exchanged NaY faujasite zeolite upon stepwise dehydration from room temperature up to 400 °C. The cation redistribution between sites and the related framework deformations taking place upon water removal are identified and quantified. Neutron diffraction allows monitoring the zeolite structure, the average cation location and the water content, whereas molecular modeling provides insights into the correlations between the positions of cations and water molecules. Importantly, we demonstrate that the migration of Ni2+ toward highly confined sites upon dehydration is the driving force behind deformation of the hexagonal prisms. The present work illustrates the relevance of combining these two experimental and theoretical approaches to clarify the complex interplay between cation hydration, cation location, and framework deformation. It also underlines th...

Formation of 5-(Hydroxymethyl)furfural by Stepwise Dehydration over TiO2 with Water-Tolerant Lewis Acid Sites

The reaction mechanism for the formation of 5-(hydroxymethyl)furfural (HMF) from glucose in water over TiO2 and phosphate-immobilized TiO2 (phosphate/TiO2) with water-tolerant Lewis acid sites was studied using isotopically labeled molecules and 13C nuclear magnetic resonance measurements for glucose adsorbed on TiO2. Scandium trifluoromethanesulfonate (Sc(OTf)3), a highly active homogeneous Lewis acid catalyst workable in water, converts glucose into HMF through aldose–ketose isomerization between glucose and fructose involving a hydrogen transfer step and subsequent dehydration of fructose. In contrast to Sc(OTf)3, Lewis acid sites on bare TiO2 and phosphate/TiO2 do not form HMF through the isomerization–dehydration route but through the stepwise dehydration of glucose via 3-deoxyglucosone as an intermediate. Continuous extraction of the evolved HMF with 2-sec-butylphenol results in the increase in the HMF selectivity for phosphate/TiO2, even in highly concentrated glucose solution. These results sugges...

Unfolding of Hydrated Alkyl Diammonium Cations Revealed by Cryogenic Ion Mobility-Mass Spectrometry.

Hydration of the ammonium ion plays a key role in determining the biomolecular structure as well as local structure of water in aqueous environments. Experimental data obtained by cryogenic ion mobility-mass spectrometry (cryo-IM-MS) show that dehydration of alkyl diammonium cations induces a distinct unfolding transition at a critical number of water molecules, n = 21 to 23, n = 24 to 26, and n = 27 to 29, for 1,7-diaminoheptane, 1,8-diaminooctane, and 1,10-diaminodecane, respectively. Results are also presented that reveal compelling evidence for unique structural transitions of hydrated ammonium ions associated with the development of the hydrogen-bond network around individual charged groups. The ability to track the evolution of structure upon stepwise dehydration provides direct insight into the intricate interplay between solvent-molecule interactions that are responsible for defining conformations. Such insights are potentially valuable in understanding how ammonium ion solvation influences conformation(s) of larger biomolecules.

From Solution to the Gas Phase: Stepwise Dehydration and Kinetic Trapping of Substance P Reveals the Origin of Peptide Conformations

Past experimental results and molecular dynamics simulations provide evidence that, under some conditions, electrospray ionization (ESI) of biomolecules produces ions that retain elements of solution phase structures. However, there is a dearth of information regarding the question raised by Breuker and McLafferty, "for how long, under what conditions, and to what extent, can solution structure be retained without solvent?" (Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 18145). Here, we use cryogenic ion mobility-mass spectrometry to experimentally probe the structural evolution of the undecapeptide substance P (SP) during the final stages of ESI. The results reveal that anhydrous SP conformers originate from evaporation of cluster ions, specifically, [SP + 2H](2+) (H2O)n (n = 0 to ∼50) and [SP + 3H](3+) (H2O)n (n = 0 to ∼30), and that major structural changes do not occur during the evaporative process. In the case of [SP + 3H](3+), the results demonstrate that a compact dehydrated conformer population can be kinetically trapped on the time scale of several milliseconds, even when an extended gas phase conformation is energetically favorable.

Hydration Dynamics for Vanadia/Titania Catalysts at High Loading: A Combined Theoretical and Experimental Study

Periodic DFT calculations are used to simulate the early stages of hydration of dehydrated high-loading vanadia/titania catalysts. The hydration of molecularly dispersed vanadia is studied by successive additions of water molecules to initially dehydrated models. Special attention is paid to the formation and transformation between different surface species, monovanadates and polyvanadates, and the role of V–OH in the hydration process. It is found that two physical surface processes occur at high vanadia coverage and that their importance depends on the surface water content. First, under mild hydration conditions, a polymerization process increases the number of polyvanadates chains. Polyvanadates formation is preceded by an initial generation of monomers with OV(OH)O2 and OVO3 pyramidal structures. Second, with higher number of water molecules, a solvation process increases the coordination number of vanadium. The interconversion between different surface vanadium oxide species occurs through a fast hydrogen transfer mechanism and depends on water content. Theoretical results are combined with in situ Raman spectra acquired at several temperatures in dry and humid environment during stepwise dehydration. The experimental data indicate that the redshift of the vanadyl band upon exposure to increasing humidity and decreasing temperature is associated to such progressive interaction with water molecules, which weakens the vanadyl V═O bond.

The role of the cation in the oxygen isotopic exchange in crystalline sulfate salt hydrates

The oxygen isotopic exchange during dehydration and decomposition of five sulfate salt hydrates (CoSO4·6H2O, NiSO4·7H2O, ZnSO4·7H2O, CaSO4·2H2O, Li2SO4·H2O) was studied in detail by temperature programmed desorption mass spectrometry (TPD-MS) in a supersonic molecular beam (SMB) inlet mode. Crystals of the 18O-enriched salts were grown and the detailed desorption steps of the various gaseous products released during dehydration and decomposition of these compounds were recorded. The desorption patterns confirmed the known characteristic stepwise dehydration of these salts, where regardless of the crystalline structure and composition, in all the salts (excluding the Li and Ca sulfates) a major group of n − 1 loosely bounded water of crystallization molecules (out of total of n molecules in the fully hydrated form) are released at adjacent temperatures in a typical low temperature range (<200 °C), while the last, most strongly bounded water molecule, consistently desorbs at relatively higher temperatures (240 < T < 440 °C). Interestingly, it is established that the oxygen isotopic exchange occurs exclusively between that latter, most strongly bound water molecule, and the salt anion. Remarkably, the results point out that the exchange process is mostly of solid-solid nature. Finally, the results point out that the probability of the isotopic exchange increases with the increment in the desorption temperature of the last dehydration step, i.e. with the bond strength in the monohydrate, between the last water molecule of crystallization and the cation.

Hexaaquaaluminium(III) tris(methanesulfonate)

The title compound, [Al(H2O)6](CH3SO3)3 (common name: aluminium methane­sulfonate hexa­hydrate), was crystallized from an aqueous solution prepared by the precipitation reaction of aluminium sulfate and barium methane­sulfonate. Its crystal structure is the first of the boron group methane­sulfonates to be determined. The characteristic building block is a centrosymmetric unit containing two hexa­aqua­aluminium cations that are connected to each other by two O atoms of the –SO3 groups in an O—H⋯O⋯H—O sequence. Further O—H⋯O hydrogen bonding links these blocks in orthogonal directions – along [010] forming a double chain array, along [10-1] forming a layered arrangement of parallel chains and along [101] forming a three-dimensional network. As indicated by the O⋯O distances of 2.600 (3)–2.715 (3) Å, the hydrogen bonds are from medium–strong to strong. A further structural feature is the arrangement of two and four methyl groups, respectively, establishing ‘hydro­phobic islands’ of different size, all positioned in a layer-like region perpendicular to [101]. The only other building block within this region is one of the –SO3 groups giving a local connection between the hydro­philic structural regions on both sides of the ‘hydro­phobic’ one. Thermal analysis indicates that a stepwise dehydration process starts at about 413 K and proceeds via the respective penta- and dihydrate until the compound completely decomposes at about 688 K.

Effect of ethanol-wet bonding with hydrophobic adhesive on caries-affected dentine

Frequently encountered in clinical practice, caries-affected dentine (CAD) is the most challenging bonding substrate. This study evaluated the effect of ethanol-wet bonding with hydrophobic adhesive to sound dentine and to CAD. In the control groups, prepared sound dentine and CAD were bonded with Adper Single Bond 2 using a traditional water-wet bonding technique. In the experimental groups, the specimens were treated as follows: Group 1, rinsed with stepwise ethanol dehydration; Group 2, immersion in 100% ethanol, three times, for 20 s each time; and Group 3, immersion in 100% ethanol for 20 s. Microtensile bond strength (μTBS) testing was used to evaluate the effects of the different protocols on bonding. The microhardness of debonded dentine surfaces was measured to ensure the presence of CAD. Interfacial nanoleakage was evaluated by field-emission scanning electron microscopy. Treatment significantly improved the μTBS in CAD in Groups 1 and 2, but had no effect on Group 3. Conversely, treatment significantly reduced the μTBS in sound dentine in Groups 2 and 3, but had no effect in Group 1. The presence of nanoleakage varied with the ethanol-wet protocol used. In conclusion, ethanol-wet bonding can potentially improve bond efficacy to CAD when an appropriate protocol is used.

Mechanisms of the Knoevenagel hetero Diels–Alder sequence in multicomponent reactions to dihydropyrans: experimental and theoretical investigations into the role of water

The role of water in a multicomponent domino reaction (MCR) involving styrene, 2,4-pentanedione, and formaldehyde was studied. Whereas anhydrous conditions produced no reaction, the MCR successfully proceeded in the presence of water, affording the targeted dihydropyran derivatives with good yield. The mechanism of this MCR (Knoevenagel hetero Diels-Alder sequence) was studied with and without explicit water molecules using the SMD continuum solvation model in combination with the B3LYP density functional and the 6-311++G** basis set to compute the water and acetone (aprotic organic solvent) solution Gibbs free energies. In the Knoevenagel step, we found that water acted as a proton relay to favor the formation of more flexible six-membered ring transition state structures both in concerted (direct H(2)O elimination) and stepwise (keto-enol tautomerization and dehydration) pathways. The inclusion of a water molecule in our model resulted in a significant decrease (-8.5 kcal mol(-1)ΔG(water)(‡)) of the direct water elimination activation barrier. Owing to the presence of water, all chemical steps involved in the MCR mechanism had activation free energies barriers lower than 39 kcal mol(-1) at 25 °C in aqueous solvent (<21 kcal mol(-1) ZPE corrected electronic energies barriers). Consequently, the MCR proceeded without the assistance of any catalyst.

Sorbitol dehydration in high temperature liquid water

Sorbitol dehydration in high temperature liquid water proceeded at 523–573 K without adding any acid catalysts. Anhydrosorbitols (1,4-anhydrosorbitol (1,4-AHSO), 2,5-anhydrosorbitol (2,5-AHSO) and 1,5-anhydrosorbitol (1,5-AHSO)) were produced by the monomolecular dehydration of sorbitol, and isosorbide was produced by the stepwise dehydration of 1,4-AHSO. The formation rates of 1,4-AHSO and 2,5-AHSO (five-membered cyclic ethers) from sorbitol dehydration were much larger than that of 1,5-AHSO (six-membered cyclic ether), and 1,4-AHSO was the main product of the monomolecular dehydration of sorbitol. The dehydration rate of sorbitol to 1,4-AHSO was faster than that of 1,4-AHSO to isosorbide; therefore, 1,4-AHSO could be obtained as an intermediate product. A kinetic analysis of sorbitol dehydration in high temperature liquid water showed that the maximum yield of 1,4-AHSO from the dehydration of sorbitol increased with decreasing reaction temperature (for example, 80% at 500 K) and that the maximum yield of isosorbide was 57% at 590 K for 1 h. 1,4-AHSO and isosorbide could be produced selectively from sorbitol dehydration in high temperature liquid water by controlling both the reaction temperature and reaction time.

Synthesis and studies of magnesium hexafluorozirconates MgZrF6 · nH2O (n = 5, 2, 0)

Crystalline magnesium hexafluorozirconate MgZrF6 · 5H2O isostructural to MnZrF6 · 5H2O, and having a chain-like structure, was synthesized and studied. According to thermogravimetry, the compound undergoes stepwise dehydration in the temperature range of 50–420°C to give the stable phase MgZrF6 · 2H2O and the final product MgZrF6 isostructural to the cubic modification of MZrF6 (M = Cu, Fe). The vibrational spectra of the initial compound and the dehydration products are analyzed and the structures of the compounds are considered.

Uranyl phosphate sheet reconstruction during dehydration of metatorbernite [Cu(UO2)2(PO4)2{middle dot}8H2O

The metatorbernite [Cu(UO2)2(PO4)2·8H2O] structure comprises autunite-type sheets of cornersharing uranyl square bipyramids and phosphate tetrahedra, with the interlayer region occupied by Cu2+ ions and molecular water. Previous studies have shown that heating induces stepwise dehydration and reduction in basal spacing. Structures of the lower hydrates have not been determined previously because suitable single crystals of these phases have yet to be prepared.

Insights into the dehydration behavior of thiamine hydrochloride (Vitamin B1) hydrates: Part I

Thiamine hydrochloride (Vitamin B1, THCl) can exist as a nonstoichiometric hydrate (NSH) and as a hemihydrate (HH). NSH can contain up to ∼1 molar equivalent of water and be dehydrated to an isomorphic desolvate (ID) with minimal change in lattice structure. Crystallographic and spectroscopic techniques were used to characterize the influence of structure and mobility on NSH dehydration. Dehydration was accompanied by lattice contraction, as noted by a decrease in the d-spacings. Dehydration also led to the development of surface cracks parallel to the (101―) and (102―) planes in the NSH single crystal, as observed by hot stage microscopy. Step-wise dehydration of NSH produced gradual shifts in XRPD and SSNMR peaks, indicating that NSH (with ∼1 mole water) and ID represent the two extremes of a continuum in the hydration state. Variable temperature 13C SSNMR studies showed that water molecules move rapidly at room temperature within the NSH crystal lattice, and the thiamine molecules transiently exist in distinct hydrated and dehydrated states. It is hypothesized that, despite the lack of continuous hydration channels in the NSH crystal lattice, cooperative deformation of the thiamine molecules allows a nondisruptive departure of water molecules from the lattice during dehydration. © 2009 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 99:816–827, 2010

Thermodynamic modelling of clay dehydration, stability and compositional evolution with temperature, pressure and H 2O activity

We propose a thermodynamic approach to model the stepwise dehydration with increasing temperature or decreasing H 2O activity of K, Na, Ca and Mg-smectite. The approach relies on the relative stability of the different solid-solutions that describe the hydration of di- or trioctahedral-smectites containing 0, 1, 2 or 3 interlayer water layers. The inclusion of anhydrous mica end-members makes it possible to cover, with the same solid-solution model, the entire range of composition from low-charge smectite to mica, through high-charge smectite and illite. Non-ideal Margules parameters were used to describe the non-ideality of the solid solutions between the hydrated and dehydrated smectite end-members. Standard state properties of all smectite end-members as well as Ca- and Mg-muscovite and -phlogopite were initially estimated by oxide summation. These values were then refined and the other non-ideal interactions were estimated on the basis of different experimental data. The stepwise dehydration of smectite, and its stability and compatibility relations were calculated by Gibbs free energy minimising. Our results account for the progressive evolution of smectite to interlayered illite/smectite and then to mica, as observed in nature and experiments, and our model provides an explanation for the thermodynamic stability of smectite and illite/smectite compared to mica + kaolinite or pyrophyllite assemblages. The results suggest that the enthalpic contribution of interlayer water is a function of the ionic potential of the interlayer cation and the number of interlayer water molecules. This evolution makes possible to estimate the standard-state thermodynamic parameters and hydration-temperature behaviour of smectite of virtually all possible compositions. For the four-interlayer cations considered in the study, our model reproduces the 3 → 2 → 1 water-layer transitions that accompany a reduction of water activity or an increase of temperature at ambient pressure. The range of water content and interlayer distance calculated for the 3w, 2w and 1w states are also in fair agreement with the experimental values at ambient pressure.

Spectrophotometry and Luminescence Spectroscopy of Acetohydroxamate Complexes of Trivalent Lanthanide and Actinide Ions

The complexation chemistries of acetohydroxamic acid (HA) with the trivalent Eu, Tb and Cm ions have been probed by combinations of Optical Absorbance Spectroscopy (OAS), Time Resolved Laser Induced Fluorescence Spectroscopy (TRLIFS) and emission spectroscopy, with some rather unexpected trends being observed. The formation of four complexed species was established for all three metal cations by OAS. The magnitudes of the formation constants of the respective M(A) 3− complexes suggested a much stronger binding efficiency of HA for the first two complexation steps than the third and fourth steps. Tb(III) and Eu(III) TRLIFS data both suggested a final octadentate tetrakis-hydroxamato complex in which the metal ion is close to being fully dehydrated. Step-wise dehydration of Tb(III) by successive ligands did not appear to proceed as expected for a bidentate ligand. Of the EuA n complexes, only the tetrakis species was found to luminesce, with HA causing an unusually strong quenching effect for all other Eu species. Cm(III) complexation appeared similar to the lanthanide analogs.

Thermal and spectroscopic investigations of new lanthanide complexes with 1,2,4-benzenetricarboxylic acid

Abstract The new 1,2,4-benzenetricarboxylates of lanthanide(III) of the formula Ln(btc)�nH2O, where btc is 1,2,4-benzenetricarboxylate; Ln is La-Lu, and n=2 for Ce; n=3 for La, Yb, Lu; and n=4 for Pr-Tm were prepared and characterized by elemental analysis, infrared spectra and X-ray diffraction patterns. Polycrystalline complexes are isotructural in the two groups: La-Tm and Yb, Lu. IR spectra of the complexes show that all carboxylate groups from 1,2,4-benzentricarboxylate ligands are engaged in coordination of lanthanide atoms. The thermal analysis of the investigated complexes in air atmosphere was carried out by means of simultaneous TG-DTA technique. The complexes are stable up to about 30�C but further heating leads to stepwise dehydration. Next, anhydrous complexes decompose to corresponding oxides. The combined TG-FTIR technique was employed to study of decomposition pathway of the investigated complexes.

A physicochemical study of compounds formed in the systems AIII(HSiUO6)3-H2O (AIII = Y, La-Lu)

Compounds in the systems AIII(HSiUO6)3-H2O (AIII = Y, La-Lu) were prepared by stepwise dehydration of uranosilicate AIII(HSiUO6)3 decahydrates (AIII = Y, La-Lu), and their structural transformations were studied by high-temperature X-ray diffraction technique and IR spectroscopy.

Synthetic Study on Telomerase Inhibitor, D8646-2-6: Synthesis of the Key Intermediate Using Sn(OTf)2 or Sc(OTf)3 Mediated Aldol-Type Reaction and Stille Coupling

The synthesis of the key intermediate (4) in the proposed route to D8646-2-6 is described. The aldol reaction of the carbohydrate-containing pyrone 7 with the aldehyde 6 was accomplished by using LiHMDS and Sc(OTf)3 or Sn(OTf)2. The stepwise dehydration reaction of the aldol adduct 14, followed by Stille coupling with vinyl stannane 5 which contained phosphonate gave the desired 4.