Hydrogen bonding: Part 19. IR and NMR study of the lower hydrates of choline fluoride and acetylcholine chloride

Acetylcholine chloride, like choline chloride, forms a liquid salt dihydrate, and a crystalline monohydrate that only exists at reduced pressure; at atmospheric pressure the monohydrate disproportionates into liquid dihydrate and anhydrous acetylcholine chloride. Both choline and acetylcholine chlorides give endothermic dissolution in water. In contrast, choline fluoride gives exothermic dissclution in water, and forms an extra-ordinarily stable monohydrate in which choline cation hydroxyls form strong hydrogen bonds to an H₄O₂F^2?₂ cluster anion. Since the hydration behavior of choline fluoride is like that of unsubstituted tetraalkylammonium fluorides, the unusual hydration behavior of choline and acetyline chlorides results from the presence of chloride ion, and is not an intrinsic property of cholinergic cations.     

Hydrogen bonding: Part 24. Dichotomous hydration behavior of quaternary ammonium halides

On dehydration in vacuo quaternary ammonium halides show two entirely different types of behavior. Type A salts give first a liquid (4–6 H₂O) of very low vapor pressure, then a series of crystalline framework clathrates (2–4 H₂O), then very stable monohydrates with water—anion dimeric clusters. Type B salts give first a hypobarogenic clathrate (solid at reduced pressure, liquid at 760 torr), then crystalline monohydrates, which, when the pressure is returned to 760 torr, disproportionate into anhydrous salt and same hypobarogenic clathrate. Liquid—solid equilibria for type A at 760 torr is always between framework clathrate and saturated solution (or possibly liquid clathrate) and for type B is between anhydrous salt and hypobarogenic clathrate. Dissolution in water is exothermic for type A salts and endothermic for type B. Examples: type A, choline fluoride, tetramethylammonium fluoride, tetraethylammonium fluoride and chloride, tetrapropylammonium fluoride and chloride; type B, choline chloride, bromide, and iodide, tetramethylammonium chloride and bromide, tetraethylammonium bromide, tetrapropylammonium bromide. Type A behavior is favored by larger cation and smaller (more electronegative) anion, and type B by smaller cation and larger (less electronegative) anion.     

Radiation-induced,solid-state polymerization of derivatives of methacrylic acid. I. Postirradiation polymerization of zinc methacrylate

The postirradiation polymerization of the crystalline, anhydrous, monohydrate, and dihydrate forms of zinc methacrylate was studied. The anhydrous salt polymerized readily in the temperature range 50–150°C., the monohydrate did not polymerize at all, and the dihydrate polymerized at about 100°C. Aging of the anhydrous salts greatly affected the rate of polymerization; this was shown to be due mainly to the formation of peroxides by reaction with air. Polymerization could be initiated thermally, without irradiation, in monomer which had been aged in contact with air, apparently by decomposition of the peroxides. The rate of the postirradiation polymerization was increased when air was present during irradiation and decreased when air was present during polymerization. The rate of polymerization increased with temperature, corresponding to an apparent activation energy of 10 kcal./mole. The dihydrate lost one molecule of water rapidly under vacuum at 20°C. and slowly on heating at 50°C. in a sealed vessel, forming a crystalline monohydrate. Slow thermal polymerization and rapid postirradiation polymerization occurred at 100°C. without the formation of any monohydrate, indicating that the polymerization was concurrent with the phase change.     

The sorption and desorption of water in lactitols

Summary Anhydrous lactitols (A1, α- and β-lactitol), lactitol monohydrate, lactitol dihydrate and lactitol trihydrate were kept for varying times in atmospheres of different relative humidity at 20°C in equivalent size plastic desiccators. The relative humidities (8-95%) were maintained with saturated salt solutions and drying agents (silica gel and phosphorous pentoxide). The composition of the samples was monitored by thermogravimetry, differential scanning calorimetry and X-ray powder diffraction. According to these measurements both lactitol monohydrate and lactitol dihydrate were substantially stable under the conditions used. Lactitol monohydrate converts to lactitol dihydrate at the highest relative humidity used. All phases of anhydrous lactitol convert into a form of lactitol monohydrate but not to lactitol dihydrate, even at the highest relative humidity used. At a high relative humidity lactitol trihydrate easily loses part of its crystal water and converts partly to lactitol dihydrate. At a lower relative humidity, the phase forming from trihydrate is difficult to identify.     

Speciation of copper(II) complexes in an ionic liquid based on choline chloride and in choline chloride/water mixtures

A deep-eutectic solvent with the properties of an ionic liquid is formed when choline chloride is mixed with copper(II) chloride dihydrate in a 1:2 molar ratio. EXAFS and UV-vis-near-IR optical absorption spectroscopy have been used to compare the coordination sphere of the cupric ion in this ionic liquid with that of the cupric ion in solutions of 0.1 M of CuCl(2)·2H(2)O in solvents with varying molar ratios of choline chloride and water. The EXAFS data show that species with three chloride ions and one water molecule coordinated to the cupric ion as well as species with two chloride molecules and two water molecules coordinated to the cupric ion are present in the ionic liquid. On the other hand, a fully hydrated copper(II) ion is formed in an aqueous solution free of choline chloride, and the tetrachlorocuprate(II) complex forms in aqueous choline chloride solutions with more than 50 wt % of choline chloride. In solutions with between 0 and 50 wt % of choline chloride, mixed chloro-aquo complexes occur. Upon standing at room temperature, crystals of CuCl(2)·2H(2)O and of Cu(choline)Cl(3) formed in the ionic liquid. Cu(choline)Cl(3) is the first example of a choline cation coordinating to a transition-metal ion. Crystals of [choline](3)[CuCl(4)][Cl] and of [choline](4)[Cu(4)Cl(10)O] were also synthesized from molecular or ionic liquid solvents, and their crystal structures were determined.     

The thermal decomposition of oxalates Part 14. Dehydration of magnesium oxalate dihydrate

The isothermal dehydration of magnesium oxalate dihydrate has been studied under various pressures of water vapour by use of a thermogravimetric balance. The rate of dehydration was found to be dependent upon the water vapour pressure.The reaction rate at temperatures below 124°C decreases sharply with an increase in water vapour pressure up to 0.5 mm Hg. With further increase in pressure an increase in rates is observed; this rises to a maximum and then falls again as the pressure is increased. The limitation of this phenomena to a limited temperature range is shown in the case of magnesium oxalate dihydrate. Above 124°C the initial fall in rate is not observed, the rate rises with increasing pressure from vacuum to a maximum and the falls. X-ray diffraction studies indicated that the anhydrous product prepared in the second region where a decrease of rate of dehydration occurred was crystalline but the sample dehydrated under vacuum or in the first region produced an amorphous anhydrous salt. A compensation effect is demonstrated with plots of the activation energy against the logarithm of the pre-exponential term in the Arrhenius equation.     

Water vapor adsorption behavior of sodium deoxycholate anhydrous forms

Pseudopolymorphism of sodium deoxycholate (NaDC) was investigated. Intact NaDC (dihydrate) was dried at 60 degrees C under reduced pressure resulting an anhydrous amorphous phase. On the other hand, intact NaDC was altered to an anhydrous crystalline form by heating at 200 degrees C. The water vapor adsorption and desorption isotherms of dehydrated NaDCs were determined using an automatic gravimetric water vapor adsorption analyzer. In the case of NaDC dehydrated at 60 degrees C, the weight was increased in rising relative humidity and it was transformed into the NaDC tetrahydrate above 60% RH, which was identified by TG/DTA and powder X-ray diffraction. During the water vapor adsorption process of the sample dehydrated at 200 degrees C, the NaDC dihydrate was obtained in the range of 50 to 70% RH and then transformed into the NaDC octahydrate above 85% RH. The NaDC octahydrate was characterized by TG/DTA and powder X-ray diffraction for the first time. During the desorption process, the octahydrate was changed to the tetrahydrate between 80 and 40% RH.     

Vibrational spectra of anhydrous and monohydrated caffeine and theophylline molecules and crystals

Density functional theory and classical molecular dynamics simulations are used to investigate the vibrational spectra of caffeine and theophylline anhydrous and monohydrate molecules and those of their crystalline anhydrous and monohydrated states, with emphasis in the terahertz region of the spectra. To better understand the influence of water in the monohydrate crystal spectra, we analyze the vibrational spectra of water monomer, dimer, tetramer, and pentamer, and also those of liquid water at two different temperatures. In small water clusters, we observe the progressive addition of translational and librational modes to the terahertz region of the spectra. The water spectra predicted by rigid and flexible water models is examined with classical molecular dynamics, and the respective peaks, especially in the terahertz region, are compared with those found in the small clusters. Similar analysis done for caffeine and theophylline monohydrate molecules using density functional theory clearly shows the presence of water modes in the librational states and in the water stretching region. Molecular dynamics of caffeine and theophylline anhydrous and monohydrate crystals reveal the influence of vibrations from the molecule-molecule (caffeine or theophylline) crystal stacks and those from the water-molecule interactions found in the monohydrate molecules and new modes from molecule-molecule, water-molecule, and water-water hydrogen bonding interactions arising from collective effects in the crystal structure. Findings illustrate challenges of terahertz technology for the detection of specific substances in condensed phases.     

Untersuchung der Bedingungen zur Gewinnung wasserfreien Zinksulfats

The three component system, zinc sulphate-sulphuric acid—water has been investigated in the region of high sulphuric acid concentrations at 15, 25, 35, 45, 60, and 80°C. In all cases a region of existence of anhydrous zinc sulphate was found. The conditions for obtaining anhydrous zinc sulphate by thermal dehydratation have been studied. The endothermic effect due to the conversion monohydrate—anhydrous salt was observed in the temperature interval from 220 to 290°C. The exact transition temperature depended on the conditions of preparation of the zinc sulphate monohydrate sample. The experimental data obtained by the physico-chemical investigation of the system permitted the determination of the equilibrium pressure of water vapour for the conversion monohydrate—anhydrous salt at 25.0°C.     

Proton‐Conducting Supramolecular Metallogels from the Lowest Molecular Weight Assembler Ligand: A Quote for Simplicity

Oxalic acid has been proven to be the lowest molecular weight organic ligand able to form robust supramolecular metallogel networks in the presence of metal salts. In particular, two novel multifunctional metallogels were readily prepared at room temperature by simple mixing of stock solutions of Cu^II acetate monohydrate or Cu^II perchlorate hexahydrate and oxalic acid dihydrate. Formation of different polymorphs and unprecedented proton conduction under anhydrous conditions were also demonstrated with some of these materials.     

Preparation,stability and acidity of difluoromethylene bis phosphonic acid

Hydrolysis of difluoromethylene phosphonate esters quantitatively yields difluoromethylene $\text{bis}$ phosphonic acid as a dihydrate. $\text{In vacuo}$ drying leads to either the monohydrate or the anhydrous acid. Titration of either the free acid or its disodium salt and computer fit of the data gives all four pKas. The disodium salt and the free acid are thermally stable, and the disodium salt is extremely stable even to strong base.     

Thermal-Dependent dehydration process and intramolecular cyclization of lisinopril dihydrate in the solid state

The pathway of dehydration and intramolecular cyclization of lisinopril dihydrate in the solid state was investigated using differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and a combination of thermal analyzer with Fourier transform infrared microspectroscopy (thermal FT-IR microscopic system). The results indicate that the dehydration from the solid-state lisinopril dihydrate had a two-step process from dihydrate to monohydrate at 76 degrees C and then from monohydrate to anhydrate at 99-101 approximately C, which could be clearly observed from the above three methods. Only the thermal FT-IR microscopic system could give vital information on diketopiperazine (DKP) formation via intramolecular cyclization in anhydrous lisinopril. A new peak at 1670 cm(-1) assigned to the carbonyl band of DKP formation was clearly evidenced. The water of reaction byproduct was liberated at a temperature >157 degrees C and appeared on the IR spectra near 3200-3400 cm(-1). Moreover, the peak at 1574 cm(-1) assigned to carboxylate shifted to 1552 cm(-1) due to the DKP formation. The peak at 1670 cm(-1) related to the DKP formation changed slightly in intensity from 147 degrees C and significantly near 157 degrees C. DSC and TGA methods were poor for use in supplying information on DKP formation in lisinopril. The thermal FT-IR microscopic system is useful from the view point that it can quickly and directly show the solid-state stability of drug.     

Rates and mechanisms of conversion of ice nanocrystals to hydrates of HCl and HBr: acid diffusion in the ionic hydrates

This FTIR study focuses on solid-state chemistry associated with formation and interconversion of the ionic HX (X = Cl, Br) hydrates. Kinetic data are reported for conversions of ice nanocrystal arrays exposed to the saturation pressure of the acids in the 110 approximately 125 K range. The product is amorphous acid dihydrate in the case of HBr, and amorphous monohydrate for HCl. The rate-determining step is identified as HX diffusion through the hydrate product crust toward the interfacial reaction zone, rather than diffusion through ice, as commonly believed. Slowing of the conversion process is thus observed with increasing thickness of the crust. The diffusion coefficient (D(e)) and activation energy values for HX diffusion through the hydrates were evaluated with the help of the shrinking-core model. Hydrate crystallization occurs as a separate step, upon heating above 130 K. Subsequently, rates of reversible transitions between crystal di- and monohydrates were observed upon exposure to acid vapor and acid evacuation. In conversion from di- to monohydrate, the rate slows after fast formation of several layers; subsequently, diffusion through the product crust appears to be the rate-controlling step. The activation energy for HBr diffusion through crystal dihydrate is found to be significantly higher than that for the amorphous analogue. Conjecture is offered for a molecular mechanism of HX transport through the crystal hydrate, based on (i) spectroscopic/computational evidence for the presence of molecular HX bonded to X(-) in each of the ionic hydrate phases and (ii) the relative E(a) values found for HBr and HCl diffusion. Monte Carlo modeling suggests acid transport to the reaction zone along boundaries between "nanocrystallites" generated by multiple hydrate nucleation events at the particle surfaces. The reverse conversion, of crystalline monohydrate particles to the dihydrate phase, as well as dihydrate to trihydrate, displays nearly constant rate throughout the particle conversion; suggesting desorption of HX from the particle surface as the rate-limiting factor. Like for D(e), the activation energies for desorption were found to be approximately 20% greater for HCl than HBr for related hydrate phases.     

Interactions in Calcium Oxalate Hydrate/Surfactant Systems

Phase transformation of calcium oxalate dihydrate (COD) into the thermodynamically stable monohydrate (COM) in anionic (sodium dodecyl sulfate (SDS)) and cationic (dodecylammonium chloride) surfactant solutions has been studied. Both surfactants inhibit, but do not stop transformation from COD to COM due to their preferential adsorption at different crystal faces. SDS acts as a stronger transformation inhibitor. The general shape of adsorption isotherms of both surfactants at the solid/liquid interface is of two-plateau-type, but differences in the adsorption behavior exist. They originate from different ionic and molecular structures of crystal surfaces and interactions between surfactant headgroups and solid surface. Copyright 1999 Academic Press.     

Spreading of partially crystallized oil droplets on an air/water interface

The influence of crystalline fat on the amount and rate of oil spreading out of emulsion droplets onto either a clean or a protein-covered air/water interface was measured for β-lactoglobulin stabilized emulsions prepared with either anhydrous milk fat or a blend of hydrogenated palm fat and sunflower oil. At a clean interface, liquid oil present in the emulsion droplets was observed to completely spread out of the droplets unimpeded by the presence of a fat crystal network. Further, the presence of a fat crystal network in the emulsion droplets had no effect on the rate of oil spreading out of the droplets. At a protein-covered interface, the spreading behavior of emulsion droplets containing crystalline fat was evaluated in terms of the value of the surface pressure (Π_AW) at the point of spreading; Π_AW at spreading was unaffected by the presence of crystalline fat. We conclude it is unlikely that the role of crystalline fat in stabilizing aerated emulsions such as whipped cream is to reduce oil spreading at the air/water interface. However, the temperature of the system did have an effect: spontaneous spreading of emulsion droplets at clean air/water interfaces occurred for systems measured at 5 °C, but not for those measured at 22 or 37 °C. Thus, temperature may play a more important role in the whipping process than commonly thought: the entering and spreading of emulsion droplets was favored at lower temperatures because the surface pressure exerted by protein adsorbed at the air/water interface was reduced. This effect may facilitate the whipping process.     

Solubility equilibria in the uric acid-sodium urate-water system

In a constant ionic medium, corresponding to a physiological environment (I_c = 0.15 mol dm⁻³ NaCl), the solubilities of anhydrous uric acid, uric acid dihydrate and monosodium urate monohydrate have been measured as a function of p[H] = −log[H⁺](2-8) and temperature (25°, 32°, 37° and 42°C). The solubility equilibria in the uric acid-sodium urate-water system are discussed on the basis of the solubility constants (K_s) and the first dissociation constant (K₁) of uric acid and the solubility product (K_s0) of monosodium urate. The quantities measured in this work are in good agreement with literature values, however, the present solubility data have a much higher precision.     

Non-isothermal kinetic and thermodynamic study for the dehydration of copper(II) chloride dihydrate and nickel chloride hexahydrate

Non-isothermal dehydration of copper chloride dihydrate and nickel chloride hexahydrate were studied by using TG, DTG, DTA and DSC measurements. The copper chloride salt loses its two water molecules in one step while nickel chloride salt dehydrates in three consecutive steps. The first two steps involve the loss of 4 water molecules in two overlapped steps while the third step involves the dehydration of the dihydrate salt to give the anhydrous NiCl₂. Activation energies (ΔE) and the frequency factor (A) were calculated from DTG and DTA results. We have also calculated the different thermodynamic parameters, e.g. enthalpy change (ΔH), heat capacity (C _p) and the entropy change (ΔS) from DSC measurements for both reactants. The isothermal rehydration of the completely dehydrated salts was studied in air and under saturated vapour pressure of water. Anhydrous nickel chloride was found to rehydrate in three consecutive steps while the copper salt rehydrated in one step.     

Mechanism of dehydration of magnesium oxalate dihydrate

The isothermal dehydration of magnesium oxalate dihydrate has been studied at various temperatures between 190 and 260°C in the presence of air. The isothermal dehydration curves show the usual sigmoidal character and display an induction period which is highest at 190°C. The dehydration velocity constant (k) values (obtained by the application of Mampel's equation) plotted vs. 1/T according to the Arrhenius equation gave a plot in which two linear sections intersect at ～215°C with activation energies of 12.3 and 18.3 kcal mole^?1 for the lower- and higher-temperature sections, respectively. This behaviour is tentatively explained in terms of a change in the mechanism of dehydration, and not the formation of some new phase other than the dihydrate and the anhydrous oxalate phases present in both the original crystalline oxalate and the sample heated at 200°C for 120 min. X-Ray patterns of the heated oxalate sample left for a few days at room temperature showed a marked sensitivity for rehydration of the anhydrous oxalate phase.     

Hydrates of sodium thiosulphate

Dehydration of sodium thiosulphate pentahydrate with organic solvents produces mixtures of the anhydrous salt and dihydrate for compositions up to 2H₂O and of di- and pentahydrates above this composition, with occasional traces of monohydrate present. This confirms the stable compounds in the Na₂S₂ O-H₂O system. The constituents in mixtures were identified qualitatively and semi-quantitatively by Raman spectroscopy. X-ray powder diffractometry is much inferior for identification purposes. Correlations between vibrational frequencies and structure are suggested.     

Enantiodivergent formation of a chiral cytosine crystal by removal of crystal water from an achiral monohydrate crystal under reduced pressure

An achiral nucleobase cytosine forms an achiral monohydrate crystal (space group: P2₁/c) by crystallization from a water solution. It was found that the removal of crystal water under reduced pressure at room temperature afforded a chiral crystal of anhydrous cytosine (P2₁2₁2₁). The crystal chirality of anhydrous cytosine corresponds to the enantiotopic crystal face of the achiral monohydrate; therefore, when the enantiotopic b¹-face is exposed to the reduced pressure, dehydration occurred in the direction from the b¹-face to provide [CD(+)310_KBr]-cytosine crystal. In contrast, dehydration from the b²-face gave the opposite enantiomorphous [CD(?)310_KBr]-cytosine crystal. The correlation between enantiotopic faces and the formed crystal chirality is opposite to that from dehydration by heating. The formed chiral cytosine crystals act as a chiral trigger for asymmetric autocatalysis with enantioenrichment amplification of pyrimidylalkanol.