Pertechnetates - A Structural Study Across the Periodic Table

The number of crystal structures of pertechnetates derived from aqueous solutions has been expanded from seven to over 30. We report the conversion of NH4TcO4 to aqueous HtcO4 via acidic cation exchange. This is followed by the synthesis and structural elucidation of pertechnetate salts of alkaline earth (AE), transition metal I and lanthanoids (Ln) elements. Various degrees of hydration and coordination are discussed. Where possible, a comparison with the perrhenate homologues is made. The described syntheses and materials may be used as novel starting materials for extended technetium research.

Praziquantel Fifty Years on: A Comprehensive Overview of Its Solid State

This review discusses the entire progress made on the anthelmintic drug praziquantel, focusing on the solid state and, therefore, on anhydrous crystalline polymorphs, amorphous forms, and multicomponent systems (i.e., hydrates, solvates, and cocrystals). Despite having been extensively studied over the last 50 years, new polymorphs and the greater part of their cocrystals have only been identified in the past decade. Progress in crystal engineering science (e.g., the use of mechanochemistry as a solid form screening tool and more strategic structure-based methods), along with the development of analytical techniques, including Synchrotron X-ray analyses, spectroscopy, and microscopy, have furthered the identification of unknown crystal structures of the drug. Also, computational modeling has significantly contributed to the prediction and design of new cocrystals by considering structural conformations and interactions energy. Whilst the insights on praziquantel polymorphs discussed in the present review will give a significant contribution to controlling their formation during manufacturing and drug formulation, the detailed multicomponent forms will help in designing and implementing future praziquantel-based functional materials. The latter will hopefully overcome praziquantel's numerous drawbacks and exploit its potential in the field of neglected tropical diseases.

On the identification of hyperhydrated sodium chloride hydrates, stable at icy moon conditions

Sodium chloride is expected to be found on many of the surfaces of icy moons like Europa and Ganymede. However, spectral identification remains elusive as the known NaCl-bearing phases cannot match current observations, which require higher number of water of hydration. Working at relevant conditions for icy worlds, we report the characterization of three "hyperhydrated" sodium chloride (SC) hydrates, and refined two crystal structures [2NaCl·17H2O (SC8.5); NaCl·13H2O (SC13)]. We found that the dissociation of Na+ and Cl- ions within these crystal lattices allows for the high incorporation of water molecules and thus explain their hyperhydration. This finding suggests that a great diversity of hyperhydrated crystalline phases of common salts might be found at similar conditions. Thermodynamic constraints indicate that SC8.5 is stable at room pressure below 235 K, and it could be the most abundant NaCl hydrate on icy moon surfaces like Europa, Titan, Ganymede, Callisto, Enceladus, or Ceres. The finding of these hyperhydrated structures represents a major update to the H2O-NaCl phase diagram. These hyperhydrated structures provide an explanation for the mismatch between the remote observations of the surface of Europa and Ganymede and previously available data on NaCl solids. It also underlines the urgent need for mineralogical exploration and spectral data on hyperhydrates at relevant conditions to help future icy world exploration by space missions.

Unusual shape-preserved pathway of a core-shell phase transition triggered by orientational disorder

The ubiquitous presence of crystal defects provides great potential and opportunities to construct the desired structure (hence with the desired properties) and tailor the synthetic process of crystalline materials. However, little is known about their regulation role in phase transition and crystallization pathways. It was generally thought that a phase transition in solution proceeds predominantly via the solvent-mediated phase-transformation pathway due to energetically high-cost solid-state phase transitions (if any). Herein, we report an unprecedented finding that an orientational disorder defect present in the crystal structure triggers an unusual pathway of a core-shell phase transition with apparent shape-preserved evolution. In the pathway, the solid-state dehydration phase transition occurs inside the crystal prior to its competitive transformation approach mediated by solvent, forming an unconventional core-shell structure. Through a series of combined experimental and computational techniques, we revealed that the presence of crystal defects, introduced by urate tautomerism over the course of crystallization, elevates the metastability of uric acid dihydrate (UAD) crystals and triggers UAD dehydration to the uric acid anhydrate (UAA) phase in the crystal core which precedes with surface dissolution of the shell UAD crystal and recrystallization of the core phase. This unique phase transition could also be related to defect density, which appears to be influenced by the thickness of UAD crystals and crystallization driving force. The discovery of an unusual pathway of the core-shell phase transition suggests that the solid-state phase transition is not necessarily slower than the solvent-mediated phase transformation in solution and provides an alternative approach to constructing the core-shell structure. Moreover, the fundamental role of orientational disorder defects on the phase transition identified in this study demonstrates the feasibility to tailor phase transition and crystallization pathways by strategically importing crystal defects, which has broad applications in crystal engineering.

Determination of BET Specific Surface Area of Hydrate-Anhydrate Systems Susceptible to Phase Transformation Using Inverse Gas Chromatography

Specific surface area (SSA) is an important parameter in drug development that affects other downstream pharmaceutical properties of interest such as reactivity, stability, dissolution, and ultimately bioavailability. Traditionally, the Brunauer-Emmett-Teller (BET) SSA of pharmaceutical powders is measured via gas adsorption (nitrogen or krypton) that is preceded by a prolonged degassing step under low pressure. This degassing step may not be suitable for certain pharmaceutical hydrates that are susceptible to dehydration and phase transformation under reduced pressure and humidity conditions. Therefore, inverse gas chromatography (IGC) was explored as a reliable alternate technique for determining the SSA of model anhydrate-hydrate systems (trehalose and thiamine hydrochloride) that are prone to such phase transformation during SSA measurement. Both trehalose dihydrate and thiamine HCl non-stoichiometric hydrate were found to undergo partial phase transformation to anhydrous forms during BET analysis via degassing and gas adsorption. In contrast, these hydrates remained stable during surface area analysis using IGC owing to measurements under controlled relative humidity. Thus, IGC proved to be a viable technique for SSA measurement of pharmaceutical hydrates without compromising their physical stability.

The Relevance of Crystal Forms in the Pharmaceutical Field: Sword of Damocles or Innovation Tools?

This review is aimed to provide to an “educated but non-expert” readership and an overview of the scientific, commercial, and ethical importance of investigating the crystalline forms (polymorphs, hydrates, and co-crystals) of active pharmaceutical ingredients (API). The existence of multiple crystal forms of an API is relevant not only for the selection of the best solid material to carry through the various stages of drug development, including the choice of dosage and of excipients suitable for drug development and marketing, but also in terms of intellectual property protection and/or extension. This is because the physico-chemical properties, such as solubility, dissolution rate, thermal stability, processability, etc., of the solid API may depend, sometimes dramatically, on the crystal form, with important implications on the drug’s ultimate efficacy. This review will recount how the scientific community and the pharmaceutical industry learned from the catastrophic consequences of the appearance of new, more stable, and unsuspected crystal forms. The relevant aspects of hydrates, the most common pharmaceutical solid solvates, and of co-crystals, the association of two or more solid components in the same crystalline materials, will also be discussed. Examples will be provided of how to tackle multiple crystal forms with screening protocols and theoretical approaches, and ultimately how to turn into discovery and innovation the purposed preparation of new crystalline forms of an API.

All-Atom Molecular Dynamics of Pure Water-Methane Gas Hydrate Systems under Pre-Nucleation Conditions: A Direct Comparison between Experiments and Simulations of Transport Properties for the Tip4p/Ice Water Model

(1) Background: New technologies involving gas hydrates under pre-nucleation conditions such as gas separations and storage have become more prominent. This has necessitated the characterization and modeling of the transport properties of such systems. (2) Methodology: This work explored methane hydrate systems under pre-nucleation conditions. All-atom molecular dynamics simulations were used to quantify the performance of the TIP4P/2005 and TIP4P/Ice water models to predict the viscosity, diffusivity, and thermal conductivity using various formulations. (3) Results: Molecular simulation equilibrium was robustly demonstrated using various measures. The Green–Kubo estimation of viscosity outperformed other formulations when combined with TIP4P/Ice, and the same combination outperformed all TIP4P/2005 formulations. The Green–Kubo TIP4P/Ice estimation of viscosity overestimates (by 84% on average) the viscosity of methane hydrate systems under pre-nucleation conditions across all pressures considered (0–5 MPag). The presence of methane was found to increase the average number of hydrogen bonds over time (6.7–7.8%). TIP4P/Ice methane systems were also found to have 16–19% longer hydrogen bond lifetimes over pure water systems. (4) Conclusion: An inherent limitation in the current water force field for its application in the context of transport properties estimations for methane gas hydrate systems. A re-parametrization of the current force field is suggested as a starting point. Until then, this work may serve as a characterization of the deviance in viscosity prediction.

The Hydrates of TEMPO: Water Vibrations Reveal Radical Microsolvation

An organic radical monohydrate complex is detected in vacuum isolation at low temperature by FTIR supersonic jet spectroscopy for the first time. It is shown to exhibit a rich conformational and vibrational coupling dynamics, which can be drastically reduced by appropriate isotope substitution. Its detection with a new gas recycling infrared spectrometer demonstrates the thermal metastability of the gaseous TEMPO radical even under humid gas conditions. Compared to its almost isoelectronic and isostructural, closed shell ketone analogue, the hydrogen bond of the solvating water is found to be less directional, but stronger and more strongly downshifting the bonded water OH stretch vibration. A second solvent water directs the first one into a metastable hydrogen bond position to solvate the nitrogen center and the first water at the same time.

New Experimental Installation to Determine Adsorptive Properties of Magnesium Sulphate

Adsorption processes are of great interest in catalysis, material separation, and thermal management. In recent decades, adsorbents have been further investigated because of their applications in adsorption refrigeration, heat pumps, and thermal energy storage. Thus, there is an increasing need to determine the macroscopic properties of the adsorbent, specifically their adsorption/desorption capacity and speed, because these two factors determine the power and size of the corresponding adsorber. Many designs have been proposed, but there is still not a generally adopted technology for the analysis of those properties. In this paper, a novel instrument is described, which is capable of determining the macrokinetic properties of an adsorbent composite, with better control and higher accuracy than gravimetric, volumetric, or barometric installations, and lower price and complexity than spectroscopic installations. The design of the installation is detailed, highlighting the main challenges and critical factors. The two working modes of the installation are described, and one example is provided and analyzed for each of them.

Crystal forms in pharmaceutical applications: olanzapine, a gift to crystal chemistry that keeps on giving

This contribution reviews the efforts of many scientists around the world to discover and structurally characterize olanzapine crystal forms, clearing up inconsistencies in the scientific and patent literature and highlighting the challenges in identifying new forms amidst 60+ known polymorphs and solvates. Owing to its remarkable solid-state chemistry, olanzapine has emerged over the last three decades as a popular tool compound for developing new experimental and computational methods for enhanced molecular level understanding of solid-state structure, form diversity and crystallization outcomes. This article highlights the role of olanzapine in advancing the fundamental understanding of crystal forms, interactions within crystal structures, and growth units in molecular crystallization, as well as influencing the way in which drugs are developed today.

Promoting the Insertion of Molecular Hydrogen in Tetrahydrofuran Hydrate With the Help of Acidic Additives

Among hydrogen storage materials, hydrogen hydrates have received a particular attention over the last decades. The pure hydrogen hydrate is generated only at extremely high-pressure (few thousands of bars) and the formation conditions are known to be softened by co-including guest molecules such as tetrahydrofuran (THF). Since this discovery, there have been considerable efforts to optimize the storage capacities in hydrates through the variability of the formation condition, of the cage occupancy, of the chemical composition or of the hydrate structure (ranging from clathrate to semi-clathrate). In addition to this issue, the hydrogen insertion mechanism plays also a crucial role not only at a fundamental level, but also in view of potential applications. This paper aims at studying the molecular hydrogen diffusion in the THF hydrate by in-situ confocal Raman microspectroscopy and imaging, and at investigating the impact of strong acid onto this diffusive process. This study represents the first report to shed light on hydrogen diffusion in acidic THF-H₂ hydrate. Integrating the present result with those from previous experimental investigations, it is shown that the hydrogen insertion in the THF hydrate is optimum for a pressure of ca. 55 bar at 270 K. Moreover, the co-inclusion of perchloric acid (with concentration as low as 1 acidic molecules per 136 water molecules) lead to promote the molecular hydrogen insertion within the hydrate structure. The hydrogen diffusion coefficient—measured at 270 K and 200 bar—is improved by a factor of 2 thanks to the acidic additive.

Pharmaceutical Hydrates Analysis-Overview of Methods and Recent Advances

This review discusses a set of instrumental and computational methods that are used to characterize hydrated forms of APIs (active pharmaceutical ingredients). The focus has been put on highlighting advantages as well as on presenting some limitations of the selected analytical approaches. This has been performed in order to facilitate the choice of an appropriate method depending on the type of the structural feature that is to be analyzed, that is, degree of hydration, crystal structure and dynamics, and (de)hydration kinetics. The presented techniques include X-ray diffraction (single crystal X-ray diffraction (SCXRD), powder X-ray diffraction (PXRD)), spectroscopic (solid state nuclear magnetic resonance spectroscopy (ssNMR), Fourier-transformed infrared spectroscopy (FT-IR), Raman spectroscopy), thermal (differential scanning calorimetry (DSC), thermogravimetric analysis (TGA)), gravimetric (dynamic vapour sorption (DVS)), and computational (molecular mechanics (MM), Quantum Mechanics (QM), molecular dynamics (MD)) methods. Further, the successful applications of the presented methods in the studies of hydrated APIs as well as studies on the excipients' influence on these processes have been described in many examples.

A Series of Discrete Dichloride Dihydrates: Characterisation and Symmetry Effects

A series of three discrete dichloride dihydrates [Cl₂ (H₂ O)₂ ]^2- have been isolated with different triaminocyclopropenium (TAC) cations and with different crystallographic symmetries. The cluster exhibits D_2h symmetry with the tris(dimethylamino)cyclopropenium cation [C₃ (NMe₂ )₃ ]⁺ , C_2h symmetry with the fluorinated cation [C₃ (N(CH₂ CF₃ )₂ )(NBu₂ )₂ ]⁺ (containing two 2,2,2-trifluoroethyl substituents) and C_2v symmetry with the more fluorinated [C₃ (N(CH₂ CF₃ )₂ )₂ (NBu₂ )]⁺ cation. The effect of symmetry on the infrared spectra of the dichloride ion-pair clusters, as well as deuterated analogues, has been investigated. The D_2h - and C_2h -symmetric clusters each exhibit two stretching bands in the infrared at 3427 and 3368 cm^-1 for D_2h symmetry and 3444 and 3392 cm^-1 for C_2h symmetry, whereas the C_2v -symmetric cluster exhibits three bands at 3475, 3426 and 3373 cm^-1 . Computational studies were carried out on a [Cl₂ (H₂ O)₂ ]^2- cluster with C_2v symmetry to aid the infrared band assignments.

Caesium propano-ate monohydrate

Caesium propano-ate monohydrate, Cs+·C3H5O2 -·H2O, is composed of two symmetry-independent Cs+ cations, which are situated on the special position 4e of space group P 21 m, one symmetry-independent propano-ate mol-ecule in a general position and a pair of water mol-ecules also situated on special position 4e. Two pairs of these symmetry-independent cations, four propano-ate mol-ecules and two pairs of symmetry-independent water mol-ecules form a repeat unit. These units form columns that are directed along the c axis and possess symmetry mm2. There are four such columns passing through each unit cell. Each column is inter-connected to its neighbours by four bifurcated three-centred Ow-H⋯Op (w = water, p = propano-ate) hydrogen bonds of moderate strength. There are also four intra-molecular Ow-H⋯Op hydrogen bonds of moderate strength within each column. One Cs+ cation is coordinated by six oxygen atoms (two water and four carboxyl-ate) in a trigonal-prismatic geometry, while the other Cs+ cation is coordinated by four water and four carboxyl-ate O atoms in a tetra-gonal-prismatic arrangement.

A Discrete Dichloride Tetrahydrate Trapped by a Cyclopropenium Cation: Structure and Spectroscopic Properties

A discrete dichloride tetrahydrate cluster, [Cl₂ (H₂ O)₄ ]^2- , was obtained as a salt of the bis(diphenylamino)diethylamino cyclopropenium cation [C₃ (NPh₂ )₂ (NEt₂ )]⁺ and characterized by single-crystal X-ray diffraction and infrared spectroscopy. This chloride-chloride ion-pair cluster consists of a [Cl₂ (H₂ O)₂ ]^2- square with opposite edges bridged by water molecules to give a chair-like structure of the non-hydrogen atoms. The solid-state structure is essentially the same as the calculated gas-phase structure. Infrared spectra were also collected on the deuterium analogue [Cl₂ (D₂ O)₄ ]^2- . Computational studies were carried out on gas-phase [Cl₂ (H₂ O)₄ ]^2- to confirm the infrared band assignments in the solid state. The structure and infrared spectrum are consistent with the discrete nature of the cluster.

pH-Dependent Hydration Change in a Gd-Based MRI Contrast Agent with a Phosphonated Ligand

The heptadentate ligand L was shown to form an extremely stable Gd complex at neutral pH with a pGd value of 18.4 at pH 7.4. The X-ray crystal structures of the complexes formed with Gd and Tb displayed two very different coordination behaviors being, respectively, octa- and nonacoordinated. The relaxometric properties of the Gd complex were studied by field-dependent relaxivity measurements at various temperatures and by ¹⁷ O NMR spectroscopy. The pH-dependence of the longitudinal relaxivity profile indicated large changes around neutral pH leading to a very large value of 10.1 mm^-1 ⋅s^-1 (60 MHz, 298 K) at pH 4.7. The changes were attributed to an increase of the hydration number from one water molecule in basic conditions to two at acidic pH. A similar trend was observed for the luminescence of the Eu complex, confirming the change in hydration state. DOSY experiments were performed on the Lu analogue, pointing to the absence of dimers in solution in the considered pH range. A breathing mode of the complex was postulated, which was further supported by ¹ H and ³¹ P NMR spectroscopy of the Yb complex at varying pH and was finally modeled by DFT calculations.

Synthesis and Antiproliferative Screening Of Novel Analogs of Regioselectively Demethylated Colchicine and Thiocolchicine

Colchicine, a pseudoalkaloid isolated from Colchicum autumnale, has been identified as a potent anticancer agent because of its strong antimitotic activity. It was shown that colchicine modifications by regioselective demethylation affected its biological properties. For demethylated colchicine analogs, 10-demethylcolchicine (colchiceine, 1) and 1-demethylthiocolchicine (3), a series of 12 colchicine derivatives including 5 novel esters (2b–c and 4b–d) and 4 carbonates (2e–f and 4e–f) were synthesized. The antiproliferative activity assay, together with in silico evaluation of physicochemical properties, confirmed attractive biological profiles for all obtained compounds. The substitutions of H-donor and H-acceptor sites at C1 in thiocolchicine position provide an efficient control of the hydration affinity and solubility, as demonstrated for anhydrate 3, hemihydrate 4e and monohydrate 4a.

Specific Interactions between Rifamycin Antibiotics and Water Influencing Ability To Overcome Natural Cell Barriers and the Range of Antibacterial Potency

Rifamycins are a group of macrocyclic antibiotics mainly used for the treatment of various bacterial infections including tuberculosis. Spectroscopic studies of rifamycins evidence the formation of temperature- and solvent-dependent equilibria between A-, B-, and C-type conformers in solutions. The B- and C-type conformers of rifamycin antibiotics are exclusively formed in the presence of water molecules. A- and B-type conformers exhibit a hydrophilic and "open" ansa-bridge nature whereas the C-type conformer is more lipophilic due to the presence of a "closed" ansa-bridge structure. The involvement of the lactam moiety of the ansa-bridge in intramolecular H-bonds within rifapentine and rifampicin implicates the formation of a more hydrophilic A-type conformer. In contrast to rifampicin and rifapentine, for rifabutin and rifaximin, the "free" lactam group enhances conformational flexibility of the ansa-bridge, thereby enabling interconversion between A- and C-type conformers. In turn, an equilibrium between A- and C-type conformers for rifamycins improves their adaptation to the changing nature of bacteria cell membranes, especially those of Gram-negative strains and/or to efflux pump systems.

Plasticity in zwitterionic drugs: the bending properties of Pregabalin and Gabapentin and their hydrates

The investigation of mechanical properties in molecular crystals is emerging as a novel area of interest in crystal engineering. Indeed, good mechanical properties are required to manufacture pharmaceutical and technologically relevant substances into usable products. In such endeavour, bendable single crystals help to correlate microscopic structure to macroscopic properties for potential design. The hydrate forms of two anticonvulsant zwitterionic drugs, Pregabalin and Gabapentin, are two examples of crystalline materials that show macroscopic plasticity. The direct comparison of these structures with those of their anhydrous counterparts, which are brittle, suggests that the presence of water is critical for plasticity. In contrast, structural features such as molecular packing and anisotropic distribution of strong and weak interactions seem less important.

Plasticity enhancement in pharmaceutical drugs by water of crystallization: unusual slip planes

Khandavilli et al. [(2019), IUCrJ, 6, 630-634] show the superior plasticity in hydrates of the pharmaceutical drugs, pregabalin and gabapetin, compared with their anhydrous forms. The water in the structure is believed to act as a lubricating agent in the packing of hydrates, thus facilitating slippage of molecules in the plastic bending of the crystals under external mechanical stress.

Structures of the hydrate and dihydrate forms of the DNA-binding radioprotector methyl-pro-amine

Methyl pro-amine {N,N,3-trimethyl-4-[6-(4-methyl-piperazin-1-yl)-1H,3'H-[2,5'-bibenzo[d]imidazol]-2'-yl]aniline}, C28H35N7O2, crystallized as both a dihydrate, C28H31N7·2H2O, and monohydrate, C28H31N7·H2O, form from water in the presence of β-cyclo-dextrin, in the P21/c and P21/n space groups, respectively. The two structures adopt different conformations and tautomeric forms as a result of the differing crystal packing as dictated by hydrogen-bonding inter-actions. The dihydrate crystallizes as a three-dimensional hydrogen-bonded network, while the monohydrate crystallizes as a two-dimensional hydrogen-bonded network.

Crystallization of Cyclophosphamide Monohydrate During Lyophilization

The purpose of this study was to investigate the phase behavior of cyclophosphamide (CPA) during various stages of lyophilization, with special emphasis on obtaining crystalline CPA monohydrate (CPA-MH) in the lyophilized product. Subambient differential scanning calorimetry and low-temperature X-ray diffractometry (LTXRD) were used to study the phase behavior of CPA solution (3.7% w/v). In situ lyophilization in LTXRD chamber was used to monitor the phase transitions occurring during the drying stages. Finally, the implications of these findings were confirmed by freeze-drying the aqueous solution in a laboratory-scale freeze-dryer. The results suggested that CPA remains amorphous during freeze concentration, with a T_g' of -50°C. However, its crystallization as CPA-MH can be induced by annealing the frozen solution between -5°C and -10°C. In situ lyophilization in LTXRD showed that the CPA-MH crystallized during annealing, rapidly dehydrated during primary drying, thereby causing structural collapse. The dehydration of CPA-MH can be prevented by lowering the escaping tendency of water molecules from the crystal lattice of CPA-MH by maintaining the chamber pressure to 300, 400, or 500 mTorr. This study highlights the relationship of process parameters used during lyophilization with the solid form of lyophilized CPA.

Syntheses, Crystallization, and Spectroscopic Characterization of 3,5-Lutidine N-Oxide Dehydrate

The synthesis of 3,5-lutidine N-oxide dehydrate, 1, was achieved in the synthesis route of 2-amino-pyridine-3,5-dicarboxylic acid. Ochiai first used the methodology for non-substituted pyridines in 1957 in a 12 h process, but no X-ray suitable crystals were obtained. The substituted ring used in the methodology presented here clearly influenced the addition of water molecules into the asymmetric unit, which confers a different nucleophilic strength in 1. The X-ray suitable crystal compound 1 was possible due to the stabilization of the negative charge in the oxygen by the presence of two water molecules where the hydrogen atoms donate positive charge into the ring; such water molecules serve well to construct a supramolecular interaction. The hydrated molecules may be possible for the alkaline system that is reached by adjusting the pH to 10. Importantly, the double methyl substituted ring and a reaction time of 5 h, makes it a more versatile method and with wider chemical applications for future ring insertions.

Survey and analysis of crystal polymorphism in organic structures

With the intention of producing the most comprehensive treatment of the prevalence of crystal polymorphism among structurally characterized materials, all polymorphic compounds flagged as such within the Cambridge Structural Database (CSD) are analysed and a list of crystallographically characterized organic polymorphic compounds is assembled. Classifying these structures into subclasses of anhydrates, salts, hydrates, non-hydrated solvates and cocrystals reveals that there are significant variations in polymorphism prevalence as a function of crystal type, a fact which has not previously been recognized in the literature. It is also shown that, as a percentage, polymorphic entries are decreasing temporally within the CSD, with the notable exception of cocrystals, which continue to rise at a rate that is a constant fraction of the overall entries. Some phenomena identified that require additional scrutiny include the relative prevalence of temperature-induced phase transitions among organic salts and the paucity of polymorphism in crystals with three or more chemical components.

Inhibition of the solid state transformation of carbamazepine in aqueous solution: impact of polymeric properties

The effects of polymers on the anhydrate-to-hydrate transformation of carbamazepine (CBZ) was investigated. The three types of polymers studied were polyvinylpyrrolidone (PVP), polyvinyl alcohol (PVA) and substituted celluloses which included hydroxypropyl methylcellulose (HPMC) and methylcellulose (MC). Anhydrous CBZ was added to dilute aqueous polymer solutions and Raman spectroscopy measurements were collected to monitor the kinetics of the solution-mediated transformation to CBZ dihydrate. Polymers exhibiting the greatest inhibition were able to reduce the growth phase of the solution-mediated transformation and change the habit of the hydrate crystal indicating polymer adsorption to the hydrate crystal surface as the mechanism of inhibition. The results of the various polymers showed that short chain substituted celluloses (HPMC and MC) inhibited the CBZ transformation to a much greater extent than longer chains. The same trend was observed for PVP and PVA, but to a lesser extent. These chain length effects were attributed to changes in polymer confirmation when adsorbed on the crystal surface. Additionally, decreasing the percentage of hydroxyl groups on the PVA polymer backbone reduced the ability of the polymer to inhibit the transformation and changing the degree of substitutions of methyl and hydroxypropyl groups on the cellulosic polymer backbone had no effect on the transformation.

Quantum Mechanics Approach to Hydration Energies and Structures of Alanine and Dialanine

A systematic approach to the phenomena related to hydration of biomolecules is reported at the state of the art of electronic-structure methods. Large-scale CCSD(T), MP4-SDQ, MP2, and DFT(M06-2X) calculations for some hydrated complexes of alanine and dialanine (Ala⋅13 H₂ O, Ala₂ H⁺ ⋅18 H₂ O, and Ala₂ ⋅18 H₂ O) are compared with experimental data and other elaborate modeling to assess the reliability of a simple bottom-up approach. The inclusion of a minimal number of water molecules for microhydration of the polar groups together with the polarizable continuum model is sufficient to reproduce the relative bulk thermodynamic functions of the considered biomolecules. These quantities depend on the adopted electronic-structure method, which should be chosen with great care. Nevertheless, the computationally feasible MP2 and M06-2X functionals with the aug-cc-pVTZ basis set satisfactorily reproduce values derived by high-level CCSD(T) and MP4-SDQ methods, and thus they are suitable for future developments of more elaborate and hence more biochemically significant peptides.

[Ag(OH2 )2 ][Ag(SO4 )2 ]: A Hydrate of a Silver(II) Salt

When exposed to air at ambient conditions, AgSO₄ slowly reacts with moisture, yielding AgSO₄ ⋅H₂ O. The crystal structure determination (powder data) shows that it may be described as [Ag(OH₂ )₂ ][Ag(SO₄ )₂ ], with some sulfate groups being shared between different Ag²⁺ cations, resembling in that way its Cu²⁺ analogue. [Ag(OH₂ )₂ ][Ag(SO₄ )₂ ], the first hydrate of a compound of Ag²⁺ , was extensively characterized using many physicochemical methods.

Constraints on oceanic methane emissions west of Svalbard from atmospheric in situ measurements and Lagrangian transport modeling

Methane stored in seabed reservoirs such as methane hydrates can reach the atmosphere in the form of bubbles or dissolved in water. Hydrates could destabilize with rising temperature further increasing greenhouse gas emissions in a warming climate. To assess the impact of oceanic emissions from the area west of Svalbard, where methane hydrates are abundant, we used measurements collected with a research aircraft (Facility for Airborne Atmospheric Measurements) and a ship (Helmer Hansen) during the Summer 2014 and for Zeppelin Observatory for the full year. We present a model-supported analysis of the atmospheric CH₄ mixing ratios measured by the different platforms. To address uncertainty about where CH₄ emissions actually occur, we explored three scenarios: areas with known seeps, a hydrate stability model, and an ocean depth criterion. We then used a budget analysis and a Lagrangian particle dispersion model to compare measurements taken upwind and downwind of the potential CH₄ emission areas. We found small differences between the CH₄ mixing ratios measured upwind and downwind of the potential emission areas during the campaign. By taking into account measurement and sampling uncertainties and by determining the sensitivity of the measured mixing ratios to potential oceanic emissions, we provide upper limits for the CH₄ fluxes. The CH₄ flux during the campaign was small, with an upper limit of 2.5 nmol m^-2 s^-1 in the stability model scenario. The Zeppelin Observatory data for 2014 suggest CH₄ fluxes from the Svalbard continental platform below 0.2 Tg yr^-1. All estimates are in the lower range of values previously reported.

Why do Hydrates (Solvates) Form in Small Neutral Organic Molecules? Exploring the Crystal Form Landscapes of the Alkaloids Brucine and Strychnine

Computational methods were used to generate and explore the crystal structure landscapes of the two alkaloids strychnine and brucine. The computed structures were analyzed and rationalized by correlating the modelling results to a rich pool of available experimental data. Despite their structural similarity, the two compounds show marked differences in the formation of solid forms. For strychnine only one anhydrous form is reported in the literature and two new solvates from 1,4-dioxane were detected in the course of this work. In contrast, 22 solid forms are so far known to exist for brucine, comprising two anhydrates, four hydrates (HyA - HyC and a 5.25-hydrate), twelve solvates (alcohols and acetone) and four heterosolvates (mixed solvates with water and alcohols). For strychnine it is hard to produce any solid form other than the stable anhydrate while the formation of specific solid state forms of brucine is governed by a complex interplay between temperature and relative humidity/water activity and it is rather a challenging to avoid hydrate formation. Differences in crystal packing and the high tendency for brucine to form hydrates are not intuitive from the molecular structure alone, as both molecules have hydrogen bond acceptor groups but lack hydrogen bond donor groups. Only the evaluation of the crystal energy landscapes, in particular the close-packed crystal structures and high-energy open frameworks containing voids of molecular (water) dimensions, allowed us to unravel the diverse solid state behavior of the two alkaloids at a molecular level. In this study we demonstrate that expanding the analysis of anhydrate crystal energy landscapes to higher energy structures and calculating the solvent-accessible volume can be used to estimate non-stoichiometric or channel hydrate (solvate) formation, without explicitly computing the hydrate/solvate crystal energy landscapes.

Stoichiometric and Non-Stoichiometric Hydrates of Brucine

The complex interplay of temperature and water activity (a_w) / relative humidity (RH) on the solid form stability and transformation pathways of three hydrates (HyA, HyB and HyC), an isostructural dehydrate (HyA_dehy ), an anhydrate (AH) and amorphous brucine has been elucidated and the transformation enthalpies quantified. The dihydrate (HyA) shows a non-stoichimetric (de)hydration behavior at RH < 40% at 25 °C and the removal of the water molecules results in an isomorphic dehydrate structure. The metastable dehydration product converts to AH upon storage at driest conditions or to HyA if exposed to moisture. HyB is a stoichiometric tetrahydrate. The loss of the water molecules causes HyB to collapse to an amorphous phase. Amorphous brucine transforms to AH at RH < 40% RH and a mixture of hydrated phases at higher RH values. The third hyrdate (HyC) is only stable at RH ≥ 55% at 25 °C and contains 3.65 to 3.85 mole equivalent of water. Dehydration of HyC occurs in one step at RH < 55% at 25 °C or upon heating and AH is obtained. The AH is the thermodynamically most stable phase of brucine at RH < 40% at 25 °C. Depending on the conditions, temperature and a_w, each of the three hydrates becomes the thermodynamically most stable form. This study demonstrates the importance of applying complimentary analytical techniques and appropriate approaches for understanding the stability ranges and transition behavior between the solid forms of compounds with multiple hydrates.

A Theoretical Study of the Hydration of Methane, from the Aqueous Solution to the sI Hydrate-Liquid Water-Gas Coexistence

Monte Carlo and molecular dynamics simulations were done with three recent water models TIP4P/2005 (Transferable Intermolecular Potential with 4 Points/2005), TIP4P/Ice (Transferable Intermolecular Potential with 4 Points/ Ice) and TIP4Q (Transferable Intermolecular Potential with 4 charges) combined with two models for methane: an all-atom one OPLS-AA (Optimal Parametrization for the Liquid State) and a united-atom one (UA); a correction for the C–O interaction was applied to the latter and used in a third set of simulations. The models were validated by comparison to experimental values of the free energy of hydration at 280, 300, 330 and 370 K, all under a pressure of 1 bar, and to the experimental radial distribution functions at 277, 283 and 291 K, under a pressure of 145 bar. Regardless of the combination rules used for σ_C,O, good agreement was found, except when the correction to the UA model was applied. Thus, further simulations of the sI hydrate were performed with the united-atom model to compare the thermal expansivity to the experiment. A final set of simulations was done with the UA methane model and the three water models, to study the sI hydrate-liquid water-gas coexistence at 80, 230 and 400 bar. The melting temperatures were compared to the experimental values. The results show the need to perform simulations with various different models to attain a reliable and robust molecular image of the systems of interest.

Disorder of Hydrofluorocarbon Molecules Entrapped in the Water Cages of Structure I Clathrate Hydrate

Water versus fluorine: Clathrate hydrates encaging hydrofluorocarbons as guests show both isotropic and anisotropic distributions within host water cages, depending on the number of fluorine atoms in the guest molecule; this is caused by changes in intermolecular interactions to host water molecules in the hydrates.

Stabilization of Organic-Inorganic Perovskite Layers by Partial Substitution of Iodide by Bromide in Methylammonium Lead Iodide

Thin films of the methylammonium lead halides CH3 NH3 Pb(I1-x Brx )3 are prepared on fluorine-doped tin oxide substrates and exposed to humid air in the dark and under illumination. To characterize the stability of the materials, UV/Vis spectra are acquired at fixed intervals, accompanied by XRD, energy-dispersive X-ray spectroscopy, SEM, and confocal laser scanning microscopy. Different degradation mechanisms are observed depending on the environmental conditions. It is found that bromide can successfully suppress the transformation of the perovskite into the monohydrate, presumably owing to stronger hydrogen-bonding interactions with the organic cation. However, under illumination in humid air, rather rapid decomposition of the perovskites was still observed, which is due to phase segregation. The use of increased bromide content in methylammonium lead halide absorbers is discussed in terms of their application in perovskite solar cells.

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

Hydration has a drastic impact on the structure and function of flexible biomolecules, such as aromatic ethylamino neurotransmitters. The structure of monohydrated protonated phenylethylamine (H(+) PEA-H2 O) is investigated by infrared photodissociation (IRPD) spectroscopy of cold cluster ions by using rare-gas (Rg=Ne and Ar) tagging and dispersion-corrected density functional theory calculations at the B3LYP-D3/aug-cc-pVTZ level. Monohydration of this prototypical neurotransmitter gives an insight into the first step of the formation of its solvation shell, especially regarding the competition between intra- and intermolecular interactions. The spectra of Rg-tagged H(+) PEA-H2 O reveal the presence of a stable insertion structure in which the water molecule is located between the positively charged ammonium group and the phenyl ring of H(+) PEA, acting both as a hydrogen bond acceptor (NH(+) ⋅⋅⋅O) and donor (OH⋅⋅⋅π). Two other nearly equivalent isomers, in which water is externally H bonded to one of the free NH groups, are also identified. The balance between insertion and external hydration strongly depends on temperature.

Spectroscopic Observation of the Hydroxy Position in Butanol Hydrates and Its Effect on Hydrate Stability

In this study, we investigate the crystal structures and phase equilibria of butanols+CH₄ +H₂ O systems to reveal the hydroxy group positioning and its effects on hydrate stability. Four clathrate hydrates formed by structural butanol isomers are identified with powder X-ray diffraction (PXRD). In addition, Raman spectroscopy is used to analyze the guest distributions and inclusion behaviors of large alcohol molecules in these hydrate systems. The existence of a free OH indicates that guest molecules can be captured in the large cages of structure II hydrates without any hydrogen-bonding interactions between the hydroxy group of the guests and the water-host framework. However, Raman spectra of the binary (1-butanol+CH₄ ) hydrate do not show the free OH signal, indicating that there could be possible hydrogen-bonding interactions between the guests and hosts. We also measure the four-phase equilibrium conditions of the butanols+CH₄ +H₂ O systems.

Structural Insights into Bound Water in Crystalline Amino Acids: Experimental and Theoretical (17)O NMR

We demonstrate here that the (17)O NMR properties of bound water in a series of amino acids and dipeptides can be determined with a combination of nonspinning and magic-angle spinning experiments using a range of magnetic field strengths from 9.4 to 21.1 T. Furthermore, we propose a (17)O chemical shift fingerprint region for bound water molecules in biological solids that is well outside the previously determined ranges for carbonyl, carboxylic, and hydroxyl oxygens, thereby offering the ability to resolve multiple (17)O environments using rapid one-dimensional NMR techniques. Finally, we compare our experimental data against quantum chemical calculations using GIPAW and hybrid-DFT, finding intriguing discrepancies between the electric field gradients calculated from structures determined by X-ray and neutron diffraction.

Neutron diffraction structures of water in crystalline hydrates of metal salts

Neutron diffraction structures of water molecules in crystalline hydrates of metal salts have been collected from the literature up to December 2011. Statistical methods were used to investigate the influence on the water structures of the position and nature of hydrogen bond acceptors and cations coordinated to the water oxygen. For statistical modelling the data were pruned so that only structures with oxygen as hydrogen acceptors, single hydrogen bonds, and no more than two metals or hydrogens coordinated to the water oxygen were included. Multiple linear regression models were fitted with the water OH bond length and bond angle as response variables. Other variables describing the position and nature of the acceptors and ions coordinated to the waters were taken as explanatory variables. These variables were sufficient to give good models for the bond lengths and angles. There were sufficient structures involving coordinated Mg(2+) or Cu(2+) for a separate statistical modelling to be done for these cases.

Crystal structure of ethyl (6-hy-droxy-1-benzo-furan-3-yl)acetate sesquihydrate

In the title hydrate, C12H12O4·1.5H2O, one of the water mol-ecules in the asymmetric unit is located on a twofold rotation axis. The mol-ecule of the benzo-furan derivative is essentially planar (r.m.s. deviation for the non-H atoms = 0.021 Å), with the ester group adopting a fully extended conformation. In the crystal, O-H⋯O hydrogen bonds between the water mol-ecules and the hy-droxy groups generate a centrosymmetric R 6 (6)(12) ring motif. These R 6 (6)(12) rings are fused, forming a one-dimensional motif extending along the c-axis direction.

Solid-state characterization and transformation of various creatine phosphate sodium hydrates

Creatine phosphate sodium (CPS) salt is a first-line cardiovascular drug for severe diastolic heart failure. The drug exists in different hydrate forms. The marketed drug form was determined as CPS·4.5H2 O (H1); however, the reference standard was supplied as CPS·6H2 O (H2). In this work, we present two newly identified hydrate forms: a thermodynamically stable low hydrate form, CPS·1.5H2 O (H3), and a pressure-sensitive transit form, CPS·7H2 O (H4). The hydrate forms were discovered through a comprehensive solid-state screening experiment and fully characterized using a range of analytical techniques including X-ray powder diffraction (XRPD), FTIR, Raman spectroscopy, hot-stage microscopy (HSM), thermogravimetric analysis, and differential scanning calorimetry. Stability tests revealed that H3 was the most stable hydrate under thermal stimulation. H4 is a pressure-sensitive hydrate and easily transforms to H2 and then H1 upon grinding. The form transformation process was closely monitored using the HSM, variable-temperature XRPD (VT-XRPD), and VT-Raman spectroscopy techniques. Specifically, the transformation of H4 to H1 is characterized in a single-crystal-to-single-crystal transformation process. The newly discovered hydrate form H3 has superior physicochemical properties than the marketed forms and is worthy of further development.

Antifreezes act as catalysts for methane hydrate formation from ice

Contrary to the thermodynamic inhibiting effect of methanol on methane hydrate formation from aqueous phases, hydrate forms quickly at high yield by exposing frozen water-methanol mixtures with methanol concentrations ranging from 0.6-10 wt% to methane gas at pressures from 125 bars at 253 K. Formation rates are some two orders of magnitude greater than those obtained for samples without methanol and conversion of ice is essentially complete. Ammonia has a similar catalytic effect when used in concentrations of 0.3-2.7 wt%. The structure I methane hydrate formed in this manner was characterized by powder X-ray diffraction and Raman spectroscopy. Steps in the possible mechanism of action of methanol were studied with molecular dynamics simulations of the Ih (0001) basal plane exposed to methanol and methane gas. Simulations show that methanol from a surface aqueous layer slowly migrates into the ice lattice. Methane gas is preferentially adsorbed into the aqueous methanol surface layer. Possible consequences of the catalytic methane hydrate formation on hydrate plug formation in gas pipelines, on large scale energy-efficient gas hydrate formation, and in planetary science are discussed.

Polymorphs, hydrates and solvates of a co-crystal of caffeine with anthranilic acid

A polymorph screen on a new 1:1 co-crystal of caffeine, C8H10N4O2, with anthranilic acid, C7H7NO2, has revealed a rich diversity of crystal forms (two polymorphs, two hydrates and seven solvates, including two sets of isostructural solvates). These forms were prepared by liquid-assisted grinding and solution crystallization, and the crystal structures of nine of these forms have been solved using either single-crystal or powder X-ray data. The structures contain O-H...N and N-H...O hydrogen bonds through which caffeine and anthranilic acid molecules assemble to form zigzag-type chains. These chains can interact in an anti-parallel and offset manner to form cage- or channel-type skeletons within which solvent molecules can be located, giving rise to the diversity of forms observed for this co-crystal. In contrast, an equivalent series of liquid-assisted grinding and solution crystallization experiments with the closely related system of theobromine, C7H8N4O2, and anthranilic acid resulted in the formation of only one 1:1 co-crystal form.