1,721,350 research outputs found
The Future of Structural Chemistry Nucleates in the Present
No full textA personal view of nearly four decades of structural chemistry is presented. The rise of molecular crystal engineering as an independent cross-disciplinary field of research at the intersection of crystallography with supramolecular chemistry and solid-state reactivity is discussed in light of the development of theoretical and instrumental tools and the need for novel molecular materials. The exchange of ideas and experience within the structural chemistry community has played a fundamental role in reaching the present high level of understanding of the factors that control crystal formation and stability. Crystal-oriented synthetic strategies, including solvent-free mechanochemical methods, have been developed, as well as screening methods and computational approaches. Some drawbacks and limitations are also discussed. The wealth of multiple crystal forms, from single-molecule crystals to solvates, co-crystals, salts and ionic co-crystals, and their polymorphs, is both an open challenge to the crystal engineering paradigm and a source of discovery and innovation
Crystal engineering: from promise to delivery
No full textTwenty years ago, I wrote a Chem. Commun. feature article entitled "Crystal Engineering: where from? Where to?": an update is in order. In this Highlight I argue that molecular crystal engineering, one of the areas of fast development of the field, has definitely reached the stage of "delivering the goods": new functional materials assembled via non-covalent interactions and/or improved properties of existing materials. As a proof of concept, the crystal engineering approach to tackle two contemporary emergencies, namely, urea fertilizer degradation and development of antimicrobial resistance by pathogens, is discussed and application-driven examples are provided.Twenty years ago, I wrote a Chem. Commun. feature article entitled "Crystal Engineering: where from? Where to?": an update is in order
International Year of Crystallography Celebration: Europe and South Africa
No full text2014 is the International Year of
Crystallography which recognises that
our understanding of the material
nature of our world is grounded in our
knowledge of crystallography. Modern
crystallography has passed its 100th
birthday with the centenary anniversary of
the seminal work of W. H. and W. L. Bragg
the father and son who determined the
first X-ray structures of the inorganic materials
sodium chloride, zinc blende and
diamond
Crystal structures of salts of transition-metal halide clusters
No full textThe ion organization in crystalline salts formed by transition-metal carbonyl cluster anions containing
halide ligands has been investigated. It has been shown that the presence of the heteroligand(s) drives
either towards the 'trapping' of two halogen atoms within a cage of counter ions or towards the formation
of anion rows insulated by cation rows. The formation of either type of pattern appears to be due to the size
and mode of bonding of the halogen atoms, the first type being preferred with small terminal halogens or
with large halogens in bridging bonding mode, while the second is observed with large halogens in
terminal bonding mode
Effect of supramolecular complexation of alkali hydrogenselenates with crown ethers and solid-solutions with their hydrogensulfate counterparts on the solid-to-solid phase transition behaviors
Full text linkThis study investigates the structural and phase transition characteristics of supramolecular complexes composed of 18-crown-6 ether and hydrogen selenate (HSeO4−) anions with various cations (K+, Rb+, Cs+). Single crystals of [18-crown-6·K]HSeO4·2H2O, [18-crown-6·Rb]HSeO4·H2O, [18-crown-6·Cs]HSeO4·H2O, [18-crown-6·K]HSeO4, and [18-crown-6·K](HSeO4)0.5(HSO4)0.5 were grown and their structures determined via single-crystal X-ray diffraction. Differential scanning calorimetry and variable-temperature powder X-ray diffraction were employed to analyse dehydration and phase transition behaviors. The inclusion of 18-crown-6 ether significantly lowered the superprotonic phase transition temperatures by approximately 40 °C compared to pure solid acids. Additionally, substituting HSO4− with HSeO4− decreased phase transition temperatures for K and Cs-complexes and modified the phase transition behavior of the Rb-complex from two-step to single-step isostructural phase transition. Attempts to form solid solutions between the HSeO4− and HSO4− complexes yielded mixed results, with notable success in modulating phase transition temperatures in K-complexes
The influence of hydrogen bonding on the planar arrangement of melamine in crystal structures of its solvates, cocrystals and salts
No full textThe hydrogen bonding patterns of melamine as well as mono- and diprotonated melamine have been analysed in five crystal structures of a solvate, a cocrystal and three organic and inorganic salts, namely, melamine DMSO solvate ([mel]·DMSO (1)), melamine theobromine cocrystal ([mel]·[TBR] 3 (2)), dimelaminium ethylenediaminetetraacetate ([mel-H] 2[EDTA-H2]·2H2O (3)), anhydrous dimelaminium sulfate ([mel-H]2[SO4] (4)), and anhydrous melaminium dinitrate ([mel-H2][NO3]2 (5)). Melamine is a versatile molecular building block (tecton) in cocrystals, solvates and salts. Depending on the degree of protonation and/or other molecules or ions present in the structure, parallels could be drawn to determine whether melamine is arranged in a cross-linked manner, in undulating sheets or in the form of perfectly planar sheets in the structure. Graph set analysis was used to compare the geometry of hydrogen bond interactions of the new structures with those of other structures published in the literature. Solvent drop-assisted solid state reactions (kneading) were performed for "green" synthesis of the compounds. The organic salt 3 has high thermal stability as shown by variable-temperature X-ray powder diffraction, which is presumably related to its extensive hydrogen bond network. Rietveld refinements were carried out on laboratory powder diffraction data to confirm the structures of the compounds obtained from single-crystal data. This journal is © the Partner Organisations 2014
Deriving kinetic insights from mechanochemically synthesized compounds using multivariate analysis (MCR-ALS) of powder X-ray diffraction data
Full text linkKinetics information on the progress of the mechanochemical reactions is key to their understanding and
subsequent scale-up. For crystalline materials, the most robust and tested method for obtaining kinetic data
is the Quantitative Phase Analysis (QPA) via Rietveld refinement. In this work, we tested the feasibility of the
Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) method on powder X-ray diffraction
(PXRD) data of mechanochemical processes by studying the system theophylline (TP) and malonic acid
(MA) in a 1 : 1 stoichiometric ratio at different milling conditions. We have highlighted the strengths and
weaknesses of the MCR-ALS method, and we demonstrated why it may be an alternative route to obtain
quantitative information on mechanochemical kinetics
Single crystal to single crystal [2+2] photoreactions in chloride and sulphate salts of 4-amino-cinnamic acid via solid-solution formation: A structural and kinetic study
Full text linkA set of molecular salts with general formula [1H]nA·xH2O (1 = 4-amino-cinnamic acid, An- = NO3-, BF4-, PF6-, SO42-, x = 0, 1) was prepared and structurally characterized. [1H]Cl and [1H]2SO4·H2O(ii) were found to undergo an SCSC stepwise [2+2] photodimerization, which was followed by X-ray diffraction; a kinetic analysis was performed on single crystals of both salts. In the case of [1H]Cl the photoreaction was also studied on polycrystalline materials
The Relevance of Crystal Forms in the Pharmaceutical Field: Sword of Damocles or Innovation Tools?
Full text linkThis 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
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