Halogen Bonding in Supramolecular Chemistry.

Trifluoromethyltrimethylsilane: nucleophilic trifluoromethylation and beyond.

The electrolyte solution structure for acetonitrile (AN)-lithium salt mixtures has been examined for highly dissociated salts. Phase diagrams are reported for (AN)n-LiN(SO2CF3)2 (LiTFSI) and -LiPF6 electrolytes. Single crystal structures and Raman spectroscopy have been utilized to provide information regarding the solvate species present in the solid-state and liquid phases, as well as the average solvation number variation with salt concentration. Molecular dynamics (MD) simulations of the mixtures have been correlated with the experimental data to provide additional insight into the molecular-level interactions. Quantum chemistry (QC) calculations were performed on (AN)n-Li-(anion)m clusters to validate the ability of the developed many-body polarizable force field (used for the simulations) to accurately describe cluster stability (ionic association). The combination of these techniques provides tremendous insight into the solution structure within these electrolyte mixtures.

https://iopscience.iop.org/article/10.1149/2.035209jes/pdf

Electrolyte Solvation and Ionic Association II. Acetonitrile-Lithium Salt Mixtures: Highly Dissociated Salts

We present an overview of very recent advances in the understanding of structure–mechanical property correlations in molecular crystals. After the introductory part on some classical two-dimensional structures from the literature, we survey recent reports (mostly since 2005) pertinent to the mechanical properties of molecular crystals studied by application of external stress using a range of techniques. This includes both qualitative (shearing, bending and brittle crystals) and quantitative (nanoindentation, powder compaction and high-pressure) studies on establishing the correlation of anisotropic mechanical behaviour with the underlying crystal structure. Section 9, emphasizes on the usefulness of crystal engineering approach to improve the mechanical properties of molecular crystals, particularly the active pharmaceutical ingredients for their better tabletability properties. The parallels of the phenomena in other class of well studied materials are also appropriately drawn and discussed in the context of structure-mechanical property relationship. In the final part we comment on the prospects and ramifications of this emerging field.

Mechanical properties of molecular crystals—applications to crystal engineering

Since the late 1950's, high-pressure structural studies have become increasingly frequent, following the inception of opposed-anvil cells, development of efficient diffractometric equipment (brighter radiation sources both in laboratories and in synchrotron facilities, highly efficient area detectors) and procedures (for crystal mounting, centring, pressure calibration, collecting and correcting data). Consequently, during the last decades, high-pressure crystallography has evolved into a powerful technique which can be routinely applied in laboratories and dedicated synchrotron and neutron facilities. The variation of pressure adds a new thermodynamic dimension to crystal-structure analyses, and extends the understanding of the solid state and materials in general. New areas of thermodynamic exploration of phase diagrams, polymorphism, transformations between different phases and cohesion forces, structure–property relations, and a deeper understanding of matter at the atomic scale in general are accessible with the high-pressure techniques in hand. A brief history, guidelines and requirements for performing high-pressure structural studies are outlined.

/pdf/high-pressure-crystallography-3j5mufzi5c.pdf

High-pressure crystallography

Hydrogen bonds NH···O are broken and restored, and their lengths changed by more than 1 A in the strained crystal environment of urea, (NH2)2CO, when exposed to high pressure. Single crystals of urea phases I, III, and IV were grown in situ in a diamond-anvil cell, and their structures were determined by X-ray diffraction. At 0.48 GPa, on transformation from phase I (tetragonal space group P421m) to phase III (orthorhombic space group P212121), the channel voids characteristic of phase I collapse, one of the NH···O bonds is broken, and the H-acceptor capacity of the oxygen atom is reduced from 4 to 3. Above 2.80 GPa, in phase IV (orthorhombic space group P21212), the H-bonding pattern of phase I and fourfold H-acceptor oxygen are restored. The thermodynamic phase transitions in urea have been rationalized by a microstructural mechanism involving the interplay of pressure-induced molecular reorientations, with hydrogen bonds competing for access to lone-electron pairs of carbonyl oxygen, and by the increa...

H-Bond Breaking in High-Pressure Urea

The balance of weak CH···N bonds involving the H3C− and −C≡N groups has been related to the structural rearrangement between centrosymmetric and polar acetonitrile structures. The linear highly polar molecules arrange antiparallel in phase α below a freezing temperature of 225 K/0.1 MPa and above a freezing pressure of 0.38(5) GPa/296 K and in a polar mode in phase β below 206 K/0.1 MPa and at pressures higher than 0.63(5) GPa/296 K. The α ↔ β phase transition has been considered as a supramolecular reaction between 2-dimensional and 3-dimensional hydrogen-bonding networks, the latter favoring the polar association. Acetonitrile has been in situ pressure-frozen, and its structure has been determined at room temperature by single-crystal X-ray diffraction at 0.57(5), 0.63(5), and 1.50(5)GPa. The crystallization pressure at 296 K has been determined as 0.38(5) GPa both by ruby fluorescence in a diamond anvil-cell and by the compressibility measurement in a cylinder-and-piston device. Acetonitrile mixed with...

Supramolecular reaction between pressure-frozen acetonitrile phases alpha and beta.

A subtle interplay of supramolecular aggregation, crystal symmetry, and bistable proton sites in NH+···N hydrogen bonds in 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI, [C6H13N2]+I−) leads to the giant dielectric response in this compound. This unique feature disappears on increasing the temperature above 410 K or on increasing the pressure above 40 MPa at 296 K. Ten polymorphs of dabcoHI have been obtained in varied thermodynamical conditions. Phase V (space group P6m2) is stable at normal conditions. For five new polymorphs, the destructive phase transitions have been circumvented by growing the single crystals of specific phases in situ in their stability regions, and their space-group symmetries are Pmm2 (phase IV), Pmc21 (phase VI), Pbcm (phase VII), Cmm2 (phase VIII), and P2/c (phase IX). Of all these 10 dabcoHI phases, the giant dielectric response and anisotropic relaxor properties have been evidenced only for phases V and X. In all the determined structures of dabcoHI, linear or nearly lin...

Ten Polymorphs of NH+···N Hydrogen-Bonded 1,4-Diazabicyclo[2.2.2]octane Complexes: Supramolecular Origin of Giant Anisotropic Dielectric Response in Polymorph V

High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH2)2CS·H2O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3(NH2)2CS·2H2O, is formed, but above 1.20 GPa anhydrous thiourea becomes stable again. The single crystals of both hydrates were grown in situ in a diamond-anvil cell and their structures were determined by X-ray diffraction. The structural factors favoring the formation of hydrates above 0.6 GPa involve new types of hydrogen bonds to water molecules and the more efficient molecular packing. The crystallization of thiourea anhydrate above 1.20 GPa coincides with the stability region of ice VI.

Pressure-Dependent Formation and Decomposition of Thiourea Hydrates

The non-solvated crystals of 1,4-diazabicyclo[2.2.2]octane hydroiodide (dabcoHI, [C6H13N2]+I−) obtained at normal conditions are hexagonal and exhibit giant dielectric response and anisotropic relaxer properties. The isochoric crystallization of dabcoHI at 1.80 GPa yields non-solvated dabcoHI crystal and a [C6H14N2]2+2I−:3CH3OH solvate (dabco2HI:3CH3OH) solvate, while pressure exceeding 2.30 GPa triggers the synthesis of N-methyl-1,4-diazabicyclo[2.2.2]octanium iodide, which co-crystallized with methanol (dabcoCH3I:2CH3OH, [C7H15N2]+I−:2CH3OH). Structures of both solvates were determined by single-crystal X-ray diffraction. Compared to non-solvated dabcoHI governed by NH+⋯N hydrogen bonds, in the two methane solvates investigated there are NH⋯O, N⋯HO, and OH⋯I hydrogen bonds, which illustrates the preference to replace the NH+⋯N hydrogen bonds with those involving oxygen atoms in high pressure.

Pressure induced transformations of 1,4-diazabicyclo[2.2.2]octane (dabco) hydroiodide: diprotonation of dabco, its N-methylation and co-crystallization with methanol

Anna Olejniczak

Papers

H-Bond Breaking in High-Pressure Urea

Supramolecular reaction between pressure-frozen acetonitrile phases alpha and beta.

Ten Polymorphs of NH+···N Hydrogen-Bonded 1,4-Diazabicyclo[2.2.2]octane Complexes: Supramolecular Origin of Giant Anisotropic Dielectric Response in Polymorph V

Pressure-Dependent Formation and Decomposition of Thiourea Hydrates

Pressure induced transformations of 1,4-diazabicyclo[2.2.2]octane (dabco) hydroiodide: diprotonation of dabco, its N-methylation and co-crystallization with methanol