Hydrogen bonds NH···O are broken and restored, and their lengths changed by more than 1 Å 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 P4̅21 m) 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 increasing role of van der Waals interactions. None of phases I, III, and IV contain the hydrogen bond types most frequently encountered in urea cocrystals.

High pressure can favor the formation of either thiourea hydrates or anhydrates. Above 0.60 GPa thiourea crystallizes as monohydrate (NH 2 ) 2 CS•H 2 O, while only anhydrous thiourea is obtained from aqueous solution at normal conditions. At 0.70 GPa another hydrate, 3(NH 2 ) 2 CS•2H 2 O, 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.

No abstract

The balance of weak CH...N bonds involving the H 3C- and -CN groups has been related to the structural rearrangement between centrosymmetric and polar acetonitrile structures. The linear highly polar molecules arrange antiparallel in phase alpha 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 beta below 206 K/0.1 MPa and at pressures higher than 0.63(5) GPa/296 K. The alpha <--> beta 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 methanol trimerized at 1.60 GPa and 473 K yielding 4-amino-2,6-dimethylpyrimidine: it was in situ crystallized, and the structure of a single crystal, recovered at ambient conditions, was determined.

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 P6̅m2) 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 linear NH+···N bonded chains are present, which shows that the relaxor properties of dabcoHI are not inherent to the NH+···N aggregation alone. The polymorphic structures differ mainly in the arrangement of the chains and iodide anions, in the dabco conformation, and in the location of protons, in phases IV, V, VIII, and IX disordered and in phases VI and VII ordered. Polymorph VI is highly metastable at normal conditions and only after days or weeks gradually transforms to phase V. The molecular-scale origin of the changes in dielectric and relaxor properties of dabcoHI results from the conjunction of short-range polarization induced by ordered protons in NH+···N bonds in nanoregions of phase V and the specific crystal environment of hexagonal symmetry, leading to frustration of the local crystal field. A series of hydrates and solvated dabcoHI crystals was obtained above 0.50 GPa.