H-Bond Breaking in High-Pressure Urea

Abstract

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.

Co-crystal/salt crystal structure disorder of trichloroacetic acid–N-methylurea complex with double system of homo- and heteronuclear O–H···O/N–H···O hydrogen bonds: X-ray investigation, ab initio and DFT studies

“…Moreover, the urea is of interest in experimental and theoretical investigations because of simplicity of its molecule [43][44][45][46], interesting highly symmetrical crystal structure [47,48] or physical properties [49,50]. In contrast, N-methylurea co-crystals are little recognized by X-ray methods and up to now only two co-crystal structures have been determined [51,52].…”

Thermal Properties and Transition Behavior of Host-Guest Compounds Under High Pressure

“…(2), the significant difference in the magnitude should therefore be due to a stronger scattering strength (shorter l) and/or lower phonon velocity in phase III. The recent comprehensive study of the phase I and III structures [57] provides further information concerning the unusual decrease of κ at the transition to the highdensity phase III. Based on their structural analysis, Olejniczak et al [57] established a major change in the hydrogen-bonded network at the phase I to phase III transition.…”

“…The recent comprehensive study of the phase I and III structures [57] provides further information concerning the unusual decrease of κ at the transition to the highdensity phase III. Based on their structural analysis, Olejniczak et al [57] established a major change in the hydrogen-bonded network at the phase I to phase III transition. In particular, some hydrogen bonds break and the dimensions of others change significantly.…”

“…The structure of phase II is unknown but phases III and IV have been studied in detail and these are both orthorhombic with space group P2 1 2 1 2 1 [54,55,57 ]. Phase V has been suggested to be orthorhombic with space group Pmcn [54,55].…”

Preparation and Evaluation of an Elusive Polymorph of Mefenamic Acid by High Pressure Techniques

“…This has successfully been used to generate new polymorphs of simple compounds e.g. new polymorphs of parabanic acid [8], urea [16], acetamide and paracetamol [8,17].…”