H-Bond Breaking in High-Pressure Urea

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 P4̅2₁m) 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.