Pressure-Promoted CH···O Hydrogen Bonds in Formamide Aggregates

The N–H···O bonded sheets in compressed formamide are reconstructed due to the formation of pressure-promoted C–H···O hydrogen bonds. Isochoric freezing of formamide in high pressure leads to a new β phase. Its structure has been determined by single-crystal X-ray diffraction at 0.88 and 1.20 GPa. The motif of N–H···O bonded sheets in phase α is modified to facilitate the C–H···O contacts and denser packing in phase β, whereas the distance between sheets increases compared to ambient pressure. Despite the strong rearrangement, phases α and β retain a remarkable similarity in the unit-cell dimensions, space-group symmetry P 2 1 / n, and structural fragments. Two symmetry-independent molecules in phase β differ in their H-bonding patterns: each molecule is involved in four N–H···O bonds; however, one of the molecules is H-bonded to four molecules within the sheet and the other to three molecules. The two molecules in phase β form similar C–H···O bonds, competing with the N–H···O bonds for access to the H-accepting C═O groups. The isothermal compressibility measured in a piston-cylinder apparatus confirms that formamide solidifies directly in phase β at 296 K.