The crystal structure of formamide

Formamide, I-ICONH~, melts at 2.5 ° C. On the basis of an X-ray diffraction investigation conducted at --50 ° C., it was found that the compound crystallizes in a monoclinic unit cell with: a ---3.69±0-01, b ---9.18+0.025, c ---6.87±0.02 /~, fl ---98±1⁄4% The space group is P21/n. There are four molecules per unit cell. The configuration and orientation of the molecule have been determined, using complete (three-dimensional) X-ray diffraction data. The structure consists of puckered sheets of formamide molecules; adjacent sheets are separated by about 3.1 /~. Within the sheets pairs of formamide molecules associate about centers of symmetry to form almost coplanar 'bimolecular' units. Puckering of the sheets results from the tilt of these bimolecular units relative to one another. Sheets of formamide molecules are approximately parallel to (101). Within each sheet hydrogen bonds of two types exist: one type, 2.93 s /~ long, links monomers together to form 'bimolecular' units; the other type, 2.880 A long, links bimolecular units together to form sheets. The structure may be alternatively described as consisting of chains of formamide molecules cross-linked by hydrogen bonds to form sheets. Bond lengths within the molecule are C-O = 1"255, C-N = 1.300 A. The bond angle O-C-N is 121.5 °. The probable error in bond lengths is 0.013 A.