The ab initio crystal structure determination of α-malonic acid from neutron powder diffraction data

The crystal structure of the α phase of malonic acid has been determined ab initio from high-resolution neutron powder diffraction data at 373 K. The structure is orthorhombic: a = 7.64800(1) Å, b = 5.02111(1) Å, c = 11.55803(2) Å, V_c = 443.84 ų with four molecules in the unit cell and is characterised by hydrogen bonded dimeric units linked through symmetry equivalent carboxyl groups. The molecules form zig-zag chains along the c-axis direction but in contrast to the triclinic ambient temperature β phase there is a twisting of the carboxylic groups rendering them equivalent and related by a twofold axis.