A new crystalline phase of nitric acid dihydrate

The crystal structure of a new high-temperature phase of nitric acid dihydrate, HNO3·2H2O, has been determined at 225 K by single-crystal X-ray diffraction. The H atom of the nitric acid is delocalized to one water molecule, leading to an association of equimolar NO3- and H5O2+ ionic groups. The asymmetric unit contains two molecules of HNO3·2H2O. The two independent molecules are related by a pseudo-twofold c axis, by a translation of 0.54 (approximately 1/2) along b, with a mean atomic distance difference of 0.3 Å, except for one H atom of the water molecules (1.5 Å), because of their different orientations in the two molecules. The two independent molecules linked by strong hydrogen bonds, are arranged in layers. These layers are linked by weaker hydrogen bonds oriented approximately along the c axis. A three-dimensional hydrogen-bond network is observed.