X-ray powder diffraction experiments are performed to prove the possible crystallization of nitric acid dihydrate (HNO(3).2H(2)O, further denoted NAD) and to determine the best thermal conditions for growing a single crystal. It is shown that the kinetic behaviour of NAD strongly depends on the preliminary thermal treatment. One good single crystal obtained by an in situ adapted Bridgman method procedure enabled determination of the crystal structure. The intensities of diffracted lines with h odd are all very weak. The H atom of nitric acid is delocalized to one water molecule leading to an association of equimolar nitrate (NO(3)(-)) and an H(5)O(2)(+) ionic group. The asymmetric unit contains two such molecules. These two molecules are related by a pseudo a/2 translation (with a 0.3 A mean atomic distance difference), except for one H atom of the water molecules (0.86 A) because of their different orientations in the two molecules. The two molecules, linked by very strong hydrogen bonds, are arranged in layers. Two layers which are linked by weaker hydrogen bonds are approximately oriented along the c axis. The structure may be described by translations of this set of two layers along the c axis without hydrogen bonds leading to a two-dimensional hydrogen-bond network. The structures of the monohydrate (NAM) and trihydrate (NAT) are re-determined for comparisons. These structures may be described by one- and three-dimensional hydrogen-bond networks, respectively.

The crystal structure of a new high-temperature phase of nitric acid dihydrate, HNO(3).2H(2)O, 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 NO(3)(-) and H(5)O(2)(+) ionic groups. The asymmetric unit contains two molecules of HNO(3).2H(2)O. 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 A, except for one H atom of the water molecules (1.5 A), 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.

3,5‐Dihydrobenz[f]indolizin‐3‐one was prepared by a novel dehydration reaction involving the heating of 1,2,3,5,10,10a‐hexahydro[f]indolizine‐3,10‐dione with polyphosphoric acid. The structure of this new compound was established by X‐ray crystallography, by nmr spectroscopy and by reduction to the known products 1,2,3,5‐tetrahydrobenz[f]indolizin‐3‐one and 1,2,3,5,10,10a‐hexahydrobenz[f]indolizin‐3‐one.

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