Differences and similarities among hypoxanthinium nitrate hydrate structures

Crystals of hypoxanthinium (6‐oxo‐1 H ,7 H ‐purin‐9‐ium) nitrate hydrates were investigated by means of X‐ray diffraction at different temperatures. The data for hypoxanthinium nitrate monohydrate (C 5 H 5 N 4 O + ·NO 3 − ·H 2 O, Hx1 ) were collected at 20, 105 and 285 K. The room‐temperature phase was reported previously [Schmalle et al. (1990). Acta Cryst. C 46 , 340–342] and the low‐temperature phase has not been investigated yet. The structure underwent a phase transition, which resulted in a change of space group from Pmnb to P 2 1 / n at lower temperature and subsequently in nonmerohedral twinning. The structure of hypoxanthinium dinitrate trihydrate (H 3 O + ·C 5 H 5 N 4 O + ·2NO 3 − ·2H 2 O, Hx2 ) was determined at 20 and 100 K, and also has not been reported previously. The Hx2 structure consists of two types of layers: the `hypoxanthinium nitrate monohydrate' layers (HX) observed in Hx1 and layers of Zundel complex H 3 O + ·H 2 O interacting with nitrate anions (OX). The crystal can be considered as a solid solution of two salts, i.e. hypoxanthinium nitrate monohydrate, C 5 H 5 N 4 O + ·NO 3 − ·H 2 O, and oxonium nitrate monohydrate, H 3 O + (H 2 O)·NO 3 − .