Differences and similarities among hypoxanthinium nitrate hydrate structures.

Crystals of hypoxanthinium (6-oxo-1H,7H-purin-9-ium) nitrate hydrates were investigated by means of X-ray diffraction at different temperatures. The data for hypoxanthinium nitrate monohydrate (CHNO·NO·HO, Hx1) were collected at 20, 105 and 285 K. The room-temperature phase was reported previously [Schmalle et al. (1990). Acta Cryst. C46, 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 P2/n at lower temperature and subsequently in nonmerohedral twinning. The structure of hypoxanthinium dinitrate trihydrate (HO·CHNO·2NO·2HO, 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 HO·HO interacting with nitrate anions (OX). The crystal can be considered as a solid solution of two salts, i.e. hypoxanthinium nitrate monohydrate, CHNO·NO·HO, and oxonium nitrate monohydrate, HO(HO)·NO.