Open AccessA variable‐temperature study of a phase transition in barbituric acid dihydrate
Author(s) -
Nichol Gary S.,
Clegg William
Publication year - 2005
Publication title -
acta crystallographica section b
Language(s) - English
Resource type - Journals
eISSN - 1600-5740
pISSN - 0108-7681
DOI - 10.1107/s0108768105017258
Subject(s) - monoclinic crystal system , orthorhombic crystal system , crystallography , crystal structure , hydrogen bond , barbituric acid , crystal (programming language) , chemistry , phase (matter) , phase transition , molecule , condensed matter physics , organic chemistry , physics , computer science , programming language
The crystal structure of barbituric acid dihydrate (C 4 H 4 N 2 O 3 ·2H 2 O) has twice been reported as orthorhombic, space group Pnma , with all atoms (except for CH 2 H atoms) lying on the mirror plane [Al‐Karaghouli et al. (1977). Acta Cryst . B 33 , 1655–1660; Jeffrey et al. (1961). Acta Cryst . 14 , 881–887]. The present study has found that at low temperatures, below 200 K, the crystal structure is no longer orthorhombic but is non‐merohedrally twinned monoclinic, space group P 2 1 / n . This phase is stable down to 100 K. Above 220 K the crystal structure is orthorhombic, and between 200 and 220 K the structure undergoes a phase change, with the monoclinic‐to‐orthorhombic phase transition itself taking place at around 216–217 K. The size of the β angle in the monoclinic structure is temperature dependent; at 100 K β is around 94° and it decreases in magnitude towards 90° as the temperature increases. Although the hydrogen‐bonding motifs are the same for both crystal systems, there are significant differences in the crystal packing, in particular the out‐of‐plane displacement of the two water molecules and the sp 3 ‐hybridized C atom of barbituric acid.