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A variable-temperature study of a phase transition in barbituric acid dihydrate

Lookup NU author(s): Gary Nichol, Professor William Clegg

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Abstract

The crystal structure of barbituric acid dihydrate (C4H 4N2O3·2H2O) 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. B33, 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 P21/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. © 2005 International Union of Crystallography Printed in Great Britain - all rights reserved.

Publication metadata

Author(s): Nichol GS, Clegg W

Publication type: Article

Publication status: Published

Journal: Acta Crystallographica Section B: Structural Science

Year: 2005

Volume: 61

Issue: 4

Pages: 464-472

ISSN (print): 0108-7681

ISSN (electronic): 1600-5740

Publisher: Wiley-Blackwell Munksgaard

URL: http://dx.doi.org/10.1107/S0108768105017258

DOI: 10.1107/S0108768105017258

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