{"doi":"10.1107/s0108768105017258","title":"A variable-temperature study of a phase transition in barbituric acid dihydrate","abstract":"<jats:p>The crystal structure of barbituric acid dihydrate (C<jats:sub>4</jats:sub>H<jats:sub>4</jats:sub>N<jats:sub>2</jats:sub>O<jats:sub>3</jats:sub>·2H<jats:sub>2</jats:sub>O) has twice been reported as orthorhombic, space group <jats:italic>Pnma</jats:italic>, with all atoms (except for CH<jats:sub>2</jats:sub> H atoms) lying on the mirror plane [Al-Karaghouli <jats:italic>et al.</jats:italic> (1977). <jats:italic>Acta Cryst</jats:italic>. B<jats:bold>33</jats:bold>, 1655–1660; Jeffrey <jats:italic>et al.</jats:italic> (1961). <jats:italic>Acta Cryst</jats:italic>. <jats:bold>14</jats:bold>, 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 <jats:italic>P</jats:italic>2<jats:sub>1</jats:sub>/<jats:italic>n</jats:italic>. 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 <jats:italic>sp</jats:italic>\n                  <jats:sup>3</jats:sup>-hybridized C atom of barbituric acid.</jats:p>","journal":"Acta Crystallographica Section B Structural Science","year":2005,"id":25600,"datarank":3.162973171122298,"base_score":3.828641396489095,"endowment":3.828641396489095,"self_citation_contribution":0.5742962094733643,"citation_network_contribution":2.588676961648934,"self_endowment_contribution":0.5742962094733643,"citer_contribution":2.588676961648934,"corpus_percentile":null,"corpus_rank":null,"citation_count":45,"citer_count":38,"citers_with_citation_signal":36,"citers_with_endowment":36,"datacite_reuse_total":14,"is_dataset":false,"is_dataset_confidence":null,"is_data_producer":false,"deposit_databanks":null,"is_oa":false,"file_count":0,"downloads":0,"has_version_chain":false,"published_date":null,"fair_score":null,"fair_percentile":null,"algorithm_id":"datarank_citation_only_1hop_v6","ranking_scope":"data_only","authors":[{"id":150213,"name":"William Clegg","orcid":null,"position":1,"is_corresponding":false},{"id":150212,"name":"Gary S. Nichol","orcid":null,"position":0,"is_corresponding":false}],"reference_count":0,"raw_metadata":{"has_enrichment":true,"base_score":3.828641396489095,"endowment":3.828641396489095,"datacite_reuse_total":14,"file_count":0,"downloads":0,"views":0,"has_version_chain":false,"is_dataset":false,"is_oa":false,"pmid":"16041097","pmcid":null,"openalex_id":"https://openalex.org/W2110991995","authors":[],"funders":[],"total_grants":0,"fwci":3.5576,"citation_percentile":0.92190997,"influential_citations":0,"citation_trend":[{"year":2012,"count":1},{"year":2013,"count":1},{"year":2014,"count":4},{"year":2016,"count":4},{"year":2017,"count":2},{"year":2018,"count":1},{"year":2019,"count":3},{"year":2020,"count":2},{"year":2021,"count":1},{"year":2023,"count":2},{"year":2024,"count":4}],"oa_status":"hybrid","license":"cc-by","oa_locations":[{"url":"http://journals.iucr.org/b/issues/2005/04/00/ws5026/ws5026.pdf","host_type":"journal"},{"url":"http://journals.iucr.org/b/issues/2005/04/00/ws5026/ws5026.pdf","host_type":"HYBRID"},{"url":"http://journals.iucr.org/b/issues/2005/04/00/ws5026/ws5026.pdf","host_type":"publisher"},{"url":"https://doi.org/10.1107/s0108768105017258","host_type":"journal"},{"url":"https://pubmed.ncbi.nlm.nih.gov/16041097","host_type":"repository"},{"url":"http://dx.doi.org/10.1107/S0108768105017258","host_type":"repository"}],"fields_of_study":["Crystallography 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