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organic compounds
Differential scanning calorimetry in the range 173–303 K reveals the presence of a previously unknown metastable phase of 2-pyrrolidone, C4H7NO, with a melting point of 286 K. Its crystal structure at 173 K is described. The molecules crystallize as centrosymmetric dimers and are linked by N—HO hydrogen bonds with each molecule adopting an unstrained envelope conformation. Theoretical calculations confirm a slight pyramidalization of the N atom.