Very small crystals of anhydrous guanine (systematic name: 2-amino-1,7-dihydro-6H-purin-6-one), C5H5N5O, were obtained from an attempted solvothermal synthesis of a potassium complex. Data were collected at 120 K using a synchrotron radiation source. There is one essentially planar molecule in the asymmetric unit. Molecules are linked to each other by N—H⋯N and N—H⋯O hydrogen bonds to form sheets, between which there are π–π stacking interactions. This crystal structure determination demonstrates conclusively that, in the absence of any solvent or other molecules, guanine exists as the amino–keto tautomer in the solid state, with H atoms attached to N1 and N7 (purine numbering), unlike its monohydrate, which has H atoms on N1 and N9.
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Guille et al. (2006) studied this question.
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