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The X-ray crystal and molecular structures of 1 -methyl-1H-perimidin-2(3H)-one (5b) and 1,3-dimethyl-IHperimidin-2(3H)-one (6) were determined. The crystals are built of piles of dimers faced head-to-head in 5b and of alternating independent head-to-tail molecules in 6, both along the 6 axis. Semiempirical calculations at the AM1 level revealed that the eclipsed conformation of the Me groups with respect to the C=O group, found in the crystals, is the most stable. The lack of planarity of the whole molecules is a consequence of the packing forces since it is not found in the calculations. A comparative NMR study was carried out in solution ('H and I3C) and in the solid state (CP/MAS I3C) for lH-perimidin-2(3H)-one (2) and 1H-benzimidazol-2(3H)-one (3) with the conclusion that in both heterocycles the 0x0 tautomer is the most abundant. The structure in the gas phase was approached by mass spectrometry. In the case of 3, the 0x0 tautomer loses CO after ionization, while the 0x0 form of 2 tautomerizes to the hydroxy form which loses H,O after a [1,3]-H shift. AM1 calculations were carried out on the ground and ionized states of 2.

The crystal structures of the title compound and that of its hemihydrate have been determined. The anhydrous compound is roughly planar due to an intramolecular hydrogen bond (IMHB). The orange color can be related to its planarity and to the greater degree of overlapping between the perimidine and the phenyl group than in the hemihydrate. The yellow hemihydrate is not planar and the water molecule joins through O--H 9 9 9 N hydrogen bonds two molecules related by a crystallographic twofold axis. In solution the compound behaves like the anhydrous form.