Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
The X-ray crystal and molecular structures of 1-methyl- 1H-perimidin-2(3H)-one (5b) and 1,3-dimethyl-1H-perimidin-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 b axis. Semiempirical calculations at the AM 1 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 (1H and 13C) and in the solid state (CP/MAS 13C) for 1H-perimidin-2(3H)-one (2) and 1H-benzimidazol-2(3H)-one (3) with the conclusion that in both heterocycles the oxo tautomer is the most abundant. The structure in the gas phase was approached by mass spectrometry. In the case of 3, the oxo tautomer loses CO after ionization, while the oxo form of 2 tautomerizes to the hydroxy form which loses, H2O after a [1,3]-H shift. AM 1 calculations were carried out on the ground and ionized states of 2.
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