Dimers of solid 4-arylhexahydro-1H,3H-pyrido[1,2-c]pyrimidine-1,3-diones.: 13C CP/MAS NMR, x-ray diffraction and semi-empirical MO studies

C-13 cross-polarization magic angle spinning NMR data are reported for eight derivatives of 4-aryl-hexahydro-1H,3H-pyrido[1,2-c]pyrimidine-1,3-diones 1-8 and the x-ray diffraction data for 4 with R = 4'-Me. The crystal structure of 4 shows the formation of centrosymmetric dimers via intermolecular hydrogen bonds with an N2 ... O11 distance of 2.797(3) Angstrom. Deshielding of carbonyl carbons in the solids 1-8 as compared with solution shows that such dimers probably also exist in other compounds. According to the PM3 calculations, three types of dimers with two Cl=O ... HN, two C3=O ... HN bonds and 'mixed,' i.e. with one Cl=O ... HN and one C3=O ... HN bond, are possible. Copyright (C) 2000 John Wiley & Sons, Ltd.