X-ray and computational studies of some 5-(perfluoroalkenyl) uracils

X-ray structures of 1,3-dimethyl-5-trifluorovinyl-, 5-E- and Z-pentafluoroa lkenyl uracils (compounds 1, 2, 3) were determined. In all cases planar rin g of uracil was slightly distorted around C2, C4, N3 and C6 atoms. The exoc yclic alkenyl group is not coplanar with uracil ring and exocyclic C=C bond was shortened when compared with typical alkenes. Using semiempirical (MND O, AM1, PM3) and ab initio (RHF 4-31 G) methods the structures of stable co nformers were determined and obtained geometries were compared with those f ound experimentally. (C) 2000 Elsevier Science S.A. All rights reserved.