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

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