The gas-phase molecular structures of 4,6-dichloropyrimidine, 2,6-dichloropyrazine and 3,6-dichloropyridazine have been determined by electron diffraction (GED) and ab initio calculations, and are compared to their respective parent compounds to demonstrate the effects of chlorination on ring geometry. The crystal structures of the three dichloro compounds are also reported; the intermolecular contacts leading to distortions in the solid phases have been identified. Gas-phase electron diffraction (GED)Sample preparation. The sample of 4,6-dichloropyrimidine was a gift from Dr R. V. H. Jones of Zeneca plc. Both 2,6dichloropyrazine and 3,6-dichloropyridazine were bought from Lancaster Synthesis at 99 and 98% purity and used in the GED analysis without further purification.GED experiments. Electron scattering intensities were recorded on Kodak Electron Image photographic plates using the Edinburgh apparatus. 15 Six plates (three from the long camera distance and three from the short distance) were recorded for each compound and traced digitally using a computer controlled Joyce Loebl MDM6 microdensitometer 16 at the EPSRC Daresbury laboratory. Standard programs were used for the data reduction 17 with the scattering factors of Fink et al. 18 The weighting points used in setting up the offdiagonal weight matrix, s range, scale factors, correlation parameters and electron wavelengths are given in Table 1. Least-squares correlation matrices for the restrained GED refinement for the compounds under study have been supplied † Scale constants were obtained from the successful scaling of the forcefield for 1,3,5-triazine against a set of experimental IR frequencies.

Gas-phase electron-diffraction scattering data and dipolar couplings from NMR experiments in four different liquid crystal solvents have been combined to give a high-accuracy molecular structure of 1,4-difluorobenzene. The anisotropic components of the CF and FF indirect couplings have been deduced directly from the experimental data. The resultant structure has standard deviations of around 0.2 pm for interatomic distances and less than 0.20 for inter-bond angles.

No abstract