CONFORMATIONAL STABILITY OF THE HALOACETYL HALIDES

The far-infrared spectra of a series of haloacetyl halides, CH2XC(O)X, in the vapour phase have been recorded at a resolution of 0.10cm -1 in the 350-35 cm-1 region. The fundamental asymmetric torsional frequencies of the more stable trans (two halogen atoms oriented trans to one another) and high-energy conformers have been observed. From these data the asymmetric torsional potential functions governing internal rotation about the C-C bond have been determined. Only fluoroacetyl fluoride and fluoroacetyl chloride have the cis conformation for the high-energy form whereas all the other haloacetyl halides have the gauche conformation for the high-energy form. For those molecules in this series which have the gauche form as the high-energy rotamer the importance of using the value for the torsional dihedral angle in the determination of the potential function governing internal rotation is reviewed. It is shown how complete equilibrium geometries, which have been determined by ab initio Hartree-Fock gradient calculations, can be used in conjunction with microwave and/or electron diffraction studies to obtain more reliable experimental structural parameters for these molecules. Finally, the reliability of the ab initio techniques used at predicting trends in the optimized geometries and potential functions governing internal rotation are assessed. © 1990 Taylor & Francis Ltd.