AB-INITIO STUDY OF FLUOROCYCLOBUTENES - AN ATTEMPT TO RESOLVE THE DIFFERENCE BETWEEN MICROWAVE SPECTROSCOPY AND ELECTRON-DIFFRACTION GEOMETRIES OF HEXAFLUOROCYCLOBUTENE

Unusually large discrepancies exist between the reported values of the experimental geometries of hexafluorocyclobutene determined from micr owave (MW) spectroscopy and those from electron diffraction (ED). In a n attempt to explain the origin of the difference, the geometries of c yclobutene, 1,2-difluorocyclobutene, 3,3,4,4-tetrafluorocyclobutene, h exafluorocyclobutene and I,2-dichloro-3,3,4,4-tetrafluorocyclobutene a re optimized by ab initio calculations at the Hartree-Fock, second-ord er Moller-Plesset perturbation levels of theory and configuration inte raction with single and double substitutions. Comparison of the calcul ated results with available MW and ED parameters imply that MW geometr ical parameters are probably more accurate for hexafluorocyclobutene t han ED ones. The difficulty associated with vibrational corrections fo r hexafluorocyclobutene appears to be responsible for the unusual patt ern of the difference between MW and ED structures. (C) 1998 Elsevier Science B.V.