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 microwave (MW) spectroscopy and those from electron diffraction (ED). In an attempt to explain the origin of the difference, the geometries of cyclobutene, 1,2-difluorocyclobutene, 3,3,4,4-tetrafluorocyclobutene, hexafluorocyclobutene and 1,2-dichloro-3,3,4,4-tetrafluorocyclobutene are optimized by ab initio calculations at the Hartree-Fock, second-order Moller-Plesset perturbation levels of theory and configuration interaction with single and double substitutions. Comparison of the calculated results with available MW and ED parameters imply that MW geometrical parameters are probably more accurate for hexafluorocyclobutene than ED ones. The difficulty associated with vibrational corrections for hexafluorocyclobutene appears to be responsible for the unusual pattern of the difference between MW and ED structures.