HEXAFLUOROCYCLOBUTENE - GAS-PHASE MOLECULAR-STRUCTURE AND QUADRATIC FORCE-FIELD FROM AN ELECTRON-DIFFRACTION AND AB-INITIO STUDY

The molecular structure of hexafluorocyclobutene has been determined by electron diffraction from its vapor at a nozzle tip temperature of 19-20°C. The data are consistent with molecular symmetry C2v. Values for the distances (rg/Å), angles (〈α,/deg), and selected rms amplitudes of vibration (l/Å) with estimated 2σ uncertainties are r(C=C) = 1.324 (23), 〈r(C-F〉 = 1.336 (3) (weighted average), Δr(C-F) = r(-C-F) - r(=C-F) = 0.032 (12), 〈r(C-C)〉 = 1.529 (4) (weighted average), Δr(C-C) = r(-C-C-) - r(=C-C-) = 0.082 (14), 〈(C=C-C) = 95.0 (5), 〈(C=C-F) = 135.2 (12), 〈(-C-C-F) = 114.5 (3), 〈(F-C-F) = 107.6 (5), l(C=C) = l(=C-F) - 0.001 = l(-C-F) - 0.003 = 0.043 (3), l(-C-C) = l(=C-C) + 0.001 = 0.053 (7). The geometry of the molecule was optimized and a quadratic force field determined at the ab initio Hartree-Fock level by using 4-21G and 6-31G* basis sets. Although the calculations are of high quality, they fail to reproduce the large splitting of the C-C single bond distances. The calculated frequencies indicate that reassignment of some of the fundamentals may be necessary. © 1990 American Chemical Society.