Rotational spectrum and structure of 1,2-dichloro-3,3,4,4-tetrafluorocyclobutene: Comparison of spectroscopy, diffraction, and ab initio results

The rotational spectra of four isotopomers of 1,2-dichloro-3,3,4,4-tetrafluorocyclobutene have been measured with a Fourier transform microwave spectrometer. The ring parameters determined from the inertial data were r(C-1=C-2)= 1.311(15) Angstrom, r(C-2-C-3) = 1.487(15) Angstrom, and r(C-3-C-4) = 1.551(15) Angstrom, with the angles (C-1=C-2-C-3) = 94.6(6)degrees and (C-2-C-3-C-4) = 85.4(6)degrees. The C-3-C-4 bond is found to be shorter by 0.05 Angstrom than the value obtained from an electron diffraction study, although consistent with a previous microwave study of the closely related molecule hexafluorocyclobutene. The C-1=C-2 bond is also 0.04 Angstrom shorter than the electron diffraction data and about 0.02-0.03 Angstrom shorter than microwave spectroscopy studies of other tetrafluorocyclobutenes. The reasons for these differences are unclear. The chlorine nuclear quadrupole coupling constants were determined; the axis of the principal nuclear quadrupole coupling tensor is essentially parallel to the carbon-chlorine bond axis. Comparisons with theoretical calculations and electron diffraction results are made, along with comparisons with similar molecules.