Rotational Spectrum and Structure of 1,2-Dichloro-3,3,4,4-tetrafluorocyclobutene: Comparison of Spectroscopy, Diffraction, and ab Initio Results

Abstract

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(C1C2) = 1.311(15) Å, r(C2C3) = 1.487(15) Å, and r(C3C4) = 1.551(15) Å, with the angles (C1C2C3) = 94.6(6)° and (C2C3C4) = 85.4(6)°. The C3C4 bond is found to be shorter by 0.05 Å than the value obtained from an electron diffraction study, although consistent with a previous microwave study of the closely related molecule hexafluorocyclobutene. The C1C2 bond is also 0.04 Å shorter than the electron diffraction data and about 0.02−0.03 Å 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 carbonchlorine bond axis. Comparisons with theoretical calculations and electron diffraction results are made, along with comparisons with similar molecules.