The microwave spectrum and structure of 1,1,1,3,3,3-hexafluoropropane, freon R236fa

The microwave spectra of three isotopic species of 1,1,1,3,3,3-hexafluoropropane (R236fa) (CF3CH2CF3, 13CF3CH2CF3 and CF3 13CH2CF3) were observed in the region from 4.5 to 18 GHz using a molecular beam Fourier transform microwave spectrometer (MBFTMW). The rotational and centrifugal distortion constants were determined. The rs (C–C) bond length and bond angle (C–C–C) of the molecule were determined to be 1.56(5) and 109(3) A ° , respectively. The ab initio calculation of R236fa was executed at the MP2/6-31G(d,p) level. Comparison of the bond length and bond angle of R236fa with those in the other fluoropropanes revealed trends for the C–C bond length and CCC angle, depending on the number of fluorine atoms attached to the central carbon. A similar idea had been noted by Mack et al. from the electron diffraction studies of fluoropropanes. q 2005 Elsevier B.V. All rights reserved