Helical C₂ structure of Perfluoropentane and the C_2v structure of Perfluoropropane

Saturated hydrocarbons have structures with completely staggered bonds and dihedral angles of 180°. Substituting hydrogen by fluorine results in a slight shift from 180°, giving rise to a helical structure. X-ray diffraction studies on fibers and computational studies on perfluoroalkanes estimate a dihedral angle of about 17° from the trans position. The rotational spectra of perfluoropentane and its three 13C isotopomers have been observed and assigned using a pulsed-jet Fourier transform microwave spectrometer. The rotational constants for the parent species are A 990.6394(3) MHz, B 314.0002(1) MHz, and C 304.3703(1) MHz, respectively. The determination of an exact dihedral angle has been challenging, as the helical twist has proven to be quite sensitive to the structural inputs and constraints. A series of r₀ structures incorporating various model constraints and a Kraitchman analysis gives a range of 13-19° for the torsional angle. An objective approach, which only assumes overall C₂ symmetry, is to scale the principal coordinates from ab initio models by the square root of the ratio of the observed second moments to the computed second moments. The scaled structures of computed models at various levels of theory reproduce the parent second moments exactly and the 13C second moments very well, giving a dihedral angle of 17 ± 1° from trans. The microwave spectrum of perfluoropropane has also been observed and assigned. The rotational constants are A 1678.5982(9) MHz, B 900.1968(10) MHz, and C 955.3216(11) MHz, respectively. Unlike longer perfluoroalkanes, perfluoropropane has a nonhelical, C_2v structure. Computations are in excellent agreement with experimental results.