Helical C₂ Structure of Perfluoropentane and the C_2v Structure of Perfluoropropane

Abstract

Saturated hydrocarbons have structures with completely staggered bonds and dihedral angles of 180 degrees . Substituting hydrogen by fluorine results in a slight shift from 180 degrees , giving rise to a helical structure. X-ray diffraction studies on fibers and computational studies on perfluoroalkanes estimate a dihedral angle of about 17 degrees from the trans position. The rotational spectra of perfluoropentane and its three (13)C 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 r(0) structures incorporating various model constraints and a Kraitchman analysis gives a range of 13-19 degrees for the torsional angle. An objective approach, which only assumes overall C(2) 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 (13)C second moments very well, giving a dihedral angle of 17 +/- 1 degrees 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.

Microwave spectroscopy and characterization of the helical conformer of perfluorohexane

“…Computations were performed at the PBE0/VTZ level of theory using the Gaussian 03 program. 18 The PBE0 functional has been shown to accurately model perfluoroalkanes 13,15 and is again evident by the excellent agreement between the observed and computed spectroscopic constants for C 6 F 14 ( Table 2). The scale factors are 0.99996 for the acoordinates, 1.00167 for the b-coordinates, and 1.00288 for the c-coordinates.…”

“…The structure of C 3 F 8 was recently determined to be non-helical (C 2v ). 15 The The trend demonstrates that as the C chain length increases more mass is displaced outside the ab plane and contributes to P cc due to the helicity of the structures. The microwave data alone do not allow for an exact determination of the CCCC dihedral angles.…”

“…Experimental and computational studies on smaller perfluoroalkane oligomers also show the lowest energy structures to be helical, beginning with perfluorobutane (C 4 F 10 ). [5][6][7][8][9][10][11][12][13][14][15] The helical (trans), gauche, and ortho (τ ~ 90 o ) conformers of C 4 F 10 were observed by nitrogen matrixisolated IR spectroscopy. 8 Only the gauche form was observed in a microwave study, 16 with the helical form apparently having too small a dipole moment to be observable and the ortho form relaxing to the lower energy gauche form.…”

“…15 The value of the dihedral angle was determined by scaling computed models to exactly reproduce the observed second moments. The CCCC dihedral angle was calculated to be 17 o ± 1 o from trans using this method, in good agreement with previous computational studies.…”

2,5-Bis[(3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluorooctyl)sulfanyl]-1,3,4-thiadiazole

“…1). The fluoroalkyl chains adopt a typical helical conformation (Fournier et al, 2010). Positional disorder of the terminal F atoms of the fluoro tail was observed with occupancies of 0.55 and 0.45.…”