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Raman and Far‐Infrared Spectra, Barriers to Internal Rotation and Ab Initio Calculations of CF 3 CF 2 X, where X=H, Cl, Br, and I
Author(s) -
Eltayeb Saadia,
Guirgis Gamil A.,
Fanning Ada R.,
Durig James R.
Publication year - 1996
Publication title -
journal of raman spectroscopy
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.748
H-Index - 110
eISSN - 1097-4555
pISSN - 0377-0486
DOI - 10.1002/(sici)1097-4555(199602)27:2<111::aid-jrs935>3.0.co;2-q
Subject(s) - raman spectroscopy , overtone , infrared , ab initio , chemistry , ab initio quantum chemistry methods , molecule , internal rotation , infrared spectroscopy , molecular physics , far infrared , spectral line , atomic physics , analytical chemistry (journal) , physics , optics , mechanical engineering , organic chemistry , astronomy , engineering , chromatography
The far‐infrared (360 to 30 cm −1 ) of gaseous CF 3 CF 2 H, CF 3 CF 2 Cl, CF 3 CF 2 Br and CF 3 CF 2 I were recorded at a resolution of 0.10 cm −1 . The fundamental torsional transitions were observed at 75, 69.92, 67.35 and 66.72 cm −1 for CF 3 CF 2 X (X=H, Cl, Br, I), respectively, and each torsional fundamental except for CF 3 CF 2 H was observed with accompanying ‘hot bands’ with decreasing intensity toward lower wavenumber. Utilizing these torsional data, along with reasonably assumed structural parameters (experimental or ab initio adjusted values) to calculate the kinetic constants, the barrier to internal rotation in these molecules was determined. For pentafluoroethane the overtone accompanied by a series of two quanta ‘hot band’ transitions of the CF 3 torsion was observed and these data were used to obtain the internal rotational barrier for this molecule. The threefold periodic barriers for the CF 3 rotors are 1190±4, 1647±4, 1872±34 and 1917±48 cm −1 for these CF 3 CF 2 X molecules, where X=H, Cl, Br and I, respectively. The barriers for the CF 3 rotors were compared with the corresponding values for the CH 3 tops. The Raman (3200 to 20 cm −1 ) and infrared spectral of CF 3 CF 2 H and CF 3 CF 2 Cl were recorded for the gases and solids. Complete vibrational assignments are proposed based on infrared band contours, relative infrared and Raman intensities and normal coordinate calculations utilizing force constants from ab initio calculations with RHF/3–21G * and MP2/6–31G * basis sets. Ab initio calculations were used to obtain optimized geometries and barriers to rotation, which were compared with the experimental values.