Low Frequency Fourier Transform Infrared (FTIR) Spectra Of Fluoroethanes

Abstract The vibrational spectra of 1, 2-dif luoroethane , 1,1, 2-trif luoroethane , and 1,1, 2 , 2-tetra- fluoroethane have been recorded In the gaseous, liquid, and solid states. The conform- ational preferences have been studied as a function of temperature and physical state. Transitions arising from the torsional vibration for the stable form of each molecule have been observed in the far infrared. Using these and other data, torsional potential func­ tions have been developed. Trends In the conformational characteristics of the fluoroe­ thanes are discussed.IntroductionMultiple conformations can exist in ethane derivatives In which there Is partial sub­ stitution on each carbon atom. For example, the staggered forms of 1,2-difluoroethane and 1,1,2,2-tetraf luoroethane can exist in either the gauche (C2 ) or tj^an_s_ (C2h) conformations, and 1,1,2-trifluoroethane can assume conformations with either GI or Cs molecular symmetry.Very Often, differences in selection rules between infrared and Raman experiments provide information necessary for distinguishing between possible conformers. The ability to use these techniques to study solids, liquids, and gases can simplify the interpretation of spectral data if the conformational behavior Is dependent upon the physical state. Addi­ tional Information concerning conformational equilibria can be found In the far infrared and low-frequency Raman spectra of vapors. Transitions arising from torsional vibrations are found in this region of the spectrum, and the frequencies of these transitions are dictated by the form of the potential energy function which governs Internal rotation. A suitable model for the potential function Is V(e) = (h) £ Vn(l-cos n0), where 6 Is the internal rotation angle and the shape of the function is determined by the relative values of the individual n-fold terms, Vn. A knowledge of the potential function Is Important for fully understanding the conformational equilibrium.Results 1, 2-DIfluoroethaneThe infrared and Raman spectra of 1,2-difluoroethane exhibit a one-to-one correspondence which Is consistent with C2 molecular symmetry (gauche) for the most abundant form. Weak bands associated with the trans (C2h) conformer are discernable in the spectra of the gas and liquid states, but these bands are absent In the spectrum of the crystalline solid. The temperature dependence of bands In the liquid state Indicate that the gauche form is more stable by 2.0 kcal/mol.The fundamental torsional vibration for gauche 1,2-difluoroethane has been observed in the Raman spectrum as a Q branch and In the far Infrared spectrum as a B-type band centered at 147 cm"1 . A second transition seems to be centered at approximately 137 cm"1 . The torsion for the trans conformer exhibits a series of Q branches at 117^ 112, 106, 101, 100, and 93 cm"1 . The transitions for both conformers have been fitted to a periodic potential function, and the torsional transitions are satisfactorily reproduced by using potential constants Vi = 215. V2 = -554 V3 = 1184 and V6 = -19 cm"1 . This function Implies that the energy separation of the gauche and trans conformers is 1.2 kcal/mol, a value which Is similar In magnitude to that for the liquid state.1,1,2-TrifluoroethaneThe difference in selection rules for GI and Cs molecular symmetries are not very dis­ tinctive. However, the abundance of Q branches In the Ramari spectrum implies that the GI form is more stable than the Cs in the gas phase, and a variable temperature study Is con­ sistent with a AH value of 1.6 ± 0.4 kcal/mol. The reverse is true for the liquid state where the Cs conformer is more stable by 1.03 ± 0.10 kcal/mol. Additionally the Cs con- former Is the only one present In the crystallne solid state.Far Infrared spectra recorded at 0.25 cm" resolution exhibit a Q branch series for the GI conformer at 117, 115, 113, 110, 106, 101, and 96 cm."1 . These are assigned to the