Determination of the conformational barriers to internal rotation of 3-fluoropropene from far infrared and low frequency Raman spectra

Abstract

The far infrared (200–40 cm−1) and low frequency Raman (1000–20 cm−1) spectra of gaseous 3-fluoropropene have been recorded. The fundamental asymmetric torsion for the conformer which has the fluorine atom cis to the double bond has been observed at 164.62 cm−1 with four excited states falling at lower frequencies, and the corresponding fundamental torsion of the gauche conformer was observed at 108.00 cm−1 with three excited states observed at lower frequencies. From these data the potential function for internal rotation of the asymmetric top has been determined and the following potential constants have been evaluated: V2=459±29, V3=830±9, V4=18±8, and V6=−37±5 cm−1, with a ΔH of 304±20 cm−1 (869 cal/mol). It has been determined that the cis conformer is the predominant form at ambient temperature in the gas phase and, from a temperature study of the Raman spectrum in this phase, the enthalpy difference between the cis and gauche conformers was determined to be 263±25 cm−1 (752 cal/mol). The calculated cis to gauche, gauche to gauche, and gauche to cis barriers are 1201 cm−1 (3.43 kcal/mol), 498 cm−1 (1.42 kcal/mol) and 869 cm−1 (2.48 kcal/mol), respectively. An alternative potential function was also calculated utilizing the successive gauche asymmetric torsional transitions proceeding to higher frequencies. A second series of asymmetric torsional transitions were observed for the cis conformer which are believed to be due to ‘‘hot bands’’ associated with the first excited state of the CCC bend. Additionally, each of the torsional transitions for the cis conformer exhibits multiple splitting which is believed to be due to excited states of the CCF bend. These results are compared to similar quantities in some related molecules.