Far infrared spectra, conformational potential function, and barrier to methyl rotation of propionyl fluoride

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Abstract
The far infrared spectra of propionyl fluoride CH3CH2CFO in the gaseous and solid states have been recorded from 500 to 40 cm−1. A rather complex spectrum of the gas was observed and a substantial number of bands have been assigned to the asymmetric torsional modes for both the s-cis (oxygen atom eclipsing the methyl group) and the high energy gauche conformers. Analysis of these bands permitted the calculation of the torsional potential function present in this molecule. The potential coefficients for the asymmetric torsional mode were calculated to be: V1=341±24, V2=236±20, V3=390±3, and V4=21±6 cm−1, with an enthalpy difference between the more stable s-cis and the high energy gauche conformers of 434±20 cm−1 (1.24±0.06 kcal/mol). This function gives a potential barrier of 692 cm−1 (1.98 kcal/mol) separating the s-cis from the gauche form and 283 cm−1 (0.81 kcal/mol) separating the two equivalent gauche forms. From a temperature study of the Raman spectrum of the gas, the enthalpy difference between the s-cis and gauche conformers was determined to be 486±45 cm−1 (1.39±0.13 kcal/mol) which is in excellent agreement with the value obtained from the potential function. The barrier to the methyl rotation for the s-cis conformer was calculated from the far infrared data to be 935±2 cm−1 (2.67 kcal/mol). Both the methyl torsion and CFO rock were observed as doublets in the spectrum of the solid which indicates that there are at least two molecules per primitive cell. These results are compared to the corresponding quantities in some related molecules.