Microwave Spectrum, Rotational Isomerism, and Internal Barrier Functions in Propionyl Fluoride

Abstract

The microwave spectrum of propionyl fluoride [CH3CH2COF] proves the existence of two stable rotameric conformations. For the cis form (methyl eclipsing oxygen) with a planar heavy-atom skeleton, rotational constants A = 10 042.53, B = 3762.19, C = 2832.69 MHz and a dipole moment of μtotal = 2.90, μa = 2.22, μb = 1.87 D have been determined. An approximate structure has been fitted to these data. Eight satellite spectra of the cis form have been analyzed yielding a torsional vibration frequency around the central carbon–carbon bond of 85 ± 3 cm−1 and an apparent barrier height V3 = 2400 ± 60 cal/mole hindering the internal rotation of the methyl group. The gauche conformation has rotational constants A = 8701.9, B = 3976.9, C = 3108.7 MHz and a total dipole moment of μtotal = 3.09 D with components μa = 2.93, μb = 0.32, and μc = 0.91 D. The dihedral angle between the CCC and COF plane is found to be 120° ± 2° and the torsional vibration frequency of the gauche form around the central carbon–carbon bond is 54 ± 5 cm−1. Energetically, the cis form is favored over the gauche form by 1290 ± 50 cal/mole. The data obtained on the two rotamers are used to derive the potential-energy function V(υ) for rotation around the central C–C bond. The first four Fourier coefficients are found to be V1 = 930, V2 = 720, V3 = 1150, and V4 = 130 cal/mole, resulting in a potential barrier of 2100 cal/mole separating the cis from the gauche form and 800 cal/mole separating the two equivalent gauche forms from each other. The modification of the υ-dependent intramolecular forces which accompanies the substitution of one hydrogen atom by a methyl group is obtained. With the assumption that these modifications are additive and from the observed approximate constancy of V3, it becomes possible to predict the potential functions of several related molecules, for later comparison with experiment.