Study of Rotational Isomerism in Fluoroacetyl Fluoride by Microwave Spectroscopy

Abstract

Fluoroacetyl fluoride has been found to have two distinct sets of pure rotational transitions in the microwave region 8 to 36 kMc/sec, which have been identified as arising from two rotamers, one with the two fluorine atoms trans to one another and the other in which they are in the cis position. These spectra have been analyzed and moments of inertia obtained. The Stark effect gives the dipole‐moment components: trans μ_a=0.456±0.010 D, μ_b=2.63±0.05 D, μ=2.67±0.05 D; cis μ_a=1.18±0.03 D, μ_b=1.68±0.04 D, μ=2.05±0.06 D; μ_c=0 for both forms. Several sets of satellite lines were observed and assigned to torsionally excited trans molecules, to a skeletal bending mode, and to a combination of these. Relative intensities then gave trans frequencies near 127 cm⁻¹ for the torsion and 265 cm⁻¹ for the bending. Intensities also gave an energy difference, E_cis—E_trans, of 910±100 cal/mole. The available data do not completely define the torsional potential function but suggest that the threefold component may be in the range 1000–1300 cal/mole as in acetyl derivatives. The twofold component must be dominant to produce the cis instead of gauche minimum.