Microwave Spectrum, Internal Barrier, Structure, Conformation, and Dipole Moment of Acetyl Fluoride

Abstract

The microwave spectra of eight isotopic species of acetyl fluoride are reported. Interaction of internal and over-all rotation splits the rotational lines into doublets. From the doublet separations of CH3COF the height of the threefold (sinusoidal) barrier to internal rotation was calculated to be 1041 cal/mole. Splittings in the spectra of C13H3COF, CH3CO18F, and CD3COF gave barriers of 1041, 1055, and 1031 cal/mole, respectively. With the assumption of a symmetrical methyl group, the following structural parameters were determined from the observed rotational constants: C–C1.503 ACCF110∘18′C–F1.348CCO128∘21′C–O1.181HCH109∘30′C–H1.084If the requirement of methyl group symmetry is dropped, the best fit to all the data is obtained with the following methyl group parameters: In−plane hydrogenOut−of−plane hydrogensC–H1.082 A1.096 ACCH110∘24′108∘48′HCH110∘51′107∘16′The observed rotational constants of CH2DCOF and CHD2COF were found to require the H(in-plane)-F trans-equilibrium conformation. From Stark effect measurements on CH3COF and CH3CO18F the dipole moment was calculated to be 2.96 D. The dipole moment makes an angle of 9°30′ with the C–C bond axis and is directed toward the oxygen atom.