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

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: $$\begin{array}{cccc}\mathrm{C}–\mathrm{C}& \text{1.503\hspace{0.17em}}\mathrm{A}& \mathrm{CCF}& {110}^{\circ }{18}^{\prime }\\ \mathrm{C}–\mathrm{F}& \text{1.348}& \mathrm{CCO}& {128}^{\circ }{21}^{\prime }\\ \mathrm{C}–\mathrm{O}& \text{1.181}& \mathrm{HCH}& {109}^{\circ }{30}^{\prime }\\ \mathrm{C}–\mathrm{H}& \text{1.084}& & \end{array}$$ If the requirement of methyl group symmetry is dropped, the best fit to all the data is obtained with the following methyl group parameters: $$\begin{array}{ccc}\mathrm{In-plane\; hydrogen}& & \mathrm{Out-of-plane\; hydrogens}\\ \mathrm{C}–\mathrm{H}& \text{1.082\hspace{0.17em}}\mathrm{A}& \text{1.096\hspace{0.17em}}\mathrm{A}\\ \mathrm{CCH}& {110}^{\circ }{24}^{\prime }& {108}^{\circ }{48}^{\prime }\\ \mathrm{HCH}& {110}^{\circ }{51}^{\prime }& {107}^{\circ }{16}^{\prime }\end{array}$$ 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.