Microwave Spectrum of Acetyl Chloride

The microwave spectrum of ten isotopic species of acetyl chloride are reported. Internal rotation of the methyl group splits some of the rotational transitions into doublets in addition to the chlorine hyperfine structure. Analysis of the internal rotation splitting gives a barrier to internal rotation of 1296±30 cal/mole. From the hyperfine structure, the chlorine nuclear quadrupole coupling constants for Cl³⁵ along the C–Cl bond axis and perpendicular to the plane of symmetry of the molecule were determined to be —58.5 and 21.5 Mc, respectively. The corresponding values for Cl³⁷ were —46.4 and 17.2 Mc. These coupling constants have been interpreted to give information about the double bond and ionic character of the C–Cl bond. The following structural parameters have been determined from the rotational constants of CH₃COCl³⁵, CH₃COCl³⁷, C¹³H₃COCl³⁵, CH₃C¹³OCl³⁵, CH₃CO¹⁸Cl³⁵, and CD₃COCl³⁵: $$\begin{array}{cccc}\mathrm{C}–\mathrm{H}& \text{1.083±0.005\hspace{0.17em}}\mathrm{A}& <\mathrm{CCH}& {110}^{\circ }{21}^{\prime }{\text{±10}}^{\prime }\\ \mathrm{C}–\mathrm{C}& \text{1.499±0.010}& <\mathrm{CCO}& {127}^{\circ }{5}^{\prime }{\text{±10}}^{\prime }\\ \mathrm{C}=\mathrm{O}& \text{1.192±0.010}& <\mathrm{CCCl}& {112}^{\circ }{39}^{\prime }{\text{±30}}^{\prime }\\ \mathrm{C}–\mathrm{Cl}& \text{1.789±0.005}& & \end{array}$$ The observed rotational constants of CH₂DCOCl³⁵ show that the equilibrium configuration of acetyl chloride has a methyl hydrogen atom opposite the oxygen atom.