Knowledge Bank

Rotational spectra of $Ar-CH2CO$ and $Ar-CD2CO$ were observed with a pulsed-beam Fourier-transform microwave spectrometer. Two states exhibiting strong {b}-type and weak {a}-type transitions were assigned and fit separately to an asymmetrical top Watson Hamiltonian for each isotopomer. Relative intensities and nuclear spin weights indicate the spectral splitting of transitions arises from an internal rotation about the C=C=O axis of ketene which exchanges the pair of hydrogen (deuterium) nuclei in the two isotopomers. The spectral constants in MHz obtained from the fit for the ground state of $Ar-CH2CO$ are A = 10447.926(1) = 1918.281(40), and C = 1606.703(36). The moments of inertia for the two isotopomers give an argon-ketene center of mass separation $Rcm$ =3.60 {\AA} and an Ar-cm-C angle of $83\circ$. Although the hydrogen positions are not well determined, the effective geometry is assumed to be planar.