Knowledge Bank

Rotational spectra of $CH2=C=O=HC=CH$ and $CD2=C=O=HC=CH$ were observed with a pulsed beam Fourier transform microwave spectrometer employing the dual flow pulsed solenoid valve described $previously.1$ Four states for each isotopic species were identified and fit separately to a quartic Watson Hamiltonian. Relative intensities and nuclear spin weights observed for these states show the splitting arises from internal rotation about the $C2$ axis in ketene and the axis perpendicular to the molecular axis of acetylene. These motions exchange the two pairs of hydrogen atoms in the monomer units. The rotational constants (in MHz) of the $In=0$ ground state are $A=8389.969(16),B=2372.179(29)$ and $C=1846.535(25)$. A planar structure is found with an inertial defect of $\Delta =+0.41023u\backslash AA2$. The geometry obtained from the moment of inertia data corresponds to the molecular symmetry axes of the monomers tilted by $-25\circ$ from parallel alignment with a center of mass distance of $3.60(1)\backslash AAC$ and an acetylenic hydrogen to oxygen distance of 2.96 \AA. These results will be compared to the related formaldehyde-acetylene $complex.2$