MICROWAVE SPECTRA AND MOLECULAR STRUCTURE OF THE KETENE-ACETYLENE COMPLEX

$^{1}$ J. Z. Gillies, C. W. Gillies, F. J. Lovas, K. Matsumura, and R. D. Suenram, 44th Symposium on Molecular Spectroscopy, paper TF3, (1989). $^{2}$ N. W. Howard and A. C. Legon, J. Chem. Phys. 88, 6793 (1988).Author Institution: Department of Chemistry, Union College. Schenectady; Department of Chemistry, Rensselaer Polytechnic Institute; Molecular Physics Division, National Institute of Standards and TechnologyRotational spectra of $CH_{2}=C=O=HC=CH$ and $CD_{2}=C=O=HC=CH$ 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 $C_{2}$ 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 $I_{n}=0$ 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 $\Delta= +0.41023 u{\AA}^{2}$. 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 $3.60(1) {\AA}C$ and an acetylenic hydrogen to oxygen distance of 2.96 \AA. These results will be compared to the related formaldehyde-acetylene $complex.^{2}