Infrared observation of OC-C<sub>2</sub>H<sub>2</sub>, OC-(C<sub>2</sub>H<sub>2</sub>)<sub>2</sub> and their isotopologues
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The fundamental band for the OC-C<sub>2</sub>H<sub>2</sub> dimer and two combination bands involving the intermolecular bending modes ν<sub>9</sub> and ν<sub>8</sub> in the carbon monoxide CO stretch region are re-examined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a mode-hop free tuneable infrared quantum cascade laser. Analogous bands for OC-C<sub>2</sub>D<sub>2</sub> and the fundamental for OC–DCCH as an impurity are also observed and analysed. A much weaker band in the same spectral region is assigned to a new mixed trimer, CO-(C<sub>2</sub>H<sub>2</sub>)<sub>2</sub>. The trimer band is composed uniquely of <i>a</i>-type transitions, establishing that the CO monomer is nearly aligned with the <i>a</i>-inertial axis. The observed rotational constants agree well with <i>ab initio</i> calculations and a small inertial defect value indicates that the trimer is planar. The structure is a compromise between the T-shaped structure of free acetylene dimer and the linear geometry of free OC-C<sub>2</sub>H<sub>2.</sub> A similar band for the fully deuterated isotopologue CO-(C<sub>2</sub>D<sub>2</sub>)<sub>2</sub> confirms our assignment.
