Infrared observation of OC-C₂H₂, OC-(C₂H₂)₂ and their isotopologues

Abstract

The fundamental band for the OC-C 2 H 2 dimer and two combination bands involving the intermolecular bending modes  9 and  8 in the carbon monoxide CO stretch region are reexamined. Spectra are obtained using a pulsed supersonic slit jet expansion probed with a modehop free tunable infrared quantum cascade laser. Analogous bands for OC-C 2 D 2 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 2 H 2 ) 2 . The trimer band is composed uniquely of a-type transitions, establishing that the CO monomer is nearly aligned with the a-inertial axis. The observed rotational constants agree well with ab initio 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 2 H 2.A similar band for the fully deuterated isotopologue CO-(C 2 D 2 ) 2 confirms our assignment.