Research paperInfrared observation of a new mixed trimer, CO - (CO2)2

•Weakly-bound trimer CO-(CO2)2 observed for first time.•b-type band in carbon monoxide C-O stretch region.•Missing levels with (Ka“, Kc”) = (e, o) and (o, e).•Structure like CO2 dimer, plus CO along symmetry axis.•C-bonded form observed, but not O-bonded.

The infrared spectrum of the weakly-bound trimer CO - (CO2)2 is observed in the carbon monoxide CO stretch region. Both 12C16O - (12C16O2)2 and 12C16O - (12C18O2)2 isotopologues are studied. The trimer band which is composed uniquely of b-type transitions and with missing levels establishes the trimer structure as that of a CO2 dimer plus a CO monomer aligned along the dimer C2 symmetry axis. The dimer to monomer center of mass separation is 3.55 Å, and the structural parameters within the CO2 dimer subunit are about 3.45 Å and 58°. It is not certain if the (CO2)2 remains planar within the trimer.

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