Infrared Spectra of (co2)2-x, Co2-x2, and Co2-x3, X = Co or N2

We previously observed the spectrum of (CO2)2-CO in the region of the CO fundamental band (2150 wn).ootnote{A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, extit{Chemical Physics Letters} extbf{677}, 127 (2017).} Its structure resembles that of a CO2 dimer (planar slipped-parallel) with a CO monomer aligned along the dimer C2 symmetry axis in a C-bonded configuration, giving a trimer with C2 symmetry. We have now observed three bands of the same species in the CO2 ub{3} region. Two of these bands are fundamentals, one a/c-type and the other b-type. The presence of this b-type band establishes that the CO2 dimer subunit within the trimer is actually not planar. The third band is a b-type combination band near 2376 wn. A new c-type fundamental near 2349.5 wn and an a-type combination band near 2365.2 wn are assigned to CO2-(CO)2. The structure has two C-bonded CO monomers in equivalent positions around the CO2 'equator', giving a C2v geometry analogous to that of CO2-Ar2. A similar fundamental is observed for CO2-(N2)2 near 2350.0 wn. The weak parallel band of a symmetric or near-symmetric rotor near 2349.2 wn is assigned to CO2-(CO)3, together with an analogous band of CO2-(N2)3 near 2349.8 wn. We think that these tetramers are accidental symmetric rotors having Cs symmetry. They are obtained by taking the C2v trimers from the previous paragraph and adding a third CO or N2 to the 'side' of the first two, in the plane bisecting them.