CO 2 -CO dimer was previously observed by high-resolution microwave and infrared spectroscopy in the form of a planar T-shaped C-bonded structure. a,b More recently, spectra of a second isomer were observed in the stretching region of carbon monoxide. c This isomer has also a planar T-shaped structure, but with the CO flipped by 180 • . Moreover, two intermolecular frequencies were measured for each isomer. These are in very good agreement with recent high level theoretical calculations. Here, CO 2 -CO dimer is observed in the carbon dioxide ν 3 asymmetric stretch region (∼2350 cm −1 ). Both C-bonded and O-bonded isomers are analyzed for the normal isotopologue as well as for 13 CO 2 -CO and 16 O 13 C 18 O-CO, the latter being the first observation of an asymmetrically substituted form for which all values of K a are allowed. Combination bands involving the lowest in-plane intermolecular mode are also observed yielding frequencies of 24.51 cm −1 for the C-bonded form and 14.37 cm −1 for the O-bonded form. In addition, two weak bands near 2337 cm −1 are assigned to CO 2 hot band transitions (01 1 1) -(01 1 0), yielding the splitting of the degenerate CO 2 bend into in-plane and out-of-plane components due to the presence of the CO. This splitting has rather different values for the C-and O-bonded isomers, 4.56 and 1.59 cm −1 , respectively, with the out-of-plane mode higher in energy than the in-plane for both cases.
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