Micro-solvation of CO in water: infrared spectra and structural calculations for (D₂O)₂-CO and (D₂O)₃-CO

The weakly-bound molecular clusters (D2O)2-CO and (D2O)3-CO are observed in the C-O stretch fundamental region (≈2150 cm-1), and their rotationally-resolved infrared spectra yield precise rotational parameters. The corresponding H2O clusters are also observed, but their bands are broadened by predissociation, preventing detailed analysis. The rotational parameters are insufficient in themselves to determine cluster structures, so ab initio calculations are employed, and good agreement between the experiment and theory is found for the most stable cluster isomers, yielding the basic cluster geometries as well as confirming the assignments to (D2O)2-CO and (D2O)3-CO. The trimer, (D2O)2-CO, has a near-planar geometry with one D atom from each D2O slightly out of the plane. The tetramer, (D2O)3-CO, has the water molecules arranged in a cyclic quasi-planar ring similar to the water trimer, with the carbon monoxide located 'above' the ring and roughly parallel to its plane. The tunneling effects observed in the free water dimer and trimer are quenched by the presence of CO. The previously observed water-CO dimer together with the trimer and tetramer reported here represent the first three steps in the solvation of carbon monoxide.