  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 experimental 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.
