  We extend studies of micro-solvation of carbon monoxide by a combination of
high-resolution IR spectroscopy and ab initio calculations. Spectra of the
(H2O)4-CO and (D2O)4-CO pentamers are observed in the C-O stretch fundamental
region (~2150 cm-1). The H2O containing spectrum is broadened by
predissociation, but that of D2O is sharp, enabling detailed analysis which
gives a precise band origin and rotational parameters. Ab initio calculations
are employed to confirm the assignment to (water)4-CO and to determine the
structure, in which the geometry of the (water)4 fragment is a cyclic ring very
similar to the isolated water tetramer. The CO fragment is located "above" the
ring plane, with a partial hydrogen bond between the C atom and one of the
"free" protons (deuterons) of the water tetramer. Together with previous
results on D2O-CO, (D2O)2-CO, and (D2O)3-CO, this represents a probe of the
four initial steps in the solvation of carbon monoxide at high resolution.
