Exploring the next step in micro-solvation of CO in water: Infrared spectra and structural calculations of (H2O)4-CO and (D2O)4-CO

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 that 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 the 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 a high resolution.