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.