THE WATER–CARBON MONOXIDE DIMER: NEW INFRARED SPECTRA, AB INITIO ENERGY LEVEL CALCULATIONS, AND A CURIOUS INTERMOLECULAR MODE

Weakly-bound H$_{2}$O-CO has a planar structure with approximately co-linear heavy atoms (O, C, O) and a hydrogen bond between the water and the carbon of the CO. Proton tunneling (H atom interchange) gives rise to two states corresponding to distinct nuclear spin modifications. The magnitude of the splitting in the ground rotational state is about 0.8 \wn for H$_{2}$O-CO and 0.04 \wn for D$_{2}$O-CO. Due to the almost linear heavy atom configuration, H$_{2}$O-CO has a large A rotational constant, equal to about 19 \wn (12 \wn for D$_{2}$O-CO), so the K quantum number is highly significant. Water-CO was first studied in the microwave and millimeter regions. Infrared spectra have been observed in the regions of the C-O stretch, the O-H stretch, the D$_{2}$O bend, and the H$_{2}$O bend. Here we study the O-D stretch region (3.6 $\mu$m) for the first time, observing D$_{2}$O-CO, HOD-CO, and DOH-CO. We also extend the C-O stretch region results to include the K = 1 $\leftarrow$ 0 subbands, thus determining A rotational constants for the v(CO) = 1 excited state. But more significantly, we also observe additional K = 1 $\leftarrow$ 0 combination bands in both regions which involve the lowest intermolecular vibration of water-CO. This mode, which lies at 43 – 49 \wn depending on isotopologue, can be identified as the in-plane CO bend. It is observed for H$_{2}$O-CO, D$_{2}$O-CO, and HOD-CO, and exhibits anomalous isotope shifts: even though their A-values are quite different, the D$_{2}$O-CO mode is only slightly lower in energy than that of H$_{2}$O-CO. Detailed rotational energy level calculations, based on a recent high-level ab initio potential energy surface \footnote {Y. N. Kalugina, A. Faure, A. van der Avoird, K. Walker, and F. Lique, Phys. Chem. Chem. Phys. 20, 5469 (2018).}, are in good agreement with experiment, including the newly observed intermolecular mode. As well, the calculations show that the unobserved K = 0 level of this mode lies above the observed K = 1 level, thus explaining the anomalous isotope shifts