The water-carbon monoxide dimer: new infrared spectra, ab initio rovibrational energy level calculations, and an interesting intermolecular mode

TL;DR

This study combines high-level theoretical calculations with new infrared spectroscopy experiments to analyze the rovibrational energy levels and intermolecular dynamics of the water-carbon monoxide dimer, revealing novel spectral features and energy level behaviors.

Contribution

It provides the first combined theoretical and experimental analysis of H2O-CO and D2O-CO dimers, including new spectra and insights into their intermolecular modes and energy levels.

Findings

Predicted K=1 levels are slightly below K=0 levels, confirmed by experiments.

Infrared spectra reveal discrete perturbations and validate energy level predictions.

Wavefunction analysis and Coriolis interactions explain anomalous energy levels.

Abstract

Rovibrational energy level calculations using a high-level intermolecular potential surface are reported for H2O-CO and D2O-CO. They predict the ground K = 1 levels to lie about 20 (12) cm-1 above K = 0 for H2O-CO (D2O-CO) in good agreement with past experiment. But the first excited K = 1 levels are predicted to lie about 3 cm-1 below their K = 0 counterparts in both cases. Intensity calculations also indicate that mid-infrared transitions from the K = 0 ground state to this seemingly anomalous excited K = 1 state should be observable. These predictions are strikingly verified by new spectroscopic measurements covering the C-O stretch region around 2200 cm-1 for H2O-CO, D2O-CO, and HOD-CO, and the O-D stretch region around 2700 cm-1 for D2O-CO, HOD-CO, and DOH-CO. The experiments probe a pulsed supersonic slit jet expansion using tunable infrared quantum cascade laser or optical parametric oscillator sources. Discrete perturbations in the O-D stretch region give an experimental lower limit of about 340 cm-1 for D2O-CO, as compared to our calculated binding energy of 368 cm-1. Wavefunction plots are presented to help understand the intermolecular dynamics of H2O-CO. Coriolis interactions are invoked to explain the seemingly anomalous energies of the first excited K = 1 levels.