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

Abstract

Bound state rovibrational energy 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 experiments. But the first excited K = 1 levels are predicted to lie about 3 cm -1 below their K = 0 counterparts in both cases. Line strength 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 to the binding energy D0 of about 340 cm -1 for D2O-CO, as compared to our calculated value 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.

The effect of CO-H2O collisions in the rotational excitation of cometary CO

“…This CO−H2O PES has a global well depth De = 646.1 cm −1 for an intermolecular center-of-mass separation R = 7.42 a0 and a planar configuration OC−H2O, with a weak hydrogen bond between the C atom of CO and one H atom of H2O. Bound-state rovibrational energy calculations using this PES have been found in very good agreement with experiments (Barclay et al 2019), confirming the good accuracy of the potential. We note that a full-dimensional CO−H2O PES was recently computed at the same level (CCSD(T)-F12a/AVTZ) by Liu & Li (2019).…”

The effect of CO−H2O collisions in the rotational excitation of cometary CO

“…The intermolecular energies were then fitted to a body-fixed expansion expressed as a sum of spherical harmonic products and a Monte Carlo estimator was employed to select the largest expansion terms. This CO−H 2 O PES has a global well depth D e = 646.1 cm −1 for an intermolecular centre-of-mass separation R = 7.42 a 0 and a planar configuration OC−H 2 O, with a weak hydrogen bond between the C atom of CO and one H atom of H 2 O. Bound-state rovibrational energy calculations using this PES have been found in very good agreement with experiments (Barclay et al 2019), confirming the good accuracy of the potential. We note that a full-dimensional CO−H 2 O PES was recently computed at the same level (CCSD(T)-F12a/AVTZ) by Liu & Li (2019).…”

HDO–CO Complex: D-Bonded and H-Bonded Isomers and Intra- and Intermolecular Rovibrational States from Full-Dimensional and Fully Coupled Quantum Calculations

“…All quantities shown are in cm –1 . For additional explanation see the text. b Reference . c Reference . d Reference . e Reference . …”

“…Water and CO are important molecules, found together in abundance as either collision partners or weakly bound complexes in diverse environments that include Earth’s atmosphere, products of combustion reactions, the interstellar medium, the coma of comets, and protoplanetary disks. − This has made the binary water–CO complex the subject of numerous high-resolution spectroscopic studies. The two isotopically symmetric isotopologues H 2 O–CO and D 2 O–CO have been investigated by means of microwave spectroscopy, , and infrared (IR) spectroscopy in the regions of the C–O stretch, , the O–H and the O–D stretches, the D 2 O bend, and the H 2 O bend . The isotopically asymmetric isotopologue HDO–CO has also been studied by microwave spectroscopy and by IR spectroscopy in the C–O stretch and O–D stretch regions.…”

“…Their quantitative description requires rigorous, high dimensional and fully coupled, quantum treatment. For H 2 O–CO and D 2 O–CO, the initial quantum bound state calculations were in 5D, assuming rigid monomers. ,, By excluding the intramolecular monomer vibrations, these calculations could not address any of the features of the (ro)­vibrational level structure of the complexes that involve intramolecular vibrational excitations, such as their frequency shifts, and the effects on the tunneling splittings and intermolecular vibrational eigenstates. This motivated us to perform, very recently, the first full-dimensional (9D) and fully coupled quantum calculations of the J = 0, 1 intra- and intermolecular rovibrational levels of H 2 O–CO and D 2 O–CO complexes, including their tunneling splittings and frequency shifts from the isolated monomer values .…”