Wide amplitude motion in the water–carbon dioxide and water–acetylene complexes

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Abstract
The optothermal detection method has been used to obtain near-infrared spectra of the H2O–CO2 and H2O –HCCH van der Waals molecules in the 3 μm region. For H2O–CO2, the observed vibrational band correlates with the asymmetric O–H stretching vibration of the water monomer and gives a direct measurement of the internal rotation tunneling splitting, from which the height of the associated barrier is estimated. This experimental estimate is compared with results obtained from both a distributed multipole analysis and an ab initio calculation. In the case of H2O –HCCH, three different vibrational bands are observed. Although these spectra are not directly sensitive to the wide amplitude motion in this molecule, they are consistent with the quasiplanar picture suggested by the ab initio calculations. These calculations show the molecule to have a bent equilibrium structure, with a small barrier located at the planar geometry.