Water–hydrocarbon interactions: Rotational spectroscopy and structure of the water–acetylene complex

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Abstract
The radiofrequency and microwave spectra of C2H2–H2O, C2H2–D2O, C2D2–H2O, and C2D2–D2O have been measured by molecular beam electric resonance spectroscopy. Rotational constants and dipole moments are reported. The structure is effectively planar with the acetylene hydrogen bonded to the oxygen of the water. The hydrogen-bond length and stretching force constant are calculated to be. 2.229 Å and 0.065 mdyn/Å, respectively. Considering the lone pair orbital structure of the water molecule, an equilibrium structure having the water plane tilted away from the a axis of the complex is expected. Even so, a comparison of the dipole moments for the different isotopically substituted species shows that the height of the barrier hindering an inversion motion is low enough that no vibrational levels lie below the top of the barrier in the double-minimum potential well.

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