STRUCTURAL STUDY OF THE H2O-H2O-CO2 TRIMER
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Date
1988
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Publisher
Ohio State University
Abstract
The microwave spectrum of the trimeric species $H_{2}O-H_{2}O-CO_{2}$ has been studied in the 718 GHz frequency range using a pulsed-molecular-beam Fabry-Perot Fourier transform microwave spectrometer. Each transition is doubled because of energy level splitting due to coupling of an internal rotation of one of the $H_{2}O$ subunits with the overall rotation of the complex. Both states can be fit using a centrifugal distortion Hamiltonian. The rotational constants and intertial defects are: [FIGURE] The electric dipole moment has also been measured yielding $\mu_{a}=1.57D, \mu_{b}=0.76D$ and $\mu_{c} <0.1D$. Several isotopically substituted species have been studied including various $^{13}C, ^{18}O$ and deuterium substitutions. All except a singly deuterated species exhibit doubled spectra. We conclude that the species has a nearly-planar triangular structure with dimer-type structures evident within the trimer complex. The oxygen of one of the $H_{2}O$ subunits is bonded to the $CO_{2}$ carbon with a bond length of 2.8A, within 0.1A of that found in the $H_{2}O-CO_{2}$ dimer. The other H2O has its oxygen bonded to one of the hydrogens of the first $H_{2}O$; the bond distance is 2.0A, within 0.1A of that found in the $H_{2}O-H_{2}O$ dimer. This second $H_{2}O$ is also hydrogen bonded to one of the oxygens in the $CO_{2}$ subunit with a bond distance of 2.0A which is what one would expect for a hydrogen bonded $H_{2}O-CO_{2}$ dimer (this species has not been observed). Evidence suggests that this H2O subunit is involved in the internal rotation.
Description
Author Institution: Chemistry Department, Pastore Hall. University of Rhode Island; Molecular Spectroscopy Division, National Bureau of Standards