THE RELIABILITY OF STRUCTURE DETERMINATIONS IN LARGE BINARY CLUSTERS: THE CASE OF $CO_{2}$-H(D)Br

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1992

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Ohio State University

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A high resolution rovibrational absorption spectrum of the weakly-bonded $CO_{2}$-H(D)Br complex has been obtained and a structure determined by using the Kraitchman equations. The microwave spectrum has also been $obtained.^{1}$ The complex has a T-shaped slipped-parallel structure with $R_{cm}=3.58$ \AA , $\theta_{BrCO}=80^{\circ}$ and $\theta_{BrCO}=93^{\circ}$. How reliable is the structure? First, this structure is corroborated by results from extensive ab initio generalised valence bond-configuration interaction calculations and by Moller-Plesset second order perturbation calculations. The theoretical equilibrium geometry is planar with parameters $R_{cm} = 3.62$\AA, $\theta_{BrCO} = 88.8^{\circ}$ and $\theta_{HBrC} = 83.7^{\circ}$. The potential energy surfaces for the in-plane and the out-of-plane one dimensional rotations of the HBr from this equilibrium structure have been determined. The calculated barrier height to the rotation out of the plane is $170 cm^{-1}$. The experimental value is $164 cm^{-1}$. The hindered rotor wave functions for the out-of-plane rotation have been calculated. The wave functions and the expectation values of $\theta$ were calculated for both the hydrogen and the deuterated species. Though the amplitude for these motions are large, the differences in values between the two isotopes are very small. Also here is no appreciable tunneling of the hydrogen for the in plane motion. We ascribe reliability of the structure determination in this case to the large coupling between the in plane rotation and the $R_{cm}$ stretching: whenever the HBr rotates in the plane, the bromine atom must move away from the $CO_{2}$ to accommodate it. Thus the reduced mass of this normal mode is larger than that of simple. hydrogen motion. We estimate errors of $\pm 0.05 $\AA and $\pm$ $5^{\circ}$ for the bond lengths and angles.

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Author Institution: Department of Chemistry, University of California at Santa Barbara; Department of Chemistry, University of Southern California

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