The $CO_{2}$-Ethylene Complex: An Example of Dual Internal Rotation
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Date
1995
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Ohio State University
Abstract
A rotationally resolved infrared spectrum of the $CO_{2}$-Ethylene complex has been obtained and analyzed. Rotational constants obtained from fitting the spectrum indicate that the geometry of the complex is stacked parallel. This places the $CO_{2}$ above the plane of the ethylene with the $CO_{2}$ axis lying parallel to the carbon-carbon double bond. Irregular spacing between the $\Delta K_{a}$ sub-bands indicated that the complex was non-rigid. To determine the floppy degree of freedom, the intermolecular potential was evaluated by various ab initio methods. The calculations agreed with the experiment in that the $CO_{2}$ appears to rotate parallel to the plane of the ethylene. Due to the low barrier and the similarity of the $CO_{2}$ and ethylene rotational constants, it was necessary to treat both the carbon-dioxide and ethylene as independent rotors with a common rotation axis. The eigenvalues associated with the internal motion were then used to calculate the origin of each $\Delta K_{a}$ sub-band. The model potential was adjusted to give $\Delta K_{a}$ spacings consistent with the spectroscopy resulting in a barrier height of $6.6 cm^{-1}$.
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