Knowledge Bank

A rotationally resolved infrared spectrum of the $CO2$-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 $CO2$ above the plane of the ethylene with the $CO2$ axis lying parallel to the carbon-carbon double bond. Irregular spacing between the $\Delta Ka$ 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 $CO2$ appears to rotate parallel to the plane of the ethylene. Due to the low barrier and the similarity of the $CO2$ 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 $\Delta Ka$ sub-band. The model potential was adjusted to give $\Delta Ka$ spacings consistent with the spectroscopy resulting in a barrier height of $6.6cm-1$.