Knowledge Bank

In our preliminary analysis of the spectrum of the $N2O$-Ethylene $complex1$ correlating with the $\nu 9$ vibrational mode of the ethylene monomer, the results suggested that, unlike the analogous $CO2$ complex which displays internal rotation, the complex was rigid. Nevertheless, there were several features of the spectrum that were inconsistent with a rigid structure. As a result, we have proceeded to record the spectrum associated with the $\nu 1+\nu 3$ mode of the $N2O$. Analysis of both spectra leads to a structure that is stacked parallel with the $N2O$ axis parallel to the carbon-carbon double bond and above the plane of the ethylene. It is now clear that the $\Delta Ka$ sub-band origins are shifted with respect to the rigid approximation, while the structure within each sub-band is well described by a rigid rotor Hamiltonian. This band shifting can be attributed to tunneling about the A inertial axis. Ab initio calculations performed for several geometries indicate that the only feasible rotation involves rotation of the $N2O$ parallel to the plane of the ethylene. Imposing this constraint on the motion it is possible to solve the problem of a double rotor around a common axis to reproduce the pattern of $\Delta Ka$ origin shifts. The barrier to internal rotation was determined to be approximately $31cm-1$, five times larger than the iso-electronic $CO2$-Ethylene complex.