Knowledge Bank

Rotational spectra of various isotopomers of the $Ar-C2H4$ and $Ne-C2H4$ van der Waals dimers were measured in the frequency range from 3.5 to 22 GHz using a pulsed jet cavity Fourier transform microwave spectrometer. The observed transitions are all of a-type. The observed spectra are consistent with planar, T-shaped equilibrium geometries. Tunneling splittings were observed for all transitions. This is attributed to an internal rotation motion of the ethylene unit within the complex. $Ne-C2H4$ shows a larger tunneling splitting than $Ar-C2H4$, which can be attributed to a lower tunneling barrier in the $Ne-C2H4$ case. Molecular symmetry group analyses using nuclear permutation inversion group theory can explain teh observed intensity variation and the absence of certain transitions. Effective separations beteen the center-of-mass of ethylene and the rare gas atoms were obtained from the ground state rotational constants. ab initio calculations at the MP4 level were carried out on $Ne-C2H4$, and the results were compared with those from the experiments.