THE GEOMETRY AND MOLECULAR PROPERTIES OF THE ETHYLENE-HCN AND ACETYLENE-HCN DIMERS

Abstract

The microwave spectra of the acetylene-HCN and ethylene-HCN dimers has been observed in the 4-18 GHz region using the pulsed Fourier transform method where the gas mixture is pulsed into a Fahry-Perot cavity. The spectra reveal both dimers to be near prolate asymmetric tops (K = -.99) in their ground vibrational states. Both dimers have a `T' shape for an equilibrium structure with the proton of HCN nearest to acetylene or ethylene. The ethylene-HCN dimer is nonplanar. Hyperfine structure originating from interaction of the nuclear electric quadrupole moment of $^{14}N$ with the surrounding electric field gradient is evident in all transitions. The nucleum quadrupole coupling analysis shows $X_{bb}$ and $X_{cc}$ to be equal within experimental uncertainty in both cases indicating that the proton of HCN experiences a nearly isotropic potential due to ethylene or acetylene. The structure of both dimers is consistent with other acetylene (ethylene)-HX dimers studied in this laboratory in structure and approximate well depth.