CHARACTERIZATION OF THE INTERMEDIATE C2H4-CENTER-DOT-CENTER-DOT-CENTER-DOT-CL-2 IN A GASEOUS MIXTURE OF ETHENE AND CHLORINE BY ROTATIONAL SPECTROSCOPY - A WEAK PI-TYPE COMPLEX

A complex of ethene and Cl-2 has been characterised in the gas phase. Rotational spectra of the isotopomers C2H4 ...Cl-35(2), C2H4 ...(ClCl)-Cl-35-Cl-37 and C2H4 ...(ClCl)-Cl-37-Cl-35 were recorded by using a fast-mixing nozzle in an FT microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl nuclear quadrupole coupling constants chi(gg)(Cl) are reported in each case. The complex is of the pi-donor-acceptor type and has a C-2, geometry in which Cl-2 lies along the C-2 axis perpendicular to the plane of the C2H4 nuclei. The binding is weak, and only small changes in the chi(gg)(Cl) attend complex formation. A simple model attributes these changes to a transfer of around 0.02 e from the inner to the outer Cl nucleus, thus confirming that the complex is of the Mulliken outer type. Similarities in the properties of C2H4 ... Cl-2 and C2H4 ... HCl indicate that the angular geometry is in both cases determined mainly by the electrostatic part of the interaction. The distance from the pi-bond midpoint to Cl decreases from C2H4 ... HCl to C2H4 ... Cl-2; this suggests that Cl-2 is ''snub-nosed''.