Characterisation of the Intermediate C 2 H 4 …︁Cl 2 in a Gaseous Mixture of Ethene and Chlorine by Rotational Spectroscopy: A Weak π‐Type Complex

A complex of ethene and Cl 2 has been characterised in the gas phase. Rotational spectra of the isotopomers C 2 H 4 … 35 Cl 2 , C 2 H 4 … 35 Cl 37 Cl and C 2 H 4 … 37 Cl 35 Cl were recorded by using a fast‐mixing nozzle in an FT microwave spectrometer. Rotational constants, centrifugal distortion constants and Cl nuclear quadrupole coupling constants. χ gg (Cl) are reported in each case. The complex is of the π‐donor–acceptor type and has a C 2 v geometry in which Cl 2 lies along the C 2 axis perpendicular to the plane of the C 2 H 4 nuclei. The binding is weak, and only small changes in the χ 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 C 2 H 4 … Cl 2 and C 2 H 4 … HCl indicate that the angular geometry is in both cases determined mainly by the electrostatic part of the interaction. The distance from the π‐bond midpoint to Cl decreases from C 2 H 4 … HCl to C 2 H 4 … Cl 2 ; this suggests that Cl 2 is “snub‐nosed”.