A π-electron donor–acceptor complex C2H4···Br2 characterised by its rotational spectrum

A complex formed from ethene and dibromine was isolated and characterised by using a pulsed-jet, Fourier-transform microwave spectrometer that incorporated a fast-mixing nozzle as a means to preclude chemical reaction between the two components. Rotational constants A, B and C, the quartic centrifugal distortion constants ΔJ, ΔJK and δJ , and the Br nuclear quadrupole coupling constants χaa(Brx) and {χbb(Brx) − χcc(Brx)} (where x is either i or o for the inner or outer Br nucleus) were determined for each of the four isotopomers C2H4···79Br79Br, C2H4···81Br79Br, C2H4···79Br81Br and C2H4···81Br81Br. Interpretation of the various spectroscopic constants shows that in the complex the Br2 internuclear axis lies along the C2 axis of C2H4 that is perpendicular to the plane of the C and H nuclei, so that Bri interacts with the centre (*) of the π bond of ethene. The distance of Bri from * is r(*···Bri) = 3.068 (2) Å. It is estimated from the changes in the coupling constants χaa(Bri) and χaa(Bro) relative to those of the free Br2 molecule that 0.026(4)e is transferred from C2H4 to Bri on complex formation while the concomitant polarization of Br2 can be represented by a net transfer of 0.053(3)e from Bri to Bro. The intermolecular stretching force constant is estimated to be kσ = 8.8(3) N m−1. The properties determined for C2H4···Br2 are compared with those similarly established for C2H4···Cl2, C2H4···BrCl and C2H4···ICl and some family relationships are identified.