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

Abstract

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 Δ_J, Δ_JK and δ_J , and the Br nuclear quadrupole coupling constants χ_aa(Br_x) and {χ_bb(Br_x) − χ_cc(Br_x)} (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 Br_i interacts with the centre (*) of the π bond of ethene. The distance of Br_i from * is r(*···Br_i) = 3.068 (2) Å. It is estimated from the changes in the coupling constants χ_aa(Br_i) and χ_aa(Br_o) relative to those of the free Br₂ molecule that 0.026(4)e is transferred from C₂H₄ to Br_i on complex formation while the concomitant polarization of Br₂ can be represented by a net transfer of 0.053(3)e from Br_i to Br_o. The intermolecular stretching force constant is estimated to be k_σ = 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.