A pi-electron donor-acceptor complex C2H4 center dot center dot center dotBr-2 characterised by its rotational spectrum

A complex formed from ethene and dibromine was isolated and characterised b y using a pulsed-jet, Fourier-transform microwave spectrometer that incorpo rated a fast-mixing nozzle as a means to preclude chemical reaction between the two components. Rotational constants A, B and C, the quartic centrifug al distortion constants Delta (J), Delta (JK) and delta (J), and the Br nuc lear quadrupole coupling constants chi (aa)(Br-x) and {chi (bb)(Br-x) -chi (cc)(Br-x)} (where x is either i or o for the inner or outer Br nucleus) we re determined for each of the four isotopomers C2H4. . . (BBr)-B-79, C2H4. . . (BBr)-B-81, C2H4. . . (BBr)-B-79 and C2H4. . . (BBr)-B-81. Interpretati on of the various spectroscopic constants shows that in the complex the Br- 2 internuclear axis lies along the C-2 axis of C2H4 that is perpendicular t o the plane of the C and H nuclei, so that Br-i interacts with the centre ( *) of the pi bond of ethene. The distance of Br-i from * is r(*. . . Br-i)= 3.068 (2) Angstrom. It is estimated from the changes in the coupling consta nts chi (aa)(Br-i) and chi (aa)(Br-o) relative to those of the free Br-2 mo lecule that 0.026(4)e is transferred from C2H4 to Br-i on complex formation while the concomitant polarization of Br-2 can be represented by a net tra nsfer of 0.053(3)e from Br-i to Br-o. The intermolecular stretching force c onstant is estimated to be k(sigma)=8.8(3) N m(-1). The properties determin ed for C2H4. . . Br-2 are compared with those similarly established for C2H 4. . . Cl-2, C2H4. . . BrCl and C2H4. . . ICl and some family relationships are identified.