The halogen bond between ethene and a simple perfluoroiodoalkane: C2H4⋯ICF3 identified by broadband rotational spectroscopy

► This work reports the first observation of a complex of C2H4 and CF3I. ► The geometry of the complex is established. ► The length of the halogen bond between the two molecules is determined by rotational spectroscopy and ab initio calculations. ► The spectra are interpreted using an empirically chosen set of Hamiltonians. ► It is demonstrated that a sophisticated model will be required to account for the internal dynamics in the complex.