The halogen bond between ethene and a simple perfluoroiodoalkane: C ₂H ₄⋯ICF ₃ identified by broadband rotational spectroscopy

An isolated complex of C ₂H ₄ and ICF ₃ has been generated in the gas phase and studied by chirped-pulse, Fourier transform microwave (CP-FTMW) spectroscopy. Ab initio calculations at the CCSD(T)(F12)/VTZ level support the experimental work and further establish the dissociation energy, D _e, and dipole moment, μ, of the complex. Rotational constants, B ₀ and C ₀, the centrifugal distortion constant, Δ _J, nuclear quadrupole coupling constants, χ _aa (I) and (χ _bb(I)-χ _cc(I)) and the nuclear spin-rotation coupling constant C _bb(I) of the complex are determined from the experimental data. The iodine atom interacts with π-electrons on C ₂H ₄ resulting in the formation of a linear (∠(CI⋯)) halogen bond (where indicates the centre of the CC bond) and a complex of C _s symmetry. The measured rotational constants allow the length of the halogen bond to be determined as 3.434(2) Å in the r ₀ geometry. The complex is formally an asymmetric top but with a very low barrier to internal rotation of C ₂H ₄ about the C ₃ axis defined by the CF ₃I sub-unit. Adopting an approach also used in a recent study of H ₂O⋯ICF ₃, other transitions are assigned using the Hamiltonian of a symmetric top molecule.