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

An isolated complex of C2H4 and ICF3 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, mu, of the complex. Rotational constants, B-0 and C-0, the centrifugal distortion constant, Delta(J), nuclear quadrupole coupling constants. chi(aa)(I) and (chi(bb)(I)-chi(cc)(I)) and the nuclear spin-rotation coupling constant C-bb(1) of the complex are determined from the experimental data. The iodine atom interacts with pi-electrons on C2H4 resulting in the formation of a linear (angle(C-I...*)) halogen bond (where * indicates the centre of the C=C bond) and a complex of Cs symmetry. The measured rotational constants allow the length of the halogen bond to be determined as 3.434(2) angstrom in the r(0) geometry. The complex is formally an asymmetric top but with a very low barrier to internal rotation of C2H4 about the C-3 axis defined by the CF3I sub-unit. Adopting an approach also used in a recent study of H2O...ICF3, other transitions are assigned using the Hamiltonian of a symmetric top molecule. (C) 2012 Elsevier Inc. All rights reserved.