Inductive effects on cation–π interactions: Structures and bond dissociation energies of alkali metal cation–halobenzene complexes

The influence of halogenation on cation–π interactions is investigated both experimentally and theoretically. Bond dissociation energies (BDEs) of alkali metal cation–halobenzene complexes, M+(C6H5X), are determined using threshold collision-induced dissociation techniques in a guided ion beam tandem mass spectrometer, where M+ = Na+ and K+, and X = Cl, Br, and I. The primary dissociation pathway for all systems is endothermic loss of the neutral halobenzene from the cation–π complex. In addition, minor production of the ligand exchange product, M+Xe, is also observed. At elevated energies elimination of NaX is also observed for the Na+(C6H5Br) and Na+(C6H5I) systems. Ab initio calculations at the B3LYP/6-31+G* level of theory are used to determine the structures of the neutral π-ligands and cation–π complexes. Theoretical BDEs are determined from single point calculations at the MP2(full)/6-311+G(2d,2p) level using the B3LYP/6-31+G* optimized geometries. Good agreement between the theoretical and experimental BDEs is found in all cases. The trends in the BDEs of the M+(C6H5X) complexes are explained in terms of the varying magnitude of the electrostatic interactions in these complexes elucidated from a binding parameter model. Comparisons are also made to BDEs previously determined for the analogous M+(C6H6) and M+(C6H5F) complexes to examine the inductive effects of the halogen substituent on the strength of the cation–π interaction.