Electrostatics and polarization determine the strength of the halogen bond: a red card for charge transfer

A series of 20 halogen bonded complexes of the types R-Br•••Br^- (R is a substituted methyl group) and R´-C≡C-Br•••Br^- are investigated at the M06-2X/6-311+G(d,p) level of theory. Computations using a point-charge (PC) model, in which Br^- is represented by a point charge in the electronic Hamiltonian, show that the halogen bond energy within this set of complexes is completely described by the interaction energy (ΔE^PC) of the point charge. This is demonstrated by an excellent linear correlation between the quantum chemical interaction energy and ΔE^PC with a slope of 0.88, a zero intercept, and a correlation coefficient of R² = 0.9995. Rigorous separation of ΔE^PC into electrostatics and polarization shows the high importance of polarization for the strength of the halogen bond. Within the data set, the electrostatic interaction energy varies between 4 and -18 kcal mol^-1, whereas the polarization energy varies between -4 and -10 kcal mol^-1. The electrostatic interaction energy is correlated to the sum of the electron-withdrawing capacities of the substituents. The polarization energy generally decreases with increasing polarizability of the substituents, and polarization is mediated by the covalent bonds. The lower (more favorable) ΔE^PC of CBr₄---Br^- compared to CF₃Br•••Br^- is found to be determined by polarization as the electrostatic contribution is more favorable for CF₃Br•••Br^-. The results of this study demonstrate that the halogen bond can be described accurately by electrostatics and polarization without any need to consider charge transfer.