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 (ΔEPC) of the point charge. This is demonstrated by an excellent linear correlation between the quantum chemical interaction energy and ΔEPC with a slope of 0.88, a zero intercept, and a correlation coefficient of R2 = 0.9995. Rigorous separation of ΔEPC 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) ΔEPC 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.