Abstract
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.
Highlights
A series of 20 halogen bonded complexes of the types R–BrBr− (R is a substituted methyl group) and R–C≡C–BrBr− are investigated at the M06-2X/6–311+G(d,p) level of theory
Despite the efforts to maximize the potential for charge transfer, our results show that the halogen bond interactions of these complexes are governed by electrostatics and polarization and that charge transfer is of negligible importance
The analysis of the potential energy surfaces (PES) of CBr4—Br− and CF3BrBr− shows that ΔEInt and ΔEPC are very similar at Br–Br− distances (R) larger than twice the equilibrium distance (Req)
Summary
A series of 20 halogen bonded complexes of the types R–BrBr− (R is a substituted methyl group) and R–C≡C–BrBr− are investigated at the M06-2X/6–311+G(d,p) level of theory. Murray, and Politzer have in recent years emphasized the importance of polarization as a natural and inseparable companion to the electrostatic interaction [9,10,11]. The σ-hole interpretation has received considerable acceptance within the scientific community, it has continuously been challenged by scientist arguing that chargetransfer and orbital interactions must be considered for a complete description of halogen bonding [14,15,16,17,18]. The ALMO-EDA is designed to provide a lower bound to the charge-transfer energy In this analysis, it was found that for any given Y− the complexation energy of CX3IY− decreases in the order F, Cl, Br, I, whereas the sum of the Pauli repulsion, electrostatic, and polarization energies increases in the same order. The decrease in the complexation energy was attributed to the charge transfer energy, which in ALMO-EDA is defined as the remaining part of the interaction energy when other energy contributions have been subtracted out
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