The Importance of Electrostatics and Polarization for Noncovalent Interactions: Ionic Hydrogen Bonds vs Ionic Halogen Bonds

A series of 26 hydrogen-bonded complexes between Br- and halogen, oxygen and sulfur hydrogen-bond (HB) donors is investigated at the M06-2X/6-311 +G(2df,2p) level of theory. Analysis using a model in which Br- is replaced by a point charge shows that the interaction energy (Delta E-Int) of the complexes is accurately reproduced by the scaled interaction energy with the point charge (Delta E-Int(PC)). This is demonstrated by Delta E-Int = 0.86 Delta E-Int(PC) with a correlation coefficient, R-2=0.999. The only outlier is (Br-H-Br)(-), which generally is classified as a strong charge-transfer complex with covalent character rather than a HB complex. Delta E-Int(PC) can be divided rigorously into an electrostatic contribution (Delta E-ES(PC)) and a polarization contribution (Delta E-pol(PC)).Within the set of HB complexes investigated, the former varies between -7.2 and -32.7 kcal mol(-1), whereas the latter varies between -1.6 and -11.5 kcal mol(-1). Compared to our previous study of halogen-bonded (XB) complexes between Br and C-Br XB donors, the electrostatic contribution is generally stronger and the polarization contribution is generally weaker in the HB complexes. However, for both types of bonding, the variation in interaction strength can be reproduced accurately without invoking a charge-transfer term. For the Br-center dot center dot center dot HF complex, the importance of charge penetration on the variation of the interaction energy with intermolecular distance is investigated. It is shown that the repulsive character of Delta E-Int at short distances in this complex to a large extent can be attributed to charge penetration.