Charged versus Neutral Hydrogen‐Bonded Complexes: Is There a Difference in the Nature of the Hydrogen Bonds?

Abstract

A theoretical study on some carboxylic acid dimers formed by positively or negatively charged molecules has been carried out by using DFT methods. The resulting dimers possess either a charge of +2 or -2. In addition, the corresponding neutral complexes have also been considered. The electron density distribution described by the atoms in molecules and the natural bond orbital methods, as well as the electric field maps of the systems, have been analyzed and compared without finding significant differences between the neutral and ionic complexes. The interaction energy along the dissociation path of the charged dimers shows both a local minimum and a local maximum, defining a stability region between them. When this energetic profile is recalculated by removing the repulsion between the charged groups, it resembles to those of the neutral molecules. Hence, the characteristics of the charged dimers are similar to those of the neutral ones: the addition of a repulsion term for the charged groups permits to retrieve the energetic profiles dependence with the distance in the charged system. The interacting quantum atom (IQA) method has been used to calculate the interaction energy terms, including the classic Coulombic term between the whole molecules and the corresponding of the carboxylic acid groups. The IQA results show repulsive electrostatic interactions when the whole molecules are considered in the ionic complexes, but attractive ones between the carboxylic groups in both neutral and ionic complexes.

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Hydrogen‐ and Halogen‐Bonds between Ions of like Charges: Are They Anti‐Electrostatic in Nature?

“…Most recently, Alkorta et al investigated neutral and charged carboxylic acid dimers, and found that the carboxylate‐ion dimers are similar to their neutral counterparts; having no significant difference in their electron density distribution as well as their electric‐field maps . Interestingly, by decomposing the total molecular energy to interatomic and intra‐atomic terms within the QTAIM method, they demonstrated that the interaction between the carboxylic groups in both neutral and ionic complexes is always attractive, though the total intermolecular electrostatic interaction in the ionic complexes is repulsive as expected . Thus, the dominance of electrostatic attraction seems universal in all kinds of H‐bonds, though the covalence must not be neglected, as it is associated with the directionality of H‐bonds.…”

“…An early example of H‐bonding between ions of like charge is the (O–H) – ···O – system, observed experimentally by Braga et al Theoretical investigations based on the quantum theory of atoms in molecules (QTAIM) method by Macchi et al suggested that the H‐bonding is contributed here by both CT interaction and electrostatic attraction . Most recently, Alkorta et al investigated neutral and charged carboxylic acid dimers, and found that the carboxylate‐ion dimers are similar to their neutral counterparts; having no significant difference in their electron density distribution as well as their electric‐field maps . Interestingly, by decomposing the total molecular energy to interatomic and intra‐atomic terms within the QTAIM method, they demonstrated that the interaction between the carboxylic groups in both neutral and ionic complexes is always attractive, though the total intermolecular electrostatic interaction in the ionic complexes is repulsive as expected .…”