Hydrogen‐Bond Cooperative Effects in Small Cyclic Water Clusters as Revealed by the Interacting Quantum Atoms Approach

Abstract

The cooperative effects of hydrogen bonding in small water clusters (H2 O)n (n=3-6) have been studied by using the partition of the electronic energy in accordance with the interacting quantum atoms (IQA) approach. The IQA energy splitting is complemented by a topological analysis of the electron density (ρ(r)) compliant with the quantum theory of atoms-in-molecules (QTAIM) and the calculation of electrostatic interactions by using one- and two-electron integrals, thereby avoiding convergence issues inherent to a multipolar expansion. The results show that the cooperative effects of hydrogen bonding in small water clusters arise from a compromise between: 1) the deformation energy (i.e., the energy necessary to modify the electron density and the configuration of the nuclei of the isolated water molecules to those within the water clusters), and 2) the interaction energy (Eint ) of these contorted molecules in (H2 O)n . Whereas the magnitude of both deformation and interaction energies is enhanced as water molecules are added to the system, the augmentation of the latter becomes dominant when the size of the cluster is increased. In addition, the electrostatic, classic, and exchange components of Eint for a pair of water molecules in the cluster (H2 O)n-1 become more attractive when a new H2 O unit is incorporated to generate the system (H2 O)n with the last-mentioned contribution being consistently the most important part of Eint throughout the hydrogen bonds under consideration. This is opposed to the traditional view, which regards hydrogen bonding in water as an electrostatically driven interaction. Overall, the trends of the delocalization indices, δ(Ω,Ω'), the QTAIM atomic charges, the topology of ρ(r), and the IQA results altogether show how polarization, charge transfer, electrostatics, and covalency contribute to the cooperative effects of hydrogen bonding in small water clusters. It is our hope that the analysis presented in this paper could offer insight into the different intra- and intermolecular interactions present in hydrogen-bonded systems.

The nature of resonance-assisted hydrogen bonds: a quantum chemical topology perspective

“…As established in previous studies on H-bonds, 13,43,44,59,60 the formation of an RAHB leads to a noticeable redistribution of electron density across the molecule. These changes are in agreement with conventional arrow pushing in conjugated unsaturated systems.…”

“…5,43 This reveals that the conjugated systems display stronger interactions than their saturated analogues, in which the H-bonds are even weaker than those between H 2 OÁ Á ÁH 2 O and H 2 OÁ Á ÁH 3 N. We stress however, that the local enhancement of E OÁ Á ÁH int in (a) and (b) is obviously dominated by the classical, not the exchange terms. In this way, IQA unveils that the increased strength of the local H-bond interaction in the RAHB moieties, despite including both exchange and classical contributions, is dominated by the latter component.…”

“…We note that (i) the formation of the H-bond induces a charge redistribution that increases the overall total atomic charge (in absolute value), this effect being considerably larger in conjugated systems; and (ii) the changes in atomic charges are slightly larger than those we have found in small water clusters with s H-bond cooperative effects. 43 In chemically appealing terms, the p channel allows for a more efficient charge density flow.…”

“…The IQA partition based on R HF 1 (r 1 ,r 1 0 ) and R HF 2 (r 1 ,r 2 ) has been successfully used recently to study the H-bond cooperative effects within small water clusters. 43 Besides, the RAHB formation energies examined in this work are considerably larger in magnitude than those associated with these H 2 O systems 43,53 and hence easier to study from an energetic perspective. Finally, the behaviour of the HF and MP2 electron densities, as reflected in the changes in delocalisation indices upon the formation of the HB, is very similar.…”

“…Their conclusions are thus free of method biases. Most importantly, IQA and QTAIM have already been used successfully to analyze the nature of the hydrogen bond 5 and to study H-bond cooperative 43 and anticooperative 44 effects in water clusters. In this sense, they open a unique reference-free window to understand the reasons behind the strengthening or weakening of HBs.…”