Visualizing Internal Stabilization in Weakly Bound Systems Using Atomic Energies: Hydrogen Bonding in Small Water Clusters

Exploring the Non-Covalent Bonding in Water Clusters

QTAIM and source function analysis were used to explore the non-covalent bonding in twelve different water clusters (H2O)n obtained by considering n = 2–7 and various geometrical arrangements. A total of seventy-seven O−H⋯O hydrogen bonds (HBs) were identified in the systems under consideration, and the examination of the electron density at the bond critical point (BCP) of these HBs revealed the existence of a great diversity of O−H⋯O interactions. Furthermore, the analysis of quantities, such as |V(r)|/G(r) and H(r), allowed a further description of the nature of analogous O−H⋯O interactions within each cluster. In the case of 2-D cyclic clusters, the HBs are nearly equivalent between them. However, significant differences among the O−H⋯O interactions were observed in 3-D clusters. The assessment of the source function (SF) confirmed these findings. Finally, the ability of SF to decompose the electron density (ρ) into atomic contributions allowed the evaluation of the localized or delocalized character of these contributions to ρ at the BCP associated to the different HBs, revealing that weak O−H⋯O interactions have a significant spread of the atomic contributions, whereas strong interactions have more localized atomic contributions. These observations suggest that the nature of the O−H⋯O hydrogen bond in water clusters is determined by the inductive effects originated by the different spatial arrangements of the water molecules in the studied clusters.

Bonding analysis of water clusters using quasi-atomic orbitals

The quasi-atomic orbital (QUAO) bonding analysis introduced by Ruedenberg and co-workers is used to develop an understanding of the hydrogen bonds in small water clusters, from the dimer through the hexamer (bag, boat, book, cyclic, prism and cage conformers). Using kinetic bond orders as a metric, it is demonstrated that as the number of waters in simple cyclic clusters increases, the hydrogen bonds strengthen, from the dimer through the cyclic hexamer. However, for the more complex hexamer isomers, the strength of the hydrogen bonds varies, depending on whether the cluster contains double acceptors and/or double donors. The QUAO analysis also reveals the three-center bonding nature of hydrogen bonds in water clusters.

Cooperative nature of the sulfur centered hydrogen bond: investigation of (H2S)n (n = 2-4) clusters using an affordable yet accurate level of theory

Existing studies have shown that appreciably high level quantum chemical calculations are required to reproduce experimental energetic and geometric features of a H₂S dimer. This condition severely restricts any practical possibility of obtaining reliable results for higher order H₂S clusters. We have shown here that the binding energies calculated at the CCSD(T)/CBS level with counterpoise corrected geometries calculated at the MP2/aug-cc-pV(Q+d)Z level of theory excellently match with the experimental results for the H₂S dimer. Subsequently, the above mentioned levels of theory were used for trimers and tetramers. (H₂S)_n (n = 2-4) clusters were found to show cooperative strengthening of S-HS hydrogen bonds, which is clearly evident from the evolution of binding energies and hydrogen bond lengths, with increasing cluster size. Localized molecular orbital energy decomposition analyses have been carried out to understand how the contributions of various energy components modulate with the size of the clusters and what are their relative contributions towards the overall stabilization of the clusters. Natural bond orbital and atoms in molecules analyses were also carried out in order to look into the evolution of the electronic charge transfer and electron density topology with cluster size.

New insights into the solubility of graphene oxide in water and alcohols

One of the main advantages of graphene oxide (GO) over its non-oxidized counterpart is its ability to form stable solutions in water and some organic solvents. At the same time, the nature of GO solutions is not completely understood; the existing data are scarce and controversial. Here, we demonstrate that the solubility of GO, and the stability of the as-formed solutions depend not just on the solute and solvent cohesion parameters, as commonly believed, but mostly on the chemical interactions at the GO/solvent interface. By the DFT and QTAIM calculations, we demonstrate that the solubility of GO is afforded by strong hydrogen bonding established between GO functional groups and solvent molecules. The main functional groups taking part in hydrogen bonding are tertiary alcohols; epoxides play only a minor role. The magnitude of the bond energy values is significantly higher than that for typical hydrogen bonding. The hydrogen bond energy between GO functional groups and solvent molecules decreases in the sequence: water > methanol > ethanol. We support our theoretical results by several experimental observations including solution calorimetry. The enthalpy of GO dissolution in water, methanol and ethanol is -0.1815 ± 0.0010, -0.1550 ± 0.0012 and -0.1040 ± 0.0010 kJ g^-1, respectively, in full accordance with the calculated trend. Our findings provide an explanation for the well-known, but poorly understood solvent exchange phenomenon.

The Transferability of Topologically Partitioned Electron Correlation Energies in Water Clusters

The electronic effects that govern the cohesion of water clusters are complex, demanding the inclusion of N-body, Coulomb, exchange and correlation effects. Here we present a much needed quantitative study of the effect of correlation (and hence dispersion) energy on the stabilization of water clusters. For this purpose we used a topological energy partitioning method called Interacting Quantum Atoms (IQA) to partition water clusters into topological atoms, based on a MP2/6-31G(d,p) wave function, and modified versions of GAUSSIAN09 and the Quantum Chemical Topology (QCT) program MORFI. Most of the cohesion in the water clusters provided by electron correlation comes from intramolecular energy stabilization. Hydrogen bond-related interactions tend to largely cancel each other. Electron correlation energies are transferable in almost all instances within 1 kcal mol^-1 . This observed transferability is very important to the further development of the QCT force field FFLUX, especially to the future modelling of liquid water.

Understanding the structure and hydrogen bonding network of (H2O)32 and (H2O)33: an improved Monte Carlo temperature basin paving (MCTBP) method and quantum theory of atoms in molecules (QTAIM) analysis

Large water clusters are of particular interest because of their connection to liquid water and the intricate hydrogen bonding networks they possess.

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

Resonance Assisted Hydrogen Bonds (RAHBs) are particularly strong H-Bonds (HBs) which are relevant in several fields of chemistry. The traditional explanation for the occurrence of these HBs is built on mesomeric structures evocative of electron delocalisation in the system. Nonetheless, there are several theoretical studies which have found no evidence of such electron delocalisation. We considered the origin of RAHBs by employing Quantum Chemical Topology tools, more specifically, the Quantum Theory of Atoms in Molecules (QTAIM) and the Interacting Quantum Atoms energy partition. Our results indicate that the π-conjugated bonds allow for a larger adjustment of electron density throughout the H-bonded system as compared with non-conjugated carbonyl molecules. This rearrangement of charge distribution is a response to the electric field due to the H atom involved in the hydrogen bonding of the considered compounds. As opposed to the usual description of RAHB interactions, these HBs lead to a larger electron localisation in the system, and concomitantly to larger QTAIM charges which in turn lead to stronger electrostatic, polarization and charge transfer components of the interaction. Overall, the results presented here offer a new perspective on the cause of strengthening of these important interactions.

On the effects of the basis set superposition error on the change of QTAIM charges in adduct formation. Application to complexes between morphine and cocaine and their main metabolites

QTAIM atomic properties variation upon interaction is analyzed by: (i) deformation; (ii) BSSE estimated by counterpoise method; and (iii) binding.

Hydrogen bond cooperativity and anticooperativity within the water hexamer

We propose a hierarchy of H-bond strength in terms of the single and double character of the involved donor and acceptors within different structures of (H₂O)₆.

Atomic partitioning of M-H2 bonds in [NiFe] hydrogenase--a test case of concurrent binding

The possibility of simultaneous addition of η(2)-H2 to both the metals (Ni and Fe) in the active site of the as isolated state of the enzyme (Ni-SI) is examined here by an atom-by-atom electronic energy partitioning based on the QTAIM method. Results show that the 4LS state prefers H2 removal than addition. Destabilization of the atomic basins of the thiolate bridges and decrease of the electrophilicity of the Fe and Ni, resulting in poor back donation to the CO ligand, are the bottlenecks that hamper dihydrogen activation simultaneously. The study helps to understand why such states are seldom accessed in the activation of dihydrogen. Moreover, Ni has been found to be the natural choice for the dihydrogen binding.

Cooperativity between hydrogen bonds and beryllium bonds in (H2O)(n)BeX2 (n = 1-3, X = H, F) complexes. A new perspective

The interaction of BeX(2) (X = H, F) with water molecules has been analyzed at the B3LYP/6-311+G(3df,2p)//B3LYP/6-311+G(d,p) level of theory. The formation of strong beryllium bonds between water molecules and the BeX(2) derivative triggers significant electron density redistribution within the whole system, resulting in significant changes in the proton donor and proton acceptor capacity of the water molecules involved. Hence, significant cooperative and anti-cooperative effects are present, explaining why there is no case in which the global minimum corresponds to a tetracoordinated beryllium atom. In fact, the most stable clusters can be viewed as the result of the attachment of BeX(2) to the water trimer and the water dimer, respectively, and not as the result of the solvation of the BeX(2) molecule. We have also shown that the decomposition of the interaction energy into atomic components is a reliable quantitative tool to describe all the closed-shell interactions present in the clusters investigated herein, namely hydrogen bonds, beryllium bonds and dihydrogen bonds. Indeed, we have shown that the changes in the atomic energy components are correlated with the changes in the strength of these interactions, and they provide a quantitative measure of cooperative effects directly in terms of energies.