Tag Archives: Imre Bakó

How can we detect hydrogen bond local cooperativity in liquid water: A simulation study

Authors: Imre Bakó, Anikó Lábas, Kersti Hermansson, Ákos Bencsura and Julianna Oláh

The significant cooperative effect between water molecules substantially affects the properties of liquid water. The cooperativity of hydrogen bonds means that the hydrogen bond strength is influenced by the neighboring water molecules. Another descriptor related to cooperativity is degree correlation (or static correlation) describing the probability of hydrogen-bonded molecule pairs participating in additional hydrogen-bonds. Herein we analyze the latter one in liquid water at various temperatures and densities in a series of molecular dynamics simulations with the help of knowledge from network science. We investigated how the applied hydrogen bond criteria (energetic or geometric) influence the obtained results, and showed that the energetic criterion is much more rigorous and reliable, therefore should be used for similar studies. We found that the structure of the subsystems of water molecules with 3 and 4 hydrogen-bonds is distinctly different at low temperature, 3‑hydrogen-bonded water molecules form branched chain structures at all temperature. Deconvolution of the descriptors of the mixing pattern of water molecules according to their donor and acceptor numbers showed that species with complementary hydrogen bonding properties are likely to correlate and form H-bonds with each other, while species with similar H-bond pattern tend to avoid each other. Pearson’s coefficient (global descriptor of the local cooperativity) of the studied networks suggests that at normal density the H-bonded network in liquid water can be described by an uncorrelated network.

Journal of Molecular Liquids, 245, 2017, pp 140-146
DOI: 10.1016/j.molliq.2017.08.023

Large polarization but small electron transfer for water around Al3+ in a highly hydrated crystal

Authors: Pavlin D. Mitev, Imre Bakó, Anders Eriksson and Kersti Hermansson



Precise molecular-level information on the water molecule is precious, since it affects our interpretation of the role of water in a range of important applications of aqueous media. Here we propose that electronic structure calculations for highly hydrated crystals yield such information. Properties of nine structurally different water molecules (19 independent OO hydrogen bonds) in the Al(NO3)3·9H2O crystal have been calculated from DFT calculations. We combine the advantage of studying different water environments using one and the same compound and method (instead of comparing a set of independent experiments, each with its own set of errors) with the advantage of knowing the exact atomic positions, and the advantage of calculating properties that are difficult to extract from experiment. We find very large Wannier dipole moments for H2O molecules surrounding the cations: 4.0–4.3 D (compared to our calculated value of 1.83 D in the gas phase). These are induced by the ions and the H-bonds, while other water interactions and the relaxation of the internal water geometry in fact decrease the dipole moments. We find a good correlation between the water dipole moment and the OO distances, and an even better (non-linear) correlation with the average electric field over the molecule. Literature simulation data for ionic aqueous solutions fit quite well with our crystalline ‘dipole moment vs. OO distance’ curve. The progression of the water and cation charges from ‘small clusterslarge clustersthe crystal’ helps explain why the net charges on all the water molecules are so small in the crystal.

Phys. Chem. Chem. Phys., 2014, 16, 9351-9363

DOI: 10.1039/C3CP55358B

The vibrating hydroxide ion in water

Kersti Hermansson, Philippe A. Bopp, Daniel Spångberg, Ljupco Pejov, Imre Bakó, Pavlin D. Mitev

scienceThe OH ion in water is studied using a CPMD/BLYP + QMelectronic + QMvibrational approach. The ion resides in a cage of water molecules, which are H-bonded among each other, and pinned by H-bonding to the ion’s O atom. The water network keeps the ‘on-top’ water in place, despite the fact that this particular ion-water pair interaction is non-binding. The calculated OH vibrational peak maximum is at ∼3645 cm−1 (experiment ∼3625 cm−1) and the shift with respect to the gas-phase is ∼ +90 cm−1 (experiment +70 cm−1). The waters molecules on each side of the ion (O and H) induce a substantial OH vibrational blueshift, but the net effect is much smaller than the sum. A parabolic ‘frequency-field’ relation qualitatively explains this non-additivity. The calculated ‘in-liquid’ ν(OH) anharmonicity is 85 cm−1.

Chemical Physics Letters, Vol. 514, 2011, Pages 1–15