© Paesani Research Group. All rights reserved.

188. Molecular insights into the influence of ions on water structure. I. Alkali metal ions in solution.

        R. Savoj, H. Agnew, R. Zhou, F. Paesani, J. Phys. Chem. B 128,1953 (2024). [link]

In this study, we explore the impact of alkali metal ions (Li+, Na+, K+, Rb+, and Cs+) on the hydration

structure of water using molecular dynamics simulations carried out with the MB-nrg potential energy

functions (PEFs). Our analyses include radial distribution functions, coordination numbers, dipole moments,

and infrared spectra of water molecules, calculated as a function of solvation shells. The results collectively

indicate a highly local influence of all the alkali metal ions on the hydrogen-bond network established by

the surrounding water molecules, with the smallest and most densely charged Li+ ion exerting the most

pronounced effect. Remarkably, the MB-nrg PEFs demonstrate excellent agreement with available

experimental data for the position and size of the first solvation shells, underscoring their potential as

predictive models for realistic simulations of ionic aqueous solutions across various thermodynamic

conditions and environments.

187. Balance between physical interpretability and energetic predictability in widely used dispersion-corrected

        density functionals. S. Dasgupta, E. Palos, Y. Pan, F. Paesani, J. Chem. Theory Comput. 20, 49 (2024). [link]

We assess the performance of different dispersion models for several popular density functionals across a diverse

set of non-covalent systems, ranging from the benzene dimer to molecular crystals. By analyzing the interaction

energies and their individual components, we demonstrate that there exists variability across different systems for

empirical dispersion models that are calibrated for reproducing the interaction energies of specific systems. Thus,

parameter fitting may undermine the underlying physics, as dispersion models rely on error compensation among

the different components of the interaction energy. Energy decomposition analyses reveal that, the accuracy of

several dispersion-corrected functionals originates from significant compensation between dispersion and charge

transfer energies. These results highlight the propensity for unpredictable behavior in various dispersion-corrected

density functionals across a wide range of molecular systems, akin to the behavior of force fields. Future development

of dispersion models should prioritize the faithful description of the dispersion energy, a shift that promises greater

accuracy in capturing the underlying physics across diverse molecular and extended systems.

189. Molecular driving forces for water adsorption in MOF-808: A comparative analysis with UiO-66.

        H.O. Frank, F. Paesani, J. Chem. Phys. 160, 094703 (2024). [link]

In this study, we investigate water adsorption in MOF-808, a prototypical MOF that shares the same

secondary building unit (SBU) as UiO-66, and elucidate how differences in topology and connectivity

between the two MOFs influence the adsorption mechanism. To this end, molecular dynamics simulations

were performed to calculate several thermodynamic and dynamical properties of water in MOF-808 as

a function of relative humidity (RH), from the initial adsorption step to full pore filling. At low RH, the

μ3-OH groups of the SBUs form hydrogen bonds with the initial water molecules entering the pores,

which triggers the filling of these pores before the μ3-OH groups in other pores become engaged in

hydrogen bonding with water molecules. Our analyses indicate that the pores of MOF-808 become

filled by water sequentially as the RH increases. A similar mechanism has been reported for water

adsorption in UiO-66. Despite this similarity, our study highlights distinct thermodynamic properties

and framework characteristics that influence the adsorption process differently in MOF-808 and UiO-66.

191. Many-body interactions and deep neural network potentials for water.

        Y. Zhai, R. Rashmi, E. Palos, F. Paesani, J. Chem. Phys. 160, 144501 (2024). [link]

We present a detailed assessment of deep neural network potentials developed within the DeePMD framework and

trained on the MB-pol data-driven many-body potential energy function. Specific focus is directed at the ability of

DeePMD-based potentials to correctly reproduce the accuracy of MB-pol across various water systems. Analyses of

bulk and interfacial properties as well as many-body interactions characteristic of water elucidate inherent limitations

in the transferability and predictive accuracy of DeePMD-based potentials. These limitations can be traced back to an

incomplete implementation of the “nearsightedness of electronic matter” principle, which may be common throughout

machine learning potentials that do not include a proper representation of self-consistently determined long-range

electric fields. These findings provide further support for the “short-blanket dilemma” faced by DeePMD-based

potentials, highlighting the challenges in achieving a balance between computational efficiency and a rigorous,

physics-based representation of the properties of water. Finally, we believe that our study contributes to the ongoing

discourse on the development and application of machine learning models in simulating water systems.

190. Excited-state rotational freedom impacts viscosity sensitivity in arylcyanoamide fluorescent molecular rotor

        dyes. R.S. Ehrlich, S. Dasgupta, R.E. Jessup, K.L. Teppang, A.L. Shiao, Kun Yong Jeoung, X. Su, A. Shivkumar,

        E.A. Theodorakis, F. Paesani, J. Yang, J. Phys. Chem. B 128, 3946 (2024). [link]

The microviscosity of intracellular environments plays an important role in monitoring cellular function. Thus,

the capability to detect changes in viscosity can be utilized for the detection of different disease states. Viscosity

sensitive fluorescent molecular rotors are potentially excellent probes for these applications; however, predictable

relationships between chemical structural features and viscosity sensitivity are poorly understood. Here, we

investigate a set of arylcyanoamide-based fluorescent probes and the effect of small aliphatic substituents on their

viscosity sensitivity. We found that the location of the substituents as well as the type of π-network of the fluorophore

can significantly affect the viscosity sensitivity of these fluorophores. Computational analysis supported that the

excited state rotational energy barrier plays a dominant role in the relative viscosity sensitivity of these fluorophores.

These findings provide valuable insight in the design of molecular rotor-based fluorophores for viscosity measurement.

192. A tribute to Gregory A. Voth. J. Jin, W.G. Noid, J. Li, R. Kumar, J. Cao, S. Jang, F. Paesani, D. Reichman,

        J. Phys. Chem. B 128, 7703 (2024). [link]

This Virtual Special Issue honors Professor Gregory A. Voth and his pioneering contributions to the field of physical chemistry.

Over the past 35 years, Greg has made a remarkable number of groundbreaking contributions that have dramatically

advanced our understanding of the structure and dynamics of complex condensed phase systems, ranging from ionic liquids

to viral capsids, while addressing both classical and quantal aspects of these phenomena. To attack these problems, Greg

and his co-workers have developed theories and simulation methodologies that are now widely adopted by the scientific

community to gain deep mechanistic insights and accurately model the behavior of strongly interacting condensed phase

systems over a wide range of length and time scales. In the following, we highlight some of these important breakthroughs..

194. Monitoring water harvesting in metal-organic frameworks, one water molecule at a time.

        K.M. Hunter, F. Paesani, Chem. Sci. 15, 5303 (2024). [link]

MOFs have gained prominence as potential materials for atmospheric water harvesting, a vital solution for arid regions and areas

experiencing severe water shortages. Among all MOFs, Ni2X2BTDD (X = F, Cl, Br) stands out as a promising water harvester due

to its ability to adsorb substantial amounts of water at low relative humidity (RH). Here, we use advanced molecular dynamics

simulations carried out with the state-of-the-art MB-pol data-driven many-body potential to monitor water adsorption in the three

Ni2X2BTDD variants as a function of RH. Our simulations reveal that the type of halide atom in the three Ni2X2BTDD frameworks

significantly influences the corresponding molecular mechanisms of water adsorption: while water molecules form strong

hydrogen bonds with the fluoride atoms in Ni2F2BTDD, they tend to form hydrogen bonds with the nitrogen atoms of the triazolate

linkers in Ni2Cl2BTDD and Ni2Br2BTDD. Importantly, the large size of the bromide atoms reduces the void volume in the

Ni2Br2BTDD pores, which enable water molecules to initiate an extended hydrogen-bond network at lower RH. These findings

not only underscore the prospect for precisely tuning structural and chemical modifications of the frameworks to optimize their

interaction with water, but also highlight the predictive power of simulations with the MB-pol data-driven many-body potential.

195. Entropy of liquid water as predicted by the two-phase thermodynamic model and data-driven many-body

        potentials. C.H. Ho, F. Paesani, J. Phys. Chem. B 128, 6885 (2024). [link]

We investigate the entropy of liquid water at ambient conditions using the two-phase thermodynamic (2PT) model, applied

to both common pairwise-additive water models and the MB-pol and MB-pol(2023) data-driven many-body potentials. Our

simulations demonstrate that the 2PT model yields entropy values in semiquantitative agreement with experimental data

when using MB-pol and MB-pol(2023). Additionally, our analyses indicate that the entropy values predicted by

pairwise-additive water models may benefit from error compensation between the inherent approximations of the 2PT

model and the known limitations of these water models in describing many-body interactions. Despite its approximate

nature, the simplicity of the 2PT model makes it a valuable tool for estimating relative entropy changes of liquid water

across various environ- ments, especially when combined with water models that provide a consistently robust

representation of the structural, thermodynamic, and dynamical properties of liquid water.

197. Current status of the MB-pol data-driven many-body potential for predictive simulations of water across

        different phases. E. Palos, E. Bull-Vulpe, X. Zhu, H. Agnew, S. Gupta, S. Saha, F. Paesani,

        J. Chem. Theory Comput. 20, 9268 (2024). [link]

Accurately modeling water through computer simulations has been a significant challenge due to the complex nature of

the hydrogen-bonding network that water molecules form under different thermodynamic conditions. The introduction of

the MB-pol data-driven many-body potential energy function marked a significant advancement toward a universal

molecular model capable of predicting the structural, thermodynamic, dynamical, and spectroscopic properties of water

across all phases. By integrating physics-based and data-driven (i.e., machine-learned) components, which correctly

capture the delicate balance among different many-body interactions, MB-pol achieves chemical and spectroscopic

accuracy, enabling realistic molecular simulations of water, from gas-phase clusters to liquid water and ice. In this review,

we present a comprehensive overview of the data-driven many-body formalism adopted by MB-pol, highlight the main

results and predictions made from computer simulations with MB-pol to date, and discuss the prospects for future

extensions to data-driven many-body potentials of generic and reactive molecular systems.

196. Concentration-dependent layer-stacking and the influence on phase-conversion in colloidally synthesized

        WSe2 nanocrystals. J.Q. Geisenhoff, Y. Pan, H. Yin, F. Paesani, A.M. Schimpf, Chem. Mater. 36, 8834 (2024). [link]

We report a synthesis of WSe2 nanocrystals in which the number of layers is controlled by varying the precursor

concentration. By altering the ratios and concentrations of W(CO)6 and Ph2Se2 in trioctylphosphine oxide,

we show that high [Se] and large Se/W ratios lead to an increased number of layers per nanocrystal. As the

number of layers per nanocrystal is increased, the nanocrystal ensembles show less phase-conversion from

the metastable 2M phase to the thermodynamically favored 2H phase. Density functional theory calculations

indicate that the interlayer binding energy increases with the number of layers, indicating that the stronger

interlayer interactions in multilayered nanocrystals may increase the energy barrier to phase-conversion. The

results presented herein provide insights for directing phase-conversion in solution-phase syntheses of

transition metal dichalcogenides.

193. Cooperative interactions with water drive hysteresis in a hydrophilic metal-organic framework. J.J. Oppenheim,

        C.-H. Ho, D. Alezi, J.L. Andrews, T. Chen, B. Dinakar, F. Paesani, M. Dincă, Chem. Mater. 36, 3395 (2024). [link]

Devices that utilize the reversible capture of water vapor provide solutions to water insecurity, increasing energy demand,

and sustainability. Herein, we synthesize a new hexagonal microporous framework, Ni2Cl2BBTQ, to elucidate these

principles. Uniquely among its known isoreticular analogues, Ni2Cl2BBTQ presents unusually high hysteresis caused by

strong wetting seeded by a particularly strong zero-coverage interaction with water. A combination of vibrational spectra

and molecular dynamics simulations reveals that this hysteretic behavior is the result of an intricate hydrogen-bonding

network, in which the monolayer consists of water simultaneously binding to open nickel sites and hydrogen bonding to

quinone sites. This latter hydrogen-bonding interaction does not exist in other isoreticular analogues: it prevents facile

water dynamics and drives hysteresis. Our results highlight an important design criterion for water sorbents: in order to

drive water uptake in progressively dry conditions, the common strategy of increasing hydrophilicity can cause strong

wetting and the formation of superclusters, which lead to undesirable hysteresis. Instead, hysteresis-free water uptake

at extremely low humidity is best promoted by decreasing the pore size, rather than increasing hydrophilicity..