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A large number of force fields have been proposed for describing the behavior of liquid water within classical atomistic simulations, particularly molecular dynamics. In the past two decades, models that incorporate molecular polarizability and even charge transfer have become more prevalent, in attempts to develop more accurate descriptions. These are frequently parameterized to reproduce the measured thermodynamics, phase behavior, and structure of water. On the other hand, the dynamics of water is rarely considered in the construction of these models, despite its importance in their ultimate applications. In this paper, we explore the structure and dynamics of polarizable and charge-transfer water models, with a focus on timescales that directly or indirectly relate to hydrogen bond (H-bond) making and breaking. Moreover, we use the recently developed fluctuation theory for dynamics to determine the temperature dependence of these properties to shed light on the driving forces. This approach provides key insight into the timescale activation energies through a rigorous decomposition into contributions from the different interactions, including polarization and charge transfer. The results show that charge transfer effects have a negligible effect on the activation energies. Furthermore, the same tension between electrostatic and van der Waals interactions that is found in fixed-charge water models also governs the behavior of polarizable models. The models are found to involve significant energy–entropy compensation, pointing to the importance of developing water models that accurately describe the temperature dependence of water structure and dynamics.
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Unraveling the Molecular Pathways for Structure “Making” and “Breaking” by Ions in Water
Aqueous anions play a crucial role in chemical and biological processes. They are traditionally classified as “structure makers” or “structure breakers” based on their impact on the viscosity of electrolyte solutions. Until now, this behavior has been assumed to stem from a single restructuring mechanism of the hydrogen (H) bonding network of water, that could align with macroscopic properties. Correlated Vibrational Spectroscopy (CVS) measurements reveal that this is not the case. Rather, anions modify water–water H-bonds through multiple distinct pathways, with frequency shifts correlating with charge transfer, and intensity changes quantifying variations in the number of interacting/orientationally cross-correlated H-bonds. The different ways through which anions impact water structure can be explained in terms of Hard–Soft-Acid–Base theory. Hard anions only affect water H-bonds through electrostatics. By contrast, soft anions weaken the H-bonds via charge transfer but simultaneously increase their concentration. The two effects for soft anions nearly cancel each other out in terms of structure breaking/making, resulting in macroscopic behavior that is similar to hard anions in spite of dramatically different molecular-level effects.
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- PAR ID:
- 10642316
- Publisher / Repository:
- American Chemical Society
- Date Published:
- Journal Name:
- Journal of the American Chemical Society
- ISSN:
- 0002-7863
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1021/jacs.5c10984
