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Computation of Atmospheric Concentrations of Molecular Clusters from ab initio Thermochemistry
Published on: April 8, 2020
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Hydrated Anions: From Clusters to Bulk Solution with Quasi-Chemical Theory
Diego T Gomez1, Lawrence R Pratt1, Dilipkumar N Asthagiri2
1Department of Chemical & Biomolecular Engineering, Tulane University, New Orleans, Louisiana 70118, United States.
Accounts of Chemical Research
|July 13, 2022
Summary
Quasi-chemical theory (QCT) accurately calculates ion hydration free energies by analyzing ion-cluster interactions. This advanced theory successfully explains anion selectivity in ion channels and aligns with experimental data.
Area of Science:
- Computational Chemistry
- Physical Chemistry
- Theoretical Chemistry
Background:
- Interactions between hydrated ions and solution/interface partners are crucial in chemical processes.
- Accurate calculation of ion hydration free energies is essential for understanding ion behavior in solution.
- Anions, particularly halides and hydroxide, present significant challenges for existing theoretical models.
Purpose of the Study:
- To present and validate the quasi-chemical theory (QCT) for calculating ion hydration free energies.
- To investigate the hydration of challenging anions like halides and hydroxide.
- To explain phenomena such as the high selectivity of fluoride over chloride in ion channels.
Main Methods:
- Utilized quasi-chemical theory (QCT), involving identification and separate treatment of inner-shell clusters.
- Employed ab initio molecular dynamics (AIMD) for sampling ion-hydration clusters and bulk solutions.
- Integrated electronic structure calculations and statistical thermodynamics for free energy evaluation.
- Compared QCT results with mass-spectrometric measurements and standard hydration free energy tabulations.
Main Results:
- QCT demonstrated excellent agreement with experimental hydration free energies using moderate computational resources.
- Identified strikingly asymmetric hydration clusters for heavier halide ions.
- An inverse QCT procedure, using AIMD from bulk solutions, significantly improved results for asymmetric hydration.
- Final QCT results showed close agreement with established hydration free energy data and were independent of coordination number.
Conclusions:
- QCT provides a numerically accurate and robust method for calculating ion hydration free energies, particularly for challenging anions.
- The theory-experiment comparison validates both QCT and experimental measurements.
- AIMD simulations revealed differences in anion hydration structures between clusters and bulk solutions, with moderated asymmetries in bulk.
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