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Challenges in predicting Δ rxn G in solution: Hydronium, hydroxide, and water autoionization
Author(s) -
Dhillon Shamneet,
East Allan L. L.
Publication year - 2018
Publication title -
international journal of quantum chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.484
H-Index - 105
eISSN - 1097-461X
pISSN - 0020-7608
DOI - 10.1002/qua.25703
Subject(s) - hydronium , autoionization , hydroxide , ion , molecular dynamics , chemistry , gibbs free energy , water cluster , cluster (spacecraft) , atomic physics , thermodynamics , computational chemistry , physics , molecule , inorganic chemistry , ionization , hydrogen bond , computer science , programming language , organic chemistry , photoionization
Standard non‐semiempirical continuum‐dielectric orbital‐based methods horribly overpredict, by 26‐50 kcal mol −1 , the Gibbs energy for the water autoionization reaction 2 H 2 O (l) → H 3 O + (aq)  + OH – (aq) . Here, we demonstrate these errors, fully investigate the reasons for these errors, and show that the use of 4 explicit solvent within the continuum (the “semicontinuum,” “cluster‐continuum,” or “hybrid” technique) can reduce the error of a standard continuum model from 50 to 2 kcal mol −1 . Results from pure cluster, pure continuum (several versions including semiempirical ones), and semicontinuum modeling are each presented and discussed. We recommend use of 3 waters around hydronium and 4 waters around hydroxide with standard continua whenever these ions are involved in reaction. To the possible surprise of some, time‐consuming molecular‐dynamics simulations are not needed to reproduce this problematic energy.

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