Estimating successive pK(a) values of polyprotic acids from ab initio molecular dynamics using metadynamics: the dissociation of phthalic acid and its isomers

Tummanapelli, Anil Kumar and Vasudevan, Sukumaran (2015) Estimating successive pK(a) values of polyprotic acids from ab initio molecular dynamics using metadynamics: the dissociation of phthalic acid and its isomers. In: PHYSICAL CHEMISTRY CHEMICAL PHYSICS, 17 (9). pp. 6383-6388.

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Abstract

Estimation of the dissociation constant, or pK(a), of weak acids continues to be a central goal in theoretical chemistry. Here we show that ab initio Car-Parrinello molecular dynamics simulations in conjunction with metadynamics calculations of the free energy profile of the dissociation reaction can provide reasonable estimates of the successive pK(a) values of polyprotic acids. We use the distance-dependent coordination number of the protons bound to the hydroxyl oxygen of the carboxylic group as the collective variable to explore the free energy profile of the dissociation process. Water molecules, sufficient to complete three hydration shells surrounding the acid molecule, were included explicitly in the computation procedure. Two distinct minima corresponding to the dissociated and un-dissociated states of the acid are observed and the difference in their free energy values provides the estimate for pK(a), the acid dissociation constant. We show that the method predicts the pK(a) value of benzoic acid in good agreement with experiment and then show using phthalic acid (benzene dicarboxylic acid) as a test system that both the first and second pK(a) values as well, as the subtle difference in their values for different isomers can be predicted in reasonable agreement with experimental data.

Item Type:	Journal Article
Publication:	PHYSICAL CHEMISTRY CHEMICAL PHYSICS
Publisher:	ROYAL SOC CHEMISTRY
Additional Information:	Copy right for this article belongs to the ROYAL SOC CHEMISTRY, THOMAS GRAHAM HOUSE, SCIENCE PARK, MILTON RD, CAMBRIDGE CB4 0WF, CAMBS, ENGLAND
Keywords:	DENSITY-FUNCTIONAL THEORY; SOLVATION FREE-ENERGIES; CLUSTER-CONTINUUM MODEL; AQUEOUS-SOLUTION; PROTON-TRANSFER; WATER CLUSTERS; QM/EFP-MD; CONSTANTS; DEPROTONATION; SIMULATIONS
Department/Centre:	Division of Chemical Sciences > Inorganic & Physical Chemistry
Date Deposited:	24 Apr 2015 06:14
Last Modified:	24 Apr 2015 06:14
URI:	http://eprints.iisc.ac.in/id/eprint/51401

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