Theoretical calculations of the relative pKa values of some selected aromatic arsonic acids in water using density functional theory

Behzad Khalili,Mehdi Rimaz

doi:10.5267/j.ccl.2015.10.002

Abstract

Article history: Received July 21, 2015 Received in revised form August 29, 2015 Accepted 8 October 2015 Available online 8 October 2015 With the focus on calculating the equilibrium constant (Ka) of arsonic acids (RAsO(OH)2) in aqueous media, the behavior of both neutral and negatively charged species of some arsonic acids have been considered through the isodesmic reaction scheme with polarized continuum model of solvation. The relative pKa values of a number of arsonic acids were calculated using density functional theory (DFT), with methylarsonic acid (CH3AsO(OH)2) used as a reference molecule. Various basis sets such as (6-31G(d), 6-31+G(d), 6-311++G(d,p) and 6311++G(2d,2p)) in conjunction with B3LYP computational method were examined. Finally the latter one was found to give better results. The results of applied B3LYP 6-311++G(2d,2p) method for pKa values of 25 arsonic acids in aqueous media showed good agreement with corresponding experimental pKa values. In this level of theory the average error was less than 0.3 pKa units. The type of substituent affected the acid strength, with electron withdrawing substituents lowering the pKa and electron releasing groups increasing it. © 2016 Growing Science Ltd. All rights reserved.

Highlights

Arsonic acids have the general formula of RAs(O)(OH)[2] in which R could be alkyl or aryl group
With the focus on calculating the equilibrium constant (Ka) of arsonic acids (RAsO(OH)2) in aqueous media, the behavior of both neutral and negatively charged species of some arsonic acids have been considered through the isodesmic reaction scheme with polarized continuum model of solvation
This work reports the calculation of the pka value of 25 arsonic acids in aqueous media using two different levels of theory

Summary

Introduction

Arsonic acids have the general formula of RAs(O)(OH)[2] in which R could be alkyl or aryl group. Organo arsenic compounds despite their toxicity gets revived attention due to pharmaceutical use.[1,2,3,4] For example Carbarsone is an organoarsenic compound with antiprotozoal activity which is used against of the amebiasis.[5,6,7] They are under consideration from the biological activity point of view yet.[8,9,10]. Organo arsenic compounds are very useful starting materials with important application in various fields of chemistry. For example Lloyd et al used the aryl arsonic acids as precursors to As(III) and As(I) species as part of their studies into the chemistry and biological activity of Salvarsan which is the historically-important arsenical drug for treatment of syphilis.[11,12]

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