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Computational determination of pKa values. A comparison of different theoretical approaches and a novel procedure

AuthorsSoriano, Elena ; Cerdán, Sebastián ; Ballesteros, Paloma
Issue Date2004
CitationJournal of Molecular Structure: THEOCHEM 684(1-3): 121-128 (2004)
AbstractWe have estimated the pKa values of different series of imidazol-1-ylalcanoic acid derivatives using combinations of semiempirical or ab initio methods and two semiempirical solvation models SM2 and SM5.4. None of these procedures was able to describe the zwitterionic structure of the carboxylic monoacids series as their most stable form determined experimentally in solution. Under these conditions, a comparison of the theoretical and experimental pKa values showed RMS differences ranging from 1.43 to 3.04 pKa units. As an alternative strategy we describe here a mathematical model for pKa determination based on quantitative structure-property relationships. The model uses two quantum chemical descriptors, the natural atomic charge on the N3 proton (qH +) and the frontier orbital energy (εL), allowing the determination of pKa values for every compound of a given series with a reasonable computational effort. A linear relationship is described among pKa, qH +, εL and the number of ester groups in the molecule n. Multiple linear regression analysis of the available series of imidazol-1-ylalcanoic compounds allowed to obtain the expression pKa=-119.435×qH++0.612×εL-1.043×n+65.204 which reproduced the experimental values within 0.1 pKa units. © 2004 Elsevier B.V. All rights reserved.
Identifiersdoi: 10.1016/j.theochem.2004.06.041
issn: 0166-1280
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