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Thermodynamic Estimate of p K a Values of the Carboxylic Acids in Aqueous Solution with the Density Functional Theory
Author(s) -
Zeng Yue,
Qian Haiyan,
Chen Xianglan,
Li Zelin,
Yu Shangci,
Xiao Xiaoming
Publication year - 2010
Publication title -
chinese journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.28
H-Index - 41
eISSN - 1614-7065
pISSN - 1001-604X
DOI - 10.1002/cjoc.201090138
Subject(s) - chemistry , aqueous solution , carboxylic acid , dissociation (chemistry) , gibbs free energy , density functional theory , thermodynamics , polarizability , standard deviation , polarizable continuum model , scaling , absolute deviation , computational chemistry , solvent , solvent effects , molecule , organic chemistry , geometry , physics , statistics , mathematics
The 96 p K a values of 85 carboxylic acids in aqueous solution were calculated with the density functional theory method at the level of B3LYP/6‐31+G(d,p) and the polarizable continuum model (PCM) was used to describe the solvent. In the calculations of p K a values, the dissociation Gibbs free energies were directly calculated using carboxylic acid dissociation reactions in aqueous solution, i . e ., no thermodynamic cycle was employed, which is different from the previous literatures. A highly significant correlation of R 2 =0.95 with a standard deviation (SD) of 0.36 between the experimental p K a values and the calculated dissociation Gibbs free energies [Δ G (calc.)] was found. The slope of p K a vs. ( G (calc.)/(20303 RT ) is only 47.6% of the theoretically expected value, which implies that the Δ G (calc.) value from the theoretical calculation is larger than the actual one for all 85 carboxylic acids studied. Thus, by adding the 0.476 scaling‐factor into the slope, we can derive a reliably procedure that can reproduce the experimental p K a values of carboxylic acids. The p K a values furnished by this procedure are in good agreement with the experimental results for carboxylic acids in aqueous solution.