Jump to main content
Jump to site search

Issue 23, 2011
Previous Article Next Article

pKa prediction from an ab initio bond length: Part 3—benzoic acids and anilines

Author affiliations

Abstract

The prediction of pKa from a single ab initio bond length has been extended to provide equations for benzoic acids and anilines. The HF/6-31G(d) level of theory is used for all geometry optimisations. Similarly to phenols (Part 2 of this series of publications), the meta-/para-substituted benzoic acids can be predicted from a single model constructed from one bond length. This model had an impressive RMSEP of 0.13 pKa units. The prediction of ortho-substituted benzoic acids required the identification of high-correlation subsets, where the compounds in the same subset have at least one of the same (e.g. halogens, hydroxy) ortho substituent. Two pKa equations are provided for o-halogen benzoic acids and o-hydroxybenzoic acids, where the RMSEP values are 0.19 and 0.15 pKa units, respectively. Interestingly, the bond length that provided the best model differed between these two high-correlation subsets. This demonstrates the importance of investigating the most predictive bond length, which is not necessarily the bond involving the acid hydrogen. Three high-correlation subsets were identified for the ortho-substituted anilines. These were o-halogen, o-nitro and o-alkyl-substituted aniline high-correlation subsets, where the RMSEP ranged from 0.23 to 0.44 pKa units. The RMSEP for the meta-/para-substituted aniline model was 0.54 pKa units. This value exceeded our threshold of 0.50 pKa units and was higher than both the m-/p-benzoic acids in this work and the m-/p-phenols (RMSEP = 0.43) of Part 2. Constructing two separate models for the meta- and para- substituted anilines, where RMSEP values of 0.63 and 0.33 pKa units were obtained respectively, revealed it was the meta-substituted anilines that caused the large RMSEP value. For unknown reasons the RMSEP value increased with the addition of a further twenty meta-substituted anilines to this model. The C–N bond always produced the best correlations with pKa for all the high-correlation subsets. A higher level of theory and an ammonia probe improved the statistics only marginally for the hydroxybenzoic acid high-correlation subsets.

Graphical abstract: pKa prediction from an ab initio bond length: Part 3—benzoic acids and anilines

Back to tab navigation

Supplementary files

Article information


Submitted
12 Feb 2011
Accepted
15 Apr 2011
First published
13 May 2011

Phys. Chem. Chem. Phys., 2011,13, 11283-11293
Article type
Paper

pKa prediction from an ab initio bond length: Part 3—benzoic acids and anilines

A.P. Harding and P.L.A. Popelier, Phys. Chem. Chem. Phys., 2011, 13, 11283
DOI: 10.1039/C1CP20380K

Social activity

Search articles by author

Spotlight

Advertisements