Issue 54, 2018, Issue in Progress

Tautomerism and antioxidant activity of some 4-acylpyrazolone-based Schiff bases: a theoretical study

Abstract

4-Acylpyrazolone Schiff bases display antimicrobial, antiprion, antioxidant, and other biological activities. They are also used as ligands and some of their complexes possess photoluminescence and anticancer properties. These Schiff bases may exist in four tautomeric forms that correspond to H at the C (imine-one(I)), N (imine-one(II)), and O (imine-ol) atoms of the pyrazolone ring or at the azomethine N atom (amine-one). While crystal structures show the amine-one form, the identity of the tautomeric form in solution and the structure–antioxidant activity relationship of these compounds are not clear. We perform quantum mechanical investigations on nine 4-acylpyrazolone-based Schiff bases at the B3LYP/6-311++G(d,p) level of theory in the gas phase and in chloroform, dimethyl sulfoxide, and water using the polarizable continuum model (PCM). Results show that the imine-ol, imine-one(I), and imine-one(II) isomers are, in respective, 6.5–8.0, 17–20, and 19–23 kcal mol−1 less stable than the amine-one form and that solvents further stabilize the later form. The energy barrier for imine-ol to amine-one conversion is only 0–1 kcal mol−1, showing that formation of the latter form is both kinetically and thermodynamically favorable. NMR calculations show that H in the amine-one and imine-ol forms appears at δ = 11.9–12.9 and 14.0–15.7 ppm, respectively, revealing that the experimentally reported 1H NMR spectra of these compounds are due to the amine-one tautomeric form. The structure–antioxidant activity relationship is investigated and structural modifications that increase the antioxidant activity are discussed. Calculations using the PCM show that the vertical ionization potential (IPV) is inversely proportional with the ferric reducing antioxidant power (FRAP) of these compounds. IPV thus presents a valuable tool for predicting the FRAP.

Graphical abstract: Tautomerism and antioxidant activity of some 4-acylpyrazolone-based Schiff bases: a theoretical study

Supplementary files

Article information

Article type
Paper
Submitted
13 Jul 2018
Accepted
28 Aug 2018
First published
03 Sep 2018
This article is Open Access
Creative Commons BY license

RSC Adv., 2018,8, 30842-30850

Tautomerism and antioxidant activity of some 4-acylpyrazolone-based Schiff bases: a theoretical study

E. A. Orabi, RSC Adv., 2018, 8, 30842 DOI: 10.1039/C8RA05987J

This article is licensed under a Creative Commons Attribution 3.0 Unported Licence. You can use material from this article in other publications without requesting further permissions from the RSC, provided that the correct acknowledgement is given.

Read more about how to correctly acknowledge RSC content.

Social activity

Spotlight

Advertisements