Ab initio studies on acidity and tautomeric equilibrium constants of some benzoxa-, benzothia-, benzoselena-zolinone derivatives

Theoretically nanoscale study on ionization of muscimol nano drug in aqueous solution

In the present work, acid dissociation constant (pKa) values of muscimol derivatives were calculated using the Density Functional Theory (DFT) method. In this regard, free energy values of neutral, protonated and deprotonated species of muscimol were calculated in water at the B3LYP/6-31G(d) basis sets. The hydrogen bond formation of all species had been analyzed using the Tomasi's method. It was revealed that the theoretically calculated pKa values were in a good agreement with the existing experimental pKa values, which were determined from capillary electrophoresis, potentiometric titration and UV-visible spectrophotometric measurements.

Spectroscopic and quantum chemical investigations of substituent effects on the azo-hydrazone tautomerism and acid-base properties of arylazo pyridone dyes

A series of 5-(4-substituted arylazo)-6-hydroxy-4-methyl-3-cyano-2-pyridone dyes was synthesized and the structure of the dyes was confirmed by UV-Vis, FT-IR, (1)H NMR and (13)C NMR spectroscopic techniques. The azo-hydrazone tautomeric equilibrium was found to depend on the substituents as well as on the acidity and basicity of the media. Ionization constant, pKa, of the dyes was determined by UV-Vis spectroscopy and correlated with the Hammett substituent constants, σp and σI. The interpretation of the effect of different substituent in phenyl ring of arylazo pyridone dyes on their spectroscopic and structural properties was based on quantum chemical calculations performed by the density functional theory (DFT/M06-2X) method. The DFT calculations confirmed the existence of two forms in water solution: hydrazone form in acidic and neutral media and anionic form in basic media. The different contribution of azo and hydrazone canonical forms of anionic form is observed for dyes with electron-donating and dyes with electron-withdrawing groups. The dependence of absorption spectra and determined pKa values to the substituent type seems to be mostly due to azo/hydrazone canonical structure ratio in their anionic form.