Effects of intramolecular hydrogen bonding and solvent composition on acidity of some dihydroxy-thioxanthone derivatives in methanol–water binary solvents

The acidity constant of four recently synthesized dihydroxy-thioxanthone derivatives (i.e., 1,4-dihydroxy; A1, 3-methyl-1,4-dihydroxy; A2, 2,3-dihydroxy; A3 and 1,3-dihydroxy; A4) in methanol–water binary mixed solvents were determined by multivariate analysis of spectrophotometric-pH-metric data. The solutes A1, A2 and A3 exhibited two ionization steps whereas one ionization step was observed for A4, attributed to the strong intramolecular hydrogen-bonding formed between the hydrogen of 1-hydroxy and its neighbor carbonyl group. The effects of molecular structure on the acid–base behavior of solutes were discussed mainly based on intra and intermolecular hydrogen bonding. The effects of solvent composition on the acidity constant were explained through linear solvation free energy relationship (LSFER) study. It was found that each ionization step of each solute can be modeled by different reduced models of Kamlet–Taft, proposing that they behave differently. Finally, the effect of solvent composition on the calculated pure absorption spectra of the different acid–base species was investigated.