Non-Covalent Interaction in Binary Thymidine/Polyamine Systems in Aqueous Solution

Electronic absorption study on acid-base equilibria for some pyrimidine derivatives containing semi- and thiosemicarbazone moiety

The UV-vis spectra of recently synthesized 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-one, (I), and 5-benzoyl-1-(methylphenylmethyleneamino)-4-phenyl-1H-pyrimidine-2-thione, (II) were studied in aqueous methanol (5%, v/v methanol). The nature of the electronic transitions and the roles of carbonyl oxygen of I and thiocarbonyl sulfur of II on the behavior of UV-vis spectra were discussed. Acid-base equilibria of the compounds against varying pH and pK(a) values related equilibria were determined at an ionic strength of 0.10 M by using the Henderson-Haselbalch equation. The mean acidity constants for the protonated forms of the compounds were determined as pK(a1)=5.121, pK(a2)=7.929 and pK(a3)=11.130 for I and pK(a1)=4.684, pK(a2)=7.245 and pK(a3)=10.630 for II. The preferred dissociation mechanisms were discussed based on UV-vis data and a mechanism was proposed for each compound.