Experiment-based physicochemical aspects for the coulombic hydration kinetics and thermodynamics of a pyrimidine and thiopyrimidine

The hydration, solvation kinetics and thermodynamics 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) are studied in due to course of time at pH values from 1.0 to 13.0 using UV/vis spectroscopic analysis of their aged solutions for the first time, at a temperature of 25 ± 0.1 °C. In this sense, a time dependent intersection point was discovered in the UV/vis spectra obtained. The time dependent intersection point method is thus improved for the first time in this study. The improved method is found to be more useful in the determination of the existence of a time dependent equilibrium and the time to reach the equilibrium at a constant pH. Methanol solvation property of the compounds is also investigated only at pH values of ca. 1.13 and ca. 13.0. Pseudo-first-order hydration kinetics and thermodynamics data comprising rate constants, hydration equilibrium constants, and the corresponding Gibbs free energy changes are calculated for water molecules which are in direct contact with the molecules of I and II. A remarkable result is discovered with II for which the hydration Gibbs energy change was calculated to be −422.64 J/mol at a pH of ca. 12, indicating the hydration is exothermic, energetically downhill and weakly in forward direction. Plausible hydration mechanisms are suggested for the compounds based on the extensive experimental UV/vis data presented.