title: Temperature and substituent effects on the dissociation constants of 5-azorhodanine derivatives. Semi-empirical quantum mechanical calculation.
abstract: Detailed analysis of the electronic structure and properties of some rhodanine derivatives (RDs) is presented. The aim of the present investigation is to pinpoint the electronic structural similarities and differences, among the series of the studied RDs that govern and determine their acidic, basic and co-ordinative properties. The geometries of the studied rhodanine were fully optimized at the level of AMI semi-empirical method. Relative stabilities of the enol/keto isomers have been calculated. Proton affinities and proton detachment energies were computed for the series of rhodanine studied, at the level of AM1 method and compared with the potentiometrically-determined proton-ligand dissociation constants. Zero-point energy and electron correlations have been taken into consideration. pK(H) have been found to increase with increasing electron-donating nature of the substituents. The resulting linear Hammett plots of pK(H) versus the Hammett constant sigma values indicate the co-planarity of the investigated molecules. The evaluated thermodynamic parameters (deltaG, deltaH and deltaS) indicate that the dissociation processes are non-spontaneous, endothermic and entropically unfavourable.
authors.affiliation: Chemistry Department, Faculty of Science (Damietta), Mansoura University, Domyat Al-Gideda, Egypt. asmasomy@mum.mans.eun.eg
authors.forename: Tarek M
authors.initials: TM
authors.lastname: El-Gogary
authors.forename: A A
authors.initials: AA
authors.lastname: El-Bindary
authors.forename: Azza S
authors.initials: AS
authors.lastname: Hilali
mesh_terms: D004583:Electrons; D006863:Hydrogen-Ion Concentration; D008024:Ligands; D008956:Models, Chemical; D011189:Potassium Chloride / Q000494:pharmacology; D011522:Protons; D011789:Quantum Theory; D012236:Rhodanine / Q000031:analogs & derivatives* / Q000737:chemistry; D012984:Software; D013053:Spectrophotometry / Q000379:methods; D013237:Stereoisomerism; D013696:Temperature; D013816:Thermodynamics
chemical_list: D008024:Ligands; D011522:Protons; D011189:Potassium Chloride; D012236:Rhodanine