Determination Of Thermodynamic Functions from the pKa Values of a Number of Schiff Bases by Employing the DFT Method: Theoretical Study

Abstract

Thermodynamic functions are normally calculated experimentally through studying the effect of temperature on the equilibrium constant. Such studies may facing experimental errors, they may require the consumption of expensive chemicals, in addition of being time consuming. The development of computer science and programing facilitated the theoretical study of such project. In this work the determination of pKa of several Schiff bases are achieved by statistical analysis using SPSS program (V.12) and by employing parameters based on quantum mechanically derived method. The Chem. Office program (V.12,2010) is used for achieving these studies. The density functional theory (DFT) is applied as an ab initio method, using the method of (B3LYP) at basis set (3-21G). The theoretical pKa values were determined by several derived equations at several temperatures (20,30,40,50, and 60 oC. These equations were obtained from applying multi-parametric regression analysis based on parameters derived from DFT method. The calculated pKa were highly consistent with the practical values in terms of R2 and SE. These consistency improved as temperature increased from 20-60Co. The best agreement between pKa(calc) and pKa(exp) are seen at 50 and 60 oc. The calculated pKa at various temperatures (20-60oC) were used to calculate the ∆Go, ∆Ho values, and ∆So. The