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Theoretical Study for the Estimation of Pka Values of a Number Ofschiff Bases Using Parameters Derived from Quantum Mechanical Methods

Author(s): Raed T.Gh. Al-Abady, Fedaa Marie Altaie, Emad A.S.Al-Hyali*

This study included correlation analysis of the experimentally determined pKa values of nine Schiff bases with theoretically derived parameters based on three models of quantum mechanical calculations. Two of them are semi empirical (Austin Method 1 (AM1), and Parameterized Model 3 (PM3)) and the third is an ab initio (Hartree Fock (HF) method, The last method is determined at 6.311 G (d, p) basis set level of theory. The suggested parameters to undergo this study are electronic and energetic in nature, involving the atomic charges (Mullikan and Lowding), the total energy of molecule, bend, stretch, torsion, length of the imine bond (C=N) and other bonds connected to imine. Other parameters are represented by the energies of the HOMO and LUMO orbitals, Kinetic Energy (KE) and two types of Vander Waals interactions, of the 1,4 type (VDW1,4) and non1, 4 type (Non 1,4 VDW). The procedure followed for achieving this work is first by simple correlation between the pKa values with each of the suggested parameters. Relying on the obtained results, multiple Linear Regression Analysis (LRA) is performed. Three sets of parameters are derived from the applied methods AM1, PM3 and HF. The selected set of each method is used to estimate the pKa values of the compounds under consideration theoretically. Comparison of the experimental pKa with those determined theoretically are achieved. A preference is given in favor of AM1 over the other two methods in terms of correlation coefficient (R2) and Standard Error (SE). Additional test is introduced for verifying this method of calculation. The pKa for a number of compounds found in the literature and not involved in the regression analysis are calculated by the derived equations and compared with their experimental results. Good agreement between the practical and calculated pKa (s) are obtained. This procedure of calculation is proved to be of greet forecasting power and sufficient confidence level.

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