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Statistical study for the prediction of pKa values of substituted benzaldoximes based on quantum chemical methods

Emad A.S.Al-Hyali*, Nezar A.Al-Azzawi, Faiz M.H.Al-Abady

Multiple regression analysis was used for the calculation of pKa values of 15 substituted benzaldoximes by using various types of descriptors as parameters. These descriptors are based on quantum mechanical treatments. They were derived by employing semi-empirical calculation represented by the PM3 model and an Ab initio method expressed byHartree-Fock (HF)model performed at the 6-311 G(d, p) level of theory. The parameters tested for their ability to represent the variations observed in the experimental pKa(s) are atomic and structural properties including Muliken charges on the atoms of hydroxyl group and C=N bond, the angle C6-C1-C7, and length of O-H bond. Molecular properties are also used like energies ofHOMO and LUMOorbitals, hardness (), chemical potential (), total energy, dipole of molecule, and electrophilicity index (W). The relation between pKa values and each of these parameters of the studied compounds in investigated. Depending on these relations, two sets of parameters were constructed for comparison between the PM3 and HF methods. The results obtained favor theAb initio method for such applications although both models proved to have high predictive power and have sufficient reliability to describe the effect of substituents on pKa values of benzaldoxime compounds under consideration which is clear from the values of correlation coefficient R2 obtained and the consistency between the experimental and the calculated values.