Canadian Journal of Pure & Applied Sciences

Abstract: Density functional theory (DFT) and ab- initio Quantum Mechanical calculations have been used to study the structures and properties of the Molecules Pyrimethamine and Sulfadoxine. Their molecular stabilities, structures, dipole moments, charges transfer, polarizability tensors, average polarizability anisotropy, energies, IR and Raman vibrational frequencies have been predicted. Tentative assignments for their intense IR active frequencies have been carried out and represented. We have used the Restricted Hartree-Fock (RHF) and density functional Becke3LYP (B3LYP) theories by employing 6- 311++G** basis set for inclusion of electron correlation. From our results we observe that the molecules are more stable at the RHF level of theory than at the B3LYP level of theory. The frequency calculations obtained at the B3LYP level are closer to some experimental values than those obtained at the RHF level due the effect of electron correlation. The magnitude of the dipole moment is higher in the RHF level and the polarizability tensor components, the average polarizability and the anisotropy are greater at the B3LYP level. This implies that the inclusion of electron correlation decreases the dipole moment and increases the polarizability tensors, the average polarizability and the anisotropy. The IR and Raman spectra of the molecules have also been presented and the IR spectrum of Pyrimethamine lies in the same range as that given by some experimental results.