Vibrational Spectroscopic Investigations, Electronic Properties, Molecular Structure and Quantum Mechanical Study of an Antifolate Drug: Pyrimethamine

The computational modelling supported by experimental results can explain the molecular structure, vibrational assignments, reactive sites and several structural properties. In this context, the spectroscopic (FT-IR, FT-Raman and NMR) analysis, electronic properties (HOMO and LUMO energies) and molecular structure of pyrimethamine (Pyr) were investigated by density functional theory (DFT) method associated with three levels of theory viz., B3LYP, MN15 and wB97XD with 6-311++G(d,p) and def2TZVPP as basis sets, respectively in the Gaussian 16 programs. The 1H and 13C NMR chemical shifts were calculated with a gauge-independent atomic orbital (GIAO) approach by also applying the same levels of theory and basis sets. All experimental results were compared with theoretical data. Although the results revealed high degrees of correlation between the theoretical and experimental values for spectroscopic properties using the three methods. Furthermore, the atomic and natural charges, energy band gap and chemical reactivity were determined, while the frontier molecular orbital (FMO) and molecular electrostatic potential (MEP) surfaces were plotted to explain the reactive nature of the title molecule.