Conformational stability, vibrational spectral studies, HOMO–LUMO and NBO analyses of 2-hydroxy-4-methyl-3-nitropyridine and 2-hydroxy-4-methyl-5-nitropyridine based on density functional theory

In this study, the conformational stability of 2-hydroxy-4-methyl-3-nitropyridine and 2-hydroxy-4-methyl-5-nitropyridine has been analyzed, and vibrational analyses of the same have been carried out by the application of B3LYP/6-311++G** basis set based on scaled quantum mechanical (SQM) method. Vibrational analyses of the most stable conformer of 2-hydroxy-4-methyl-3-nitropyridine and 2-hydroxy-4-methyl-5-nitropyridine are conducted by means of infrared absorption and Raman spectroscopy in combination with theoretical (DFT) simulations. The observed and simulated spectra agreed well. Molecular stability and bond strength were investigated by applying the natural bond orbital analysis (NBO). The calculated HOMO and LUMO energies show that charge transfer occurs in the molecules. Information about the size, shape, charge density distribution, and site of chemical reactivity of the molecules has been obtained by mapping electron density isosurface with electrostatic potential (ESP).