Theoretical, Spectroscopic and biological studies of some pyridine thiol derivatives

Research Abstract

The electronic structure of 6-phenyl-2-thioxo-1,2-dihydropyridine-3-carbonitrile (2-mercapto-6-phenylpyridine-3-carbonitrile) and its some derivatives have been studied both theoretically and experimentally. The choice of these compounds was motivated by their biological importance and relevance. The corresponding proposed structures of all studied derivatives in this work were confirmed by FT-IR spectrophotometer, 1HNMR spectra and XRD patterns. The Density Functional Theory (DFT) has been used to investigate the effect of substituents with different strengths of all studied compounds on the geometry structures, natural bond orbital (NBO) properties, electrostatic potential (ESP) and the global properties such as (the chemical hardness (η), global softness (S), and electronegativity (χ) by analysis of the charge distribution and extent of charge transfer in the molecule in the gas phase. Non-linear optical properties (NLO) such as (static dipole moment (μ), polarizability (α), anisotropy polarizability (Δα), first order hyperpolarizability (β) and mean second order hyperpolarizability (γ)) also, were computed by DFT in the gas and solvent (benzene and ethanol) phases. The effect of the different substitutions and solvent polarity on the NLO properties was also investigated to show their ability to be used as NLO compounds. We find that all studied compounds exhibited higher NLO properties compared to urea (reference materials) in the gas phase and showed higher NLO properties in ethanol than in the benzene phase. The effect of solvent polarity on the electronic absorption spectra of the studied molecules was measured experimentally and calculated theoretically at the Time-Dependent Density Functional Theory (TD-DFT) level of theory. The interactions of different solvents (ethanol and benzene) with studied compounds were also studied by molecular dynamics (MD) simulations. The radial distribution functions (RDFs) and coordination numbers (CNs) of all studied compounds in the different solvent are computed. The diffusion coefficient (D) also was calculated to investigate the influence of substitutions on the mobility of the studied compounds in the surrounding solvent. Also, the cytotoxicity of these compounds expressed EC50 (the effective concentration of a compound that causes about a 50% reduction in bacteria viability) toward the four human pathogens bacteria, A. hydrophila, E. Coli, L. monocytogenes, and S. aureus, was computed. Excitation energy transfer along molecular wires depends on the orientation of the transition dipole moments of the neighboring chromophores. This transition dipole moment is changed upon the proton transfer in the electronic ground state. To this end, a quantum chemical characterization of 6-phenyl-2-thioxo-1,2-dihydropyridine-3-carbonitrile compound and its derivatives is performed using Density Functional Theory (DFT). To study the effect of the substitutions in the parent compound on the energy barrier between thiol-thione intraconversion as well as on the transition dipole moment. The intramolecular proton transfer (IMPT) process based on the intrinsic reaction coordinate IRC calculations was energetically favorable in the excited state (S1) than the ground state (S0) for all studied compounds. The effect of different substituents on the transition dipole moment orientation between thiol and thione was calculated. It was found that the electron withdrawing groups leads to a high change in the transition dipole moment orientation through thiol/thione intra-conversion. The excited state energy transfer can be controlled by 20 % - 100 % through the intramolecular proton transfer (IMPT).

Research Keywords

Theoretical, Spectroscopic and biological studies of some pyridine thiol derivatives