Synthesis, spectroscopic characterization and DFT/TD-DFT computations of a novel charge transfer complex via hydrogen bonding between 3-amino-1,5-dimethylpyrazole with chloranilic acid in different solvents

Abstract

A novel charge transfer complex including hydrogen bonding between the e-donor (H-acceptor) 3-amino-1,5-dimethylpyrazole (3-ADMP) with the e-acceptor (H-donor) chloranilic acid (CLA) has been synthesized and characterized experimentally and theoretically. The experimental work was carried out in different solvents including chloroform (CHL), ethanol (EtOH), methanol (MeOH) and acetonitrile (AN). The solution characterizations included the determining of the molecular composition of the formed CT complex where it recorded 1:1 (donor:acceptor) in all solvent. Also, estimating its formation constant by Benesie-Hildebrand equation where it recorded high values in all solvents, suggesting its high stability. Thermodynamic and some spectroscopic physical parameters were introduced and interpreted where they confirmed the presence of charge transfer besides hydrogen bonding in the formed complex. The solid complex was prepared and characterized by elemental analysis, infrared and NMR (1H and 13C) spectroscopies, the complex was formed in 1:1 ratio, with good evidences for existing both charge transfer and hydrogen bonding in its molecular structure. Density functional theory B3LYP-DFT at the basis set 6-31G (d,p) has been running out in gas phase and solution using chloroform and acetonitrile as solvents to compliment the measured results. The complex optimized structure included two hydrogen bonds between OH and CO of chloranilic acid with the adjacent ring nitrogen and amino group of pyrazole ring. The optimization energy, complexation energy, geometrical parameters, Mullikan atomic charges as well as molecular electrostatic potential maps (MEP) were calculated and interpreted; they consisted with the experimental results where the complex stability is attributed to the presence of H-bond beside e-transfer. The electronic spectra were computed using TD-DFT through adding polarizable continuum solvation method PCM, PCM-TD-DFT to compare with the experimental results. It has been found that, the measured and computed λmax are coming close to each other. The allowed singlet transitions are located and their HOMO and LUMO contributions are presented. The pictures of the frontier HOMO and LUMO molecular orbitals, participating in the first six singlet transitions were presented. A good consistency between experimental and DFT computations has been found.