The synthesis, spectroscopic characterization, DFT/TD-DFT/PCM calculations of the molecular structure and NBO of the novel charge-transfer complexes of pyrazine Schiff base derivatives with aromatic nitro compounds

Abstract

The novel charge-transfer (CT) solid complexes of pyrazine Schiff bases, derived from 2-aminopyrazine and substituted benzaldehydes (N-benzylidenepyrazin-2-amine, (NBPA)) and N-(((4-dimethylamino)benzylidene)pyrazin-2-amine) (NDMABPA) with some aromatic nitro compounds have been synthesized and characterized experimentally using ultraviolet-visible (UV-Vis) absorption, infrared spectra and proton nuclear magnetic resonance (¹HNMR) spectroscopy. Complexes were formed in a molar ratio of 1 : 1 with good indications for the existence of charge-transfer in its molecular structure. Theoretical studies were done on donors and acceptors, elucidating their structures and active sites where the charge-transfer occurs. The experimental work was done in ethanol. Solution characterizations included the determination of the molecular structure of formed CT complexes, verifying the 1 : 1 (donor:acceptor) ratio in ethanol. The quantum mechanical calculations of geometries and energies were attained using the density functional theory with Becke's three parameter exchange functional method. The Lee–Yang–Parr correlation functional approach (B3LYP/DFT) combined with the 6-31G(d,p) basis set has been consecutively carried out in solution using ethanol as a solvent to compliment measured results, and to justify CT within donors and acceptors. The optimized energy, complexation energy, geometrical parameters, natural atomic charges, as well as the 3D-plots of the molecular electrostatic potential maps (MEP) were computed and elucidated. They agreed with the experimental results, wherein complex stabilities are attributed to the occurence of charge-transfer. The electronic spectra were computed and executed using time dependent-density functional theory (TD-DFT) via the addition of polarizable continuum solvation method PCM, PCM-TD-DFT. The allowed singlet transitions are positioned, and their highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) involvement is represented. The descriptions of frontier HOMO and LUMO molecular orbitals attributed to the first four singlet transitions, are shown. For all formed solid complexes, the main relationship between donor and acceptor molecules is through the π–π* interaction. A secondary n–π* transition was noticed in some complexes. The vibrational wavenumbers were also determined using B3LYP/6-31G(d,p), and the results match with the experiment. The small energy gap between HOMO and LUMO energies indicate that CT occurs within donors and acceptors. The hyperconjugative interactions, molecular stability, bond strength and intramolecular CT have been investigated applying natural bond orbital (NBO) analysis. The mean polarizability, total static dipole moment, anisotropy of polarizability, and mean first-order hyperpolarizability have also been attained. The obtained values show that CT complexes are accomplished candidates to non-linear optical (NLO) materials.