Spectroscopic studies and molecular orbital calculations of charge transfer complexation between 3,5-dimethylpyrazole with DDQ in acetonitrile

• Spectroscopic studies on CT reaction between DMP and DDQ have been presented.
• The reaction proceeded through 1:2 M ratio.
• Stability constant and molar absorptivity were estimated.
• Molecular orbital calculations were carried out using GAMESS computations.
• A good consistency between experimental and theoretical results was obtained.

Charge transfer (CT) interaction between 3,5-dimethylpyrazole (DMP) with the π-acceptor 2,3-dichloro-5,6-dicyano-p-benzoquinon (DDQ) has been investigated spectrophotometrically in acetonitrile (AN). Simultaneous reddish brown color has been observed upon mixing donor with acceptor solutions attributing to CT complex formation. The electronic spectra of the formed complex exhibited multi-charge transfer bands at 429, 447, 506, 542 and 589 nm, respectively. Job’s method of continuous variations and spectrophotometric titration methods confirmed the formation of the studied complex in 1:2 ratio between DMP and DDQ. Benesi–Hildebrand equation has been applied to calculate the stability constant of the formed complex where it recorded high value supporting formation of stable complex. Molecular orbital calculations using MM2 method and GAMESS (General Atomic and Molecular Electronic Structure System) interface computations as a package of ChemBio3D Ultra12 software were carried out for more analysis of the formed complex in the gas phase. The computational analysis included energy minimisation, stabilisation energy, molecular geometry, Mullikan charges, molecular electrostatic potential (MEP) surfaces of reactants and complex as well as characterization of the higher occupied molecular orbitals (HOMO) and lower unoccupied molecular orbitals (LUMO) surfaces of the complex. A good consistency between experimental and theoretical results has been recorded.

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