Structural characterization of molecular complexes formed by trimethoprim and cimitidine with 2,3,5,6-tetrachloro-1,4-benzoquinone

Spectroscopic and spectrofluorimetric techniques have been employed to investigate the structure of the charge transfer (CT) complexes of Trimethoprim (TMP) and Cimitidine (CTD) drugs with 2,3,5,6-tetrachloro-1,4-benzoquinone (p-chloranil, p-CHL). The stoichiometry of the complexes was found to be 1:2 for TMP-p-CHL system and 1:1 for CTD-p-CHL system. The thermodynamic results indicated that the formation of molecular complex between the donors and the acceptor is spontaneous and endothermic. The results of electronic spectral studies indicated that the formation constant for CTD-p-CHL system is found to be higher than that for TMP-p-CHL system. The observation is well supported by the results of fluorescence quenching studies and the association constants calculated for CTD-p-CHL system is 36.2 × 103 mol L−1 and that for TMP-p-CHL system is 2.6 × 103 mol L−1. The kinetic results, in both the cases, indicated that the interaction is first order each with respect to the concentration of the donor and the acceptor. The physico-chemical parameters viz. oscillator strength, dipole moment, ionization potential and dissociation energy of the complexes were also determined and discussed. Structural characterization of the complexes were done using FT-IR and 1H NMR spectral techniques and the results indicated that, in TMP, the free NH2 group while in CTD the pyrazole NH moiety involves in complexation with the acceptor, p-CHL.