Electronic spectral studies and DFT computational analysis of hydrogen bonded charge transfer complexes between chloranilic acid and 2,5-dihydroxy-p-benzoquinone with 2-amino-4-methylbenzothiazole in methanol

Abstract

Two new hydrogen bonded charge transfer complexes between two electron acceptors (hydrogen bond donors), chloranilic acid (CA) and 2,5-dihydroxybenzoquinone (DHBQ) with an electron donor (hydrogen bond acceptor) 2-amino-4-methylbenzothiazole (AMBTZ) have been experimentally and theoretically studied in methanol. The measured electronic spectra of these two complexes, CA-AMBTZ and DHBQ-AMBTZ, were characterized by new absorption bands appearing at λmax of 529 and 489nm, respectively. The molecular compositions of the formed complexes were determined using Job and photometric titration methods to be 1:1. Furthermore, the formation constants and some spectroscopic physical parameters were determined. These revealed the formation of stable complexes and that CA-AMBTZ is more stable than DHBQ-AMBTZ. Using the B3LYP/6-31++ G(d,p) method, the geometries of CA, DHBQ, AMBTZ, CA-AMBTZ and DHBQ-AMBTZ were calculated in the gas phase and in methanol. In AMBTZ, the S and N (ring) atoms act as sites that can accept a proton from CA or DHBQ. In both complexes, the proton from CA or DHBQ was preferentially transferred to the ring N-atom of AMBTZ. Geometrical and reactivity parameters, molecular electrostatic potential (MEP) maps and natural atomic charges were computed and analyzed. The time-dependent density functional theory (TD-DFT) method has been applied to explain the origin of the electronic spectra. Natural bond orbital (NBO) analysis at the same level of the theory has been applied to shed more light on the origin of the charge transfer in the investigated complexes. The theoretical results obtained in this study proved highly consistent with the experimental ones.