Theoretical near UV and VIS electronic spectra for the Zn^II–anhydrotetracycline complex

Abstract

The UV–VIS electronic spectra for the free ligand and complexed forms of the Zn^II–AHTC system have been analyzed using the configuration interaction (CI) procedure as implemented in the ZINDO program. The main transitions in the UV and VIS region are assigned and compared with the experimental data available. The electronic spectrum for the free ligand, the tautomeric form ionized at O¹¹ [LH^–(O¹¹)], is very close to that observed experimentally, with the calculated absorption band centered at 429 nm and the experimental one at 428 nm. This absorption is attributed to a π→π* transition of the BCD ring system. Comparing the visible spectra for the complexed forms with that obtained for the free ligand, a blue shift is observed for all structures analyzed, except for the complex II-B, where a red shift from 429 nm (free ligand) to 436 nm (complexed form) was calculated. Experimentally, a bathochromic shift from 428 nm to 440 nm was observed with the [Zn^II]/[AHTC] ratio ranging from 0 to 3. These values are very close to those calculated for the complex II-B, in which the AHTC acts as a tridentate ligand. The possibility of the formation of the binuclear complex (M₂L) was discarded, once it was observed that the second metal complexation shifts the VIS band to lower wavelength values. This effect is not observed in the experimental spectrum at high Zn^II concentration.

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