Exploring the ground and low-lying excited states of gatifloxacin

In this work, we present an investigation on the photoinduced degradation of the micropollutant gatifloxacin (GFX). Ground state properties (such as bond lengths, hydrogen-bond interactions, and relative energies) for one protonated and two deprotonated forms of the compound were determined through the use of the density functional theory (DFT), while time-dependent DFT (TD-DFT) was used for probing their excited state parameters (vertical excitation energies, oscillator strengths — OS, and structures). The CAM-B3LYP, M06-2X, and LC-$\omega$PBE exchange–correlation functionals were employed along with the 6-311+G(d,p), 6-311++G(d,p), def2-TZVP, and aug-cc-pVTZ basis sets. Solvent effects were incorporated with the polarizable continuum model. Considering the parent GFX molecule as instance, all the results for its ground state computed using the CAM-B3LYP exchange–correlation functional with a given basis set were found to be in excellent agreement to those corresponding determined using the M06-2X along with any of the basis sets used; excellent agreement was also observed in the case of the excited state properties. Three states (S₁, S₄, and S₅) were probed to be likely accessible among the five lowest-lying excited singlets, with S₁ being determined at 4.22 eV with OS = 0.1159, S₄ at 4.60 eV with OS = 0.7243, and S₅ at 4.94 eV with OS = 0.1672, at the CAM-B3LYP/6-311+G(d,p) level of theory in water, for example. These results were used for establishing a correlation with the findings presented in previous (recent) experimental work involving the photoinduced degradation of the GFX molecule. In this sense, S₁ was assigned as the excited state connected to the degradation path of the GFX molecule when irradiated with the UVA light while the existence of two additional excited singlet states with non-zero OS (S₄ and S₅, with S₄ having a considerably large OS) was associated to the faster degradation provided by the more energetic UVC when compared to the UVA radiation. In addition, the excited state structures suggested GFX as undergoing photodecomposition (chemical reactions occurring in the excited state) rather than direct photolysis when irradiated with UVA and UVC light.