Structural elucidation of Levofloxacin and Ciprofloxacin using density functional theory and Raman spectroscopy with inexpensive lab-built setup

In the present work, we have investigated the structures of fluoroquinolones Levofloxacin and Ciprofloxacin using Raman spectroscopy and density functional theory calculations. The Raman spectra of Levofloxacin and Ciprofloxacin were recorded with lab-built inexpensive Raman spectroscopy setup that uses 638 nm laser diode. Raman spectra in the fingerprint spectral region (900–1800 cm⁻¹) were investigated for both the molecules. The density functional theory (DFT) utilizing B3LYP functional with 6-31+G(d,p) basis set was used to investigate the minimum energy structures of two molecules along with the calculation of Raman spectrum. Molecular complexes of Levofloxacin and Ciprofloxacin with one and two water molecules were also studied to see the effect of H-bonding on Raman spectra. It was found that the conformation and orientation of piperazine ring along with the orientation of hydroxyl group plays a vital role in determination of the minimum energy structure. The calculated Raman spectra of bare molecules and their complexes with water molecules were compared to the experimental Raman spectra. The calculated Raman spectrum of minimum energy structure was found to be in good agreement with the experimental spectrum. Further it was observed that the experimental Raman spectrum is better explained when H-bonding is considered, which could be due to the water molecule or the dimer formation of the molecules under investigation.