Ciprofloxacin (Zwitterion, Chloride, and Sodium Forms): Experimental and DFT Characterization by Vibrational and Solid-State NMR Spectroscopies

Ciprofloxacin (Cipro), a widely used fluoroquinolone antibiotic, exists in multiple protonation states, which influence its structural and spectroscopic properties. Despite its pharmaceutical relevance and concerns regarding its accumulation in the environment, a comprehensive characterization of its zwitterionic, cationic, and anionic solid forms remains limited, particularly in terms of their vibrational and nuclear magnetic resonance (NMR) spectral assignments. The focus of this study was to identify spectral signatures that differentiate Cipro and its cationic form (as a chloride salt) and anionic form (as a sodium salt). All samples were characterized in solid-state using X-ray diffraction, thermogravimetric analysis coupled with mass spectrometry, infrared and Raman spectroscopies, and cross-polarized magic-angle spinning (CP-MAS) solid-state NMR, with support from density functional theory (DFT) calculations. Both salts were synthesized in this study. The cipro sodium was isolated as a monohydrate salt, a previously unreported phase. Six spectral regions were identified to distinguish the Cipro zwitterion from its cationic and anionic forms by using vibrational spectroscopy. Both experimental ¹³C CP-MAS solid-state NMR and theoretical analyses revealed pronounced chemical shifts induced by protonation and counterion interactions, which also differentiate the three forms. The analysis presented here provides clear fingerprints of the three Cipro forms, which can be used to support reference spectroscopic data, with direct implications in pharmaceutical formulations as well as for environmental studies.