Noushi Zaidi¹, M Amin Mir² , Sook Keng Chang³, Nouara Abdelli⁴, K Andrews⁵

Abstract

Purpose: To study the structural, vibrational and electronic properties of ampicillin using a computational approach supported by measurements. Methods: The structural optimization and vibrational frequencies for density functional theory (DFT; B3LYP/6-311++G (d,p)) calculations were utilized. Calculated frequencies were compared with measured FT-IR and FT-Raman vibrational spectra, while molecular electrostatic potential (MEP) mapping and highest occupied molecular orbit-lowest unoccupied molecular orbit (HOMO-LUMO) modeling were used to characterize the charge distribution in the compound to evaluate chemical reactivity. Finally, quantum chemical modeling was used to characterize the interaction of the ampicillin molecule on silver colloids and predictions from the DFT calculations were then verified using surface-enhanced Raman scattering (SERS) from a hydroxylamine-reduced silver colloid at 532 nm. Results: The calculated vibrational wavenumbers corresponded closely with FT-IR and FT-Raman data, confirming a valid computational model. The MEP clearly delineated electropositive regions near -NH? group and water molecules, which may serve as sites of reactivity in monomer and dimer ampicillin forms. The HOMO-LUMO models clarified the electronic transitional nature of the compound while DFT modeling on ampicillin adsorption showed good agreement with an effective interaction with silver colloids using SERS. Conclusion: This study demonstrates that DFT and spectroscopic studies accurately characterize the structural, vibrational and electronic properties of ampicillin, as well as an interaction with silver nanoparticles that supports its use as a diagnostic agent in surface-enhanced spectroscopy.