This work presents a density functional theory study of Ofloxacin (OFL) as a medicine with antibiotic activity. The Conformational behaviour, gas-phase acidity and metal ion affinity (MIA) analysis of the Ofloxacin with some selected cations from alkali (Li⁺, Na⁺, and K⁺), alkaline-earth (Be²⁺, Ca²⁺ and Mg²⁺) and transition (Zn²⁺, Cu²⁺, Ni²⁺, Co²⁺ and Fe²⁺) metals were explored. All structural optimizations were performed at the M06–2X level of theory with 6-311++G(d,p) using the basis set. The calculations demonstrated the following MIA order: OFL-Be²⁺ > OFL-Ni²⁺ > OFL-Co²⁺ > OFL-Cu²⁺ > OFL-Fe²⁺ > OFL-Zn²⁺ > OFL-Mg²⁺ > OFL-Ca²⁺ > OFL-Li⁺ > OFL-Na⁺ > OFL-K+. The results also showed that the MIA values are decrease in aqueous solution because of solvation effects. Natural bond orbital theory (NBO) and quantum theory of atoms in molecules (QTAIM) were used to probe the charge transfer process and the nature of interactions in the formed complexes. The results of the NBO analysis revealed that the charge transfer occurs from OFL and Mn+ with the metal cations acting as charge acceptors and the amount of charge transfer is in agreement with MIA. The QTAIM analysis shows that the (M–O) coordination interactions are quasi-covalent in complexes formed between OFL and alkali or alkaline-earth metal cations, whereas they are covalent in complexes formed with the transition metal cations and Be²⁺. The results confirm that the strengths of the interactions are in good agreement with electron densities at the bond critical points (BCP).

A molecular mechanism of the [3+2] cycloaddition has been explored using various DFT theoretical levels. It was found that the reaction proceeds via transition states with different synchronicity, but no intervention of the theoretical possible zwitterionic intermediates. Additionally, regioselectivity of the cycloaddition process has been analysed using vibrational analysis of localised TSs

With the focus on calculating the equilibrium constant (Ka) of arsonic acids (RAsO(OH)2) in aqueous media, the behavior of both neutral and negatively charged species of some arsonic acids have been considered through the isodesmic reaction scheme with polarized continuum model of solvation. The relative pKa values of a number of arsonic acids were calculated using density functional theory (DFT), with methylarsonic acid (CH3AsO(OH)2) used as a reference molecule. Various basis sets such as (6-31G(d), 6-31+G(d), 6-311++G(d,p) and 6-311++G(2d,2p)) in conjunction with B3LYP computational method were examined. Finally the latter one was found to give better results. The results of applied B3LYP 6-311++G(2d,2p) method for pKa values of 25 arsonic acids in aqueous media showed good agreement with corresponding experimental pKa values. In this level of theory the average error was less than 0.3 pKa units. The type of substituent affected the acid strength, with electron withdrawing substituents lowering the pKa and electron releasing groups increasing it.