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We provide a review on the status of theoretical modeling of tautomerization in free-base porphyrin. We focus our discussion on several aspects, namely: (1) potential surfaces for both the stepwise and concerted mechanisms calculated at different levels of theory, (2) solvent effects and (3) kinetics. The importance of quantum mechanical tunneling in this double hydrogen atoms transfer process is analyzed.

The results of use of chemical kinetics receptions, approaches and methods for the study of porphyrins and their metal complexes reactivity are discussed on an example of oxidation, acid-basic, and catalytic reactions of rhodium, palladium, and rhenium complexes of porphyrin in liquid solutions. The peculiarity of the porphyrin reaction rates is analyzed in a brief context of general provisions of the chemical kinetics. The opportunity to use the quasistationarity principle at the definition of the kinetic equation of the reactions with participation of metal porphyrins is shown. The transition from the process kinetic description to consideration of its mechanism is explored.

Sorption of meso-tetrakis(4-sulfonatophenyl)porphyrin (H₂tpps) onto chitosan has been investigated in aqueous medium. Kinetic and isotherm studies were carried out by considering the effects of various parameters, such as pH, initial concentration of H₂tpps solution, and temperature. The kinetic data obtained from different batch experiments were analyzed using pseudo first-, second-order, intraparticle, and film diffusion kinetic models. The equilibrium sorption data was analyzed by using Tempkin, Langmuir and Freundlich models. The best results were achieved with the pseudo second-order kinetic, Langmuir and Freundlich isotherm models. The intraparticle diffusion and film diffusion are the rate limiting steps. The amount of sorbate adsorbed at equilibrium (q_e) increased with increasing the initial concentration of H₂tpps solution, showing maximum sorption capacity of 445.21 μmol.g^-1. The activation energy (E_a) of sorption kinetics was found to be 19.47 kJ.mol^-1. Thermodynamic parameters such as change in free energy (ΔG), enthalpy (ΔH) and entropy (ΔS) were evaluated by applying the Van't Hoff equation. Thermodynamic activation parameters such as change in enthalpy of activation (ΔH^‡), entropy of activation (ΔS^‡), and free energy of activation (ΔG^‡) were also calculated. The thermodynamics of H₂tpps sorption onto chitosan in aqueous medium indicates its spontaneous and endothermic nature.