DFT investigation on the carbonate radical formation in the system containing carbon dioxide and hydroxyl free...

A density functional theory (DFT) study was performed to explore the reaction mechanisms of CO₂-containing systems(CO₂,HCO₃^-,H₂CO₃) with hydroxyl radical using UM06-2X/aug-cc-pVTZ for geometry optimization and UCCSD(T)-F12/cc-pVDZ-F12 for electronic energies, the effect of solvation is considered using the implicit solvent model with two explicit water molecule. The final transformation of hydroxyl radical into carbonate anion radical by reaction with CO₂-containing system was studied. The energy barriers, Gibbs free energy, reaction enthalpy changes and reaction rate constants for reaction of CO₂-containing systems by hydroxyl radical are calculated. The results show that the reaction of hydroxyl with HCO₃^- has the lowest energy barrier (5.6 kcal/mol) and maximum reaction rate constant (7.60 × 10⁷dm³mol^-1s^-1). However, the reaction of hydroxyl radical with CO₂ has the highest energy barrier (20.3 kcal/mol) and the minimum reaction rate constant (8.88 × 10^-10 dm³mol^-1s^-1) respectively. The equilibrium constant and rate ratio were calculated by the concentration ratio and free energy of the two conformations of H₂CO₃ in aqueous solution. The results show that the cc conformation of carbonate has a little higher reaction energy barrier and a higher reaction rate than ct conformation. ∙HCO₃ generated by the reaction of CO₂ and H₂CO₃ with hydroxyl radical is further dissociated in aqueous solution to form carbonate radical anion. The calculated free energy of the dissociation process is -2.5 kcal/mol and pKa is -1.9, indicating that HCO₃^- is a strong acid. Therefore, hydroxyl radicals are spontaneously converted to carbonate radical anions in CO₂-containing systems.