Quantum tunnelling effect on the trans-trans isomerization of uranyl tetra hydroxide in alkaline aqueous media

Following the analysis of the cis-trans isomerization reaction of uranyl tetra hydroxide (UTH, Dalton Trans. 2024, 53, 16271) we studied the impact of the quantum tunnelling (QT) effect on the kinetics of the complete trans-trans isomerization. This involves two intra-molecular proton transfers between the hydroxide ligands and the oxo atoms, going through the cis complex intermediate. Six reaction pathways were examined, with one being a direct proton transfer and the others involving a water molecule as a bridge for the proton transfer. As seen before, herein it was also observed that the presence of water molecules vastly reduces the activation barrier. QT was found to accelerate the reaction in all pathways especially at low temperatures, but still evident at room temperature (RT). Combining thermodynamic and kinetic considerations, the fifth reaction pathway (“UTH⋯4H₂O_anc”, that is UTH with four water molecules bound by hydrogen bonds to the complex) has been found to be the most reactive. In the most reactive pathway, the QT included rate constant (k_QT) was two times higher than that predicted by semi-classical methods (k_SC) at RT, with a lifetime of about a half year. The most significant H/D KIE occurs in the direct proton transfer reactions, which are nevertheless too slow to measure. In faster reactions with water-assisted proton transfer the KIE is almost 2000 at cryogenic temperatures, but almost negligible at RT.