Quantum Tunnelling Effect in the Cis-Trans Isomerization of Uranyl Tetra Hydroxide

The role of Quantum Tunnelling (QT) in the proton transfer kinetics of the Uranyl Tetra Hydroxide (UTH, [UO₂(OH)₄]^2-) cis to trans isomerization was computationally studied under three possible reaction pathways. The first involved a direct proton transfer between the hydroxide ligand to the oxo atom. In the other two one or two water molecules were added to the second sphere. The first H₂O, bound by hydrogen bonds to the ligands, acts as a bridge enabling a proton shuttling, a concerted hoping of a proton from the hydroxide to the oxo atom similar to the Grotthuss mechanism. In the third pathway the second water molecule does not participate in the H-transfer chain, but works as an anchor for the first water, limiting its movement and therefore enhancing the QT. Since experimentally the reaction occurs in water, the first two pathways (no water or one H₂O) serve only as models of the gas phase behaviour, while the third pathway will always be thermodynamically and kinetically preferred. The effects were investigated in gas phase as well as in a continuum aqueous model, including the H/D Kinetic Isotope Effect (KIE). The results indicate that at very low temperatures QT is the only mechanism that permits the reaction kinetics, consistent with the large computed KIE. At higher temperatures, thermally-activated tunnelling competes with the classical crossing over the potential barrier.