Computational Prediction of All Lanthanide Aqua Ion Acidity Constants

The protonation state of lanthanide-ligand complexes, or lanthanide-containing porous materials, with many Brønsted acid sites can change due to proton loss/gain reactions with water or other heteroatom-containing compounds. Consequently, variations in the protonation state of lanthanide-containing species affect their molecular structure and desired properties. Lanthanide(III) aqua ions undergo hydrolysis and form hydroxides; they are the best characterized lanthanide-containing species with multiple Brønsted acid sites. We employed constrained ab initio molecular dynamics simulations and electronic structure calculations to determine all acidity constants of the lanthanide(III) aqua ions solely from computation. The first, second, and third acidity constants of lanthanide(III) aqua ions were predicted, on average, within 1.2, 2.5, and 4.7 absolute pK_a units from experiment, respectively. A table includes our predicted pK_a values alongside most experimentally measured pK_a values known to date. The approach presented is particularly suitable to determine the Brønsted acidity of lanthanide-containing systems with multiple acidic sites, including those whose measured acidity constants cannot be linked to specific acid sites.