A combined UV–vis spectroscopy and DFT study of uranyl–nicotinate complexation in aqueous solution

Uranyl complexation with nitrogen-containing model ligands provides a useful molecular basis for predicting uranium(VI) interactions with chemically heterogeneous natural organic matter. Yet, the reaction thermodynamics of aqueous uranyl–nicotinate system, a representative model scheme, remains poorly constrained as its complexation stoichiometry has not been consistently established. Here, the speciation of this system was elucidated by integrating UV–vis spectroscopy with density functional theory (DFT) calculations. Spectrophotometric titrations showed an enhancement of the uranyl absorption upon complexation, and statistical analysis of the absorbance dataset confirmed the formation of a 1:1 complex while recovering its molar absorptivity spectrum, with conditional stability constants of log β = 3.93 ± 0.02 and 3.83 ± 0.02 in 0.1 and 0.5 mol∙L −1 NaClO 4 media, respectively. Since UV–vis spectroscopy alone cannot establish the protonation state of pyridine group, complementary DFT calculations were carried out with the B3LYP-D4, PBE0-D4, and ωB97X-D4 functionals in a cluster–continuum solvation framework. Evaluating the Gibbs free energy of an isodesmic proton-exchange reaction referenced to the acidity of free nicotinate (pK a ≈ 4.9 at I = 0) gave convergent pK a values of 1.7–2.1 for the pyridinium site within the complex. This acidification stems from inductive electron withdrawal by uranyl, favoring unprotonated UO 2 Nic + species under the examined conditions and helping reconcile inconsistent proton stoichiometric assignments. The resulting reaction thermodynamic data could, in turn, serve as input parameters for future discrete-fragment modeling of uranyl–natural organic matter interactions, where such remote uranyl coordination-induced acid–base shifts may further modulate the charge-controlled environmental behavior.