The relative stability constants of Tb(III) complexes exhibiting binding to a series of 4-substituted analogues of dipicolinic acid (2,6-pyridinedicarboxylic acid) (DPA) were calculated using density functional theory (DFT) with the standard thermodynamic cycle (in vacuo complexation followed by solvation/desolvation of the reference state and of the resulting complexes). DFT (Stuttgart small-core relativistic effective core potential and basis set for Tb(III) with B3LYP/6-311++g**/COSMO, M06/6-311++g**/COSMO, B3LYP/6-311++g**/SMD, and M06/6-311++g**/SMD levels of theories) calculations showed that the strengths of the stability constants were modified by the substituents in the following (decreasing) order: -NH2 &gt; -OH ~ -CH2OH &gt; -imidazole ~ -Cl ~ -Br ~ -H &gt; -F &gt; -I, with the differences among them falling within one to two log units except for -NH2. The calculated order was consistent with available experimental data from the literature comparing quantum yields of 4-substituted DPA and Tb(III) complexes. Through molecular orbital analysis we observed that the -NH2, -OH, and halide substituents can donate resonance electrons to the pyridine ring of DPA while inductively withdrawing electrons with different strengths, thus resulting in the different binding strengths of the 4-substituted DPAs to the Tb ions. On the other hand, the -CH2OH and -imidazole substituents displayed extended resonance between the pyridine ring and the -CH2OH or -imidazole moieties distinct from that of the -NH2, -OH, and halide substituents’ resonance structures. We believe these observations may possess utility not only in the ongoing development of luminescent probes for bioanalytical studies but also for more recent cross-industrial efforts to enhance reservoir surveillance capabilities using chemical tracers within the oil & gas sector.

Predicting stability constants for terbium(III) complexes with 4-substituted dipicolinic acid analogues using density functional theory

2019 Fall National Meeting

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