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Interactions Between Hydrated Cerium(III) Cations and Carboxylates in an Aqueous Solution: Anomalously Strong Complex Formation with Diglycolate, Suggesting a Chelate Effect.

ABSTRACT: Interactions between hydrated Ce³⁺ and various carboxylates are of fundamental interest. Anomalously strong interactions with Ce³⁺ occur when diglycolic acid (DGA) is added into a Ce³⁺ aqueous solution, unlike various other carboxylic acids. Herein, the complex-formation constants of Ce³⁺ with these acids are evaluated via absorption and emission spectra. Hydrated Ce³⁺ emits fluorescence with unity quantum yield; however, addition of various carboxylates statically quenches the fluorescence when Ce³⁺-carboxylate complexes form because the fluorescence lifetime is constant irrespective of the carboxylate concentration. In the observed static quenching, the complex-formation constants obtained from the absorption and emission spectra (K _abs and K _em) agree well. The binding of Ce³⁺ by the conjugate Lewis bases, i.e., carboxylates, is approximately inversely proportional to the pH. Adding DGA into the system also statically quenches the fluorescence, but far more efficiently, even in a much weaker solution. We rigorously deduce K _abs and K _em of Ce³⁺ with DGA without any approximation using comparable concentrations. Careful fittings provide equivalent K _em and K _abs values, and by varying the pH and ionic strength, we confirm that this equivalence is an inherent property of the Ce³⁺-DGA system. The Lewis acid-base theory cannot explain why DGA binds to Ce³⁺ ?1000 times more strongly than the other carboxylates. This anomalously strong binding may be due to a chelate effect caused by the DGA's central oxygen atom, which forms a five-membered ring with the conjugate Lewis bases of DGA; double chelate rings can also form, while bis-deprotonated DGA binds to Ce³⁺, facilitated by the central oxygen. Therefore, DGA enables efficient quenching through the chelate effect when it binds to Ce³⁺.

SUBMITTER: Oh-E M

PROVIDER: S-EPMC7745448 | biostudies-literature | 2020 Dec

REPOSITORIES: biostudies-literature

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Interactions Between Hydrated Cerium(III) Cations and Carboxylates in an Aqueous Solution: Anomalously Strong Complex Formation with Diglycolate, Suggesting a Chelate Effect.

Oh-E Masahito M Nagasawa Akira A

ACS omega 20201201 49

Interactions between hydrated Ce3+ and various carboxylates are of fundamental interest. Anomalously strong interactions with Ce3+ occur when diglycolic acid (DGA) is added into a Ce3+ aqueous solution, unlike various other carboxylic acids. Herein, the complex-formation constants of Ce3+ with these acids are evaluated via absorption and emission spectra. Hydrated Ce3+ emits fluorescence with unity quantum yield; however, addition of various ...[more]

PMID: 33344842

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Project description:Theoretical ab initio quantum mechanical charge field molecular dynamics (QMCF MD) has been applied in conjunction with experimental large angle X-ray scattering (LAXS) and EXAFS measurements to study structure and dynamics of the hydrated oxo chloro anions chlorite, ClO2(-), chlorate, ClO3(-), and perchlorate, ClO4(-). In addition, the structures of the hydrated hypochlorite, ClO(-), bromate, BrO3(-), iodate, IO3(-) and metaperiodate, IO4(-), ions have been determined in aqueous solution by means of LAXS. The structures of the bromate, metaperiodate, and orthoperiodate, H2IO6(3-), ions have been determined by EXAFS as solid sodium salts and in aqueous solution as well. The results show clearly that the only form of periodate present in aqueous solution is metaperiodate. The Cl-O bond distances in the hydrated oxo chloro anions as determined by LAXS and obtained in the QMCF MD simulations are in excellent agreement, being 0.01-0.02 Å longer than in solid anhydrous salts due to hydration through hydrogen bonding to water molecules. The oxo halo anions, all with unit negative charge, have low charge density making them typical structure breakers, thus the hydrogen bonds formed to the hydrating water molecules are weaker and more short-lived than those between water molecules in pure water. The water exchange mechanism of the oxo chloro anions resembles those of the oxo sulfur anions with a direct exchange at the oxygen atoms for perchlorate and sulfate. The water exchange rate for the perchlorate ion is significantly faster, ?0.5 = 1.4 ps, compared to the hydrated sulfate ion and pure water, ?0.5 = 2.6 and 1.7 ps, respectively. The angular radial distribution functions show that the chlorate and sulfite ions have a more complex water exchange mechanism. As the chlorite and chlorate ions are more weakly hydrated than the sulfite ion the spatial occupancy is less well-defined and it is not possible to follow any well-defined migration pattern as it is difficult to distinguish between hydrating water molecules and bulk water in the region close to the ions.

Dataset Information

Interactions Between Hydrated Cerium(III) Cations and Carboxylates in an Aqueous Solution: Anomalously Strong Complex Formation with Diglycolate, Suggesting a Chelate Effect.

Publications

Interactions Between Hydrated Cerium(III) Cations and Carboxylates in an Aqueous Solution: Anomalously Strong Complex Formation with Diglycolate, Suggesting a Chelate Effect.

Similar Datasets

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