Europium(III) lactate structure determination using spectroscopic (ATR FT-IR, NMR) and theoretical (DFT) methods


Europium(III) lactate structure determination using spectroscopic (ATR FT-IR, NMR) and theoretical (DFT) methods

Barkleit, A.; Kretzschmar, J.; Tsushima, S.; Acker, M.

Small organic molecules like lactic acid (HO-CH(CH3)-COOH), which can bind heavy metal ions, are ubiquitous in nature. They can be found in nearly all biological systems as a product of various biochemical processes and in the geosphere as well, i.e. as part of organic matter of argillaceous rocks. This renders lactate a suitable model molecule for a multi-technique structure determination. Eu(III) was chosen as non-radioactive model for trivalent actinides.
Current structural suggestions for the Eu(III) lactate are only assumptions from indirect methods [1,2]. We want to provide direct structural information. ATR FT-IR spectroscopy combined with calculations of structure and spectroscopic data using DFT reveals structural features. Lanthanide induced shifts (LIS) in NMR spectroscopy as caused by the interaction of nuclear spins with electronic unpaired spins can be used as a helpful tool for signal separation, probing the potential binding sites and structure including geometries and distances [3].
The combination of all these methods offers new insights concerning the structure of the Eu(III) lactate 1:1 complex thereby resolving contradictions in the previous works whether the hydroxyl group is protonated or not.
From ATR FT-IR measurements, bidentate coordination of exclusively the carboxylate group could be ruled out because of the characteristic degree of spectral splitting of the asymmetric and symmetric stretching vibrations νas and νs of the carboxylate group. The best accordance of the DFT calculated vibrational spectra to the measured spectrum is given for monodentate coordinating carboxylate group and additional coordination of the deprotonated hydroxyl group (Fig. 1). NMR findings strongly support the results obtained from ATR FT-IR measurements in combination with DFT calculations. The correlation between the chemical shift changes of the 13C NMR signals and the Eu-C distances calculated by DFT suites perfectly this structure model [4].
The finding that the hydroxyl group seems to be deprotonated under complex formation [4] contradicts former structure suggestions, which suppose a coordination of the trivalent metal ion with the protonated hydroxyl group [1,2]. Both experimental methods, ATR FT-IR and NMR, as well as the DFT calculations yielded an impressively homogeneous structural explanation of the investigated Eu(III) lactate 1:1 species.

[1] Tian, G.X. et al. (2010) Inorg. Chem. 49, 10598-10605.
[2] Dickins, R.S. et al. (2002) J. Am. Chem. Soc. 124, 12697-12705.
[3] Mayo, B.C. (1973) Chem. Soc. Rev. 2, 49-74.
[4] Barkleit, A. et al. (2014) Dalton Trans. DOI: 10.1039/c4dt00440j

  • Poster
    Advanced Techniques in Actinide Spectroscopy 2014 (ATAS 2014), 03.-07.11.2014, Dresden, Deutschland
  • Open Access Logo Contribution to proceedings
    Advanced Techniques in Actinide Spectroscopy 2014 (ATAS 2014), 03.-07.11.2014, Dresden, Deutschland
    Wissenschaftlich-Technische Berichte HZDR-054: Helmholtz-Zentrum Dresden-Rossendorf

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Publ.-Id: 20519