Lanthanide-Induced Nuclear Magnetic Resonance Shifts and Molecular Structure of L-Azetidine-2-Carboxylic Acid. II. Determination of Molecular Conformation

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Lanthanide-induced shifts (LIS) in <SUP>1</SUP>H NMR spectra of L-azetidine-2-carboxylic acid were observed in D<SUB>2</SUB>O solution using Pr(III), Nd(III), and Eu(III) nitrates as shift reagents. By taking the LIS of one of the protons as reference, the shift ratios for the other four protons were calculated. The shifts ratio obtained were nearly constant for the three lanthanide ions, except for that with respect to the α proton which is located closest to the lanthanide ion. This indicates that the contribution of the contact shift is appreciable only for the observed LIS of the α proton and is negligible for the LIS's of the β and γ protons. The constancy of shift ratios assures, in addition, that the magnetic tensor is axially symmetric. Therefore, the LIS's of the β and γ protons may be used for the structure determination. For the β and γ protons, the ratios of LIS's intrinsic to the lanthanide-substrate 1:1 complex were corrected for the contribution of diamagnetic shifts due to complex formation which were estimated by the use of La(III) nitrate. The line broadening induced by Gd(III) was also observed. The structure analysis based on these data shows that the four-membered ring of L-azetidine-2-carboxylic acid is puckered with the dihedral angle of 25° and the carboxylate is in the equatorial position.
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