Paramagnetic ions as structural probes in solid-state NMR: Distance measurements in crystalline lanthanide acetates

Article abstract of DOI:10.1021/ja00069a034

The rare earth acetates M(O₂CCH₃)₃·4H₂O (M = Nd, Sm, Eu, Y) and Pr(O₂CCH₃)₃·H₂O, and the analogous deuterated compounds, have been studied by ¹³C MAS-NMR. The paramagnetic materials show a large range of isotropic ¹³C chemical shifts which result largely from contact interactions with the rare earth electronic moments. They often show substantial linebroadening, which appears to result predominantly from anisotropic bulk magnetic susceptibility broadening for the deuterated compounds; the line widths for the protonated materials are increased further because of incomplete proton decoupling. Proton spectra acquired from a largely deuterated sample indicated that the spread in proton frequencies (40 kHz for Sm(O₂CCH₃₎3·4H₂O at 4.7 T, and calculated to be approximately 200 kHz for Eu(O₂CCH₃)₃·4H₂O) is too large for decoupling to be effective with attainable ¹H power levels. The deuterated materials exhibit sufficiently good resolution to allow analysis of the large ¹³C spinning sideband manifolds; these result mainly from dipolar coupling to the paramagnetic centers. X-ray diffraction shows that the solid solutions Y_(1-x)Ln_x(O₂CCH₃) ₃·4H₂O (Ln = Pr, Nd, Sm, Eu; x ≤ 0.1) crystallize with the Y(O₂CCH₃)₃·4H₂O crystal structure. MAS-NMR spectra of the ¹³C enriched deuterated analogues contain remarkably narrow resonances of only 100 Hz line width despite having spinning sideband envelopes spanning 1000 ppm. The paramagnetic shift anisotropies could be predicted with reasonable accuracy by considering the interactions of the nuclei with the single nearest paramagnetic ion. Ln-¹³C distances could also be estimated from the spectra and agreed to within 5% for the carboxyl carbons and 15% for the methyl carbons with the distances available from single-crystal X-ray diffraction studies. The different resonances in the solid solutions could then be assigned by the use of these estimated distances despite the large number of different ¹³C local environments. These results suggest that in favorable circumstances, paramagnetic centers can provide a means for the determination of structural information and distances from MAS-NMR.