PARAMAGNETIC-IONS AS STRUCTURAL PROBES IN SOLID-STATE NMR - DISTANCE MEASUREMENTS IN CRYSTALLINE LANTHANIDE ACETATES

The rare earth acetates M(O2CCH3)3.4H2O (M = Nd, Sm, Eu, Y) and Pr(O2C CH3)3.H2O, and the analogous deuterated compounds, have been studied b y C-13 MAS-NMR. The paramagnetic materials show a large range of isotr opic C-13 chemical shifts which result largely from contact interactio ns with the rare earth electronic moments. They often show substantial linebroadening, which appears to result predominantly from anisotropi c bulk magnetic susceptibility broadening for the deuterated compounds ; the line widths for the protonated materials are increased further b ecause of incomplete proton decoupling. Proton spectra acquired from a largely deuterated sample indicated that the spread in proton frequen cies (40 kHz for SM(O2CCH3)3.4H2O at 4.7 T, and calculated to be appro ximately 200 kHz for Eu(O2CCH3)3.4H2O) is too large for decoupling to be effective with attainable H-1 power levels. The deuterated material s exhibit sufficiently good resolution to allow analysis of the large C-13 spinning sideband manifolds; these result mainly from dipolar cou pling to the paramagnetic centers. X-ray diffraction shows that the so lid solutions Y(1-x)Lnx(O2CCH3)3.4H2O (Ln = Pr, Nd, Sm, Eu; x less-tha n-or-equal-to 0.1) crystallize with the Y(O2CCH3)3.4H2O crystal struct ure. MAS-NMR spectra of the C-13 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 consideri ng the interactions of the nuclei with the single nearest paramagnetic ion. Ln-C-13 distances could also be estimated from the spectra and a greed to within 5% for the carboxyl carbons and 15% for the methyl car bons with the distances available from single-crystal X-ray diffractio n 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 C-13 local environments. These results suggest tha t in favorable circumstances, paramagnetic centers can provide a means for the determination of structural information and distances from MA S-NMR.