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

The rare earth acetates M(O[sub 2]CCH[sub 3])[sub 3][center dot]4H[sub 2]O (M = Nd, Sm, Eu, Y) and Pr(O[sub 2]CCH[sub 3])[sub 3][center dot]H[sub 2]O, and the analogous deuterated compounds, have been studied by [sup 13]C MAS-NMR. The paramagnetic materials show a large range of isotropic [sup 13]C 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(O[sub 2]CCH[sub 3])[sub 3][center dot]4H[sub 2]O at 4.7 T, and calculated to be approximately 200 kHz for Eu(O[sub 2]CCH[sub 3])[sub 3][center dot]4H[sub 2]O) is too large for decoupling to be effective with attainable [sup 1]H power levels. The deuterated materials exhibit sufficiently good resolution to allow analysis of the large [sup 13]C spinning sideband manifolds; these result mainly from dipolar coupling to the paramagnetic centers. X-ray diffraction shows that the solid solutions Y[sub (1-x)]Ln[sub x](O[sub 2]CCH[sub 3])[sub 3][center dot]4H[sub 2]O (Ln = Pr, Nd, Sm, Eu; xmore » [le] 0.1) crystallize with the Y(O[sub 2]CCH[sub 3])[sub 3][center dot]4H[sub 2]O crystal structure. MAS-NMR spectra of the [sup 13]C enriched deuterated analogues contain remarkably narrow resonances of only 100 Hz line width despite having spinning sideband envelopes spanning 1000 ppm. 53 refs., 6 figs., 3 tabs.« less