Carbon-13 spectral line broadening in paramagnetic solids

Abstract

Sensitivity enhancement by cross-polarization and resolution enhancement by dipolar decoupling and magic-angle sample spinning has become an increasingly important method for structural and dynamic characterization of solid materials (I). Paramagnetic compounds, with several notable exceptions (24, have resisted characterization using line-narrowing methods. A common assumption about the failure of this experiment is that an efficient nuclear electron dipolar relaxation mechanism, modulated by the rapid electron relaxation times, r,, and Th, can render the relaxation times of the observed carbon nucleus, fi and T$ , sufficiently short that an unusually broad carbon spectrum results. Our experiments on several compounds demonstrate that this assumption is incorrect, at least among the paramagnetic lanthanide systems studied. Short carbon relaxation times as a source of the line broadening may be tested by measuring the carbon relaxation times in question, fi and E. We have previously reported the spin-lattice relaxation times for the carbon atoms of praseodymium acetate monohydrate and find that the carboxyl and methyl carbons have T, values of 125155 and 600 ms, respectively (3). These times predict a linewidth less than 3 Hz, far narrower than those observed in the MASS spectra of this compound, about 280 Hz for the carbonyl. A convenient and efficient method for estimating the transverse relaxation times for the carbon atoms is to take the Fourier transform of the echo train (5, 6) accumulated on the paramagnetic compound. The resulting spectrum consists of a series of sharp lines the intensity of which maps the powder pattern and the width of which correspond to the linewidth implied by the decay time of the whole echo train. The width of these lines is typically about 25 Hz for the carbonyl lines, far narrower than the MASS lines or the single crystal lines. Thus, the width of the carbon lines in these paramagnetic solids is not determined by the spin-lattice or spin-spin relaxation times at the carbon atoms. The limiting linewidths of diamagnetic organic solids have been considered by VanderHart, Earl, and Garroway (7) and by Alla and Lippmaa (8). The broadening in both the static and MASS spectra may be attributed to the anisotropy of the diamagnetic susceptibility in the aromatic compounds. The situation is aggravated by paramagnetic materials where the anisotropy of the magnetic susceptibility may be far larger (9-11). Following Alla and Lippmaa (8) the bulk effects are demonstrated in Fig. 1, which shows the spectrum of adamantane and adamantane in samples where praseodymium