High-Resolution NMR Spectroscopy in Solids by Truly Magic-Angle Spinning

摘要

There is an ever increasing interest in obtaining high-resolution NMR spectra of S=1/2 nuclei, such as 13C, in solids. Solid-state NMR spectroscopy is important for material science, for (bio)organic chemistry, for protein structure determination,[1]-[3] and for the characterization of pharmaceutical products (e.g., crystalline polymorphism).[4] The combination of magic-angle spinning (MAS) with heteronuclear dipolar decoupling leads to line narrowing, and hence to an improvement of both resolution and sensitivity (peak-height-to-noise ratio). Herein, we show that the line width of 13C resonances can be narrowed to 0.039 ppm (3.9 Hz for 13C at 100.6 MHz or 9.4 T). Such a narrow resonance is observed for carbonyl carbon atoms of polycrystalline cholesteryl acetate if the magic angle (m=arccos 3-1/254.736°) is adjusted very accurately, that is, within ||=|-m|=0.004°, as commonly done for satellite-transition magic-angle spinning (STMAS) NMR spectroscopy of quadrupolar nuclei.[5], [6] The lower limit of the line width (which is inversely proportional to the effective spin-echo decay time constant ) can be as little as 0.09 Hz for carbonyl carbon atoms in choresteryl acetate. We also demonstrate by 207Pb NMR spectroscopy that temperature gradients across the sample (which lead to a distribution of isotropic chemical shifts) can provide an important contribution to the line width, in addition to imperfect decoupling,[7] structural disorder,[8] and magnetic susceptibility effects.