Seeking Higher Resolution and Sensitivity for NMR of Quadrupolar Nuclei at Ultrahigh Magnetic Fields Zhehong Gan,* ,† Peter Gor’kov, † Timothy A. Cross, † Ago Samoson, ‡ and Dominique Massiot § National High Magnetic Field Laboratory, 1800 East Paul Dirac DriVe, Tallahassee, Florida 32310, National Institute of Chemical Physics and Biophysics, Tallinn, 12618, Estonia, and CRMHT-CNRS, 45071 Orle ´ ans Cedex 2, France. Received February 7, 2002 The quest for higher spectral resolution and sensitivity is a common feature of many NMR techniques and has drawn develop- ment of constantly higher homogeneous magnetic fields (super- conducting 900 MHz spectrometers are now commercially avail- able). For solid-state NMR, high spectral resolution requires averaging of the anisotropic part of spin interactions to ideally obtain a spectrum with sharp lines positioned at their individual isotropic chemical shifts. For most spin ) 1 / 2 nuclei except protons that are strongly coupled by homonuclear dipolar interactions, high resolu- tion can be obtained at usual magnetic fields using magic angle spinning (MAS) 1 at conventional spinning rates (now up to 30 kHz) combined with heteronuclear decoupling when necessary. For nuclei with I > 1 / 2 , a supplemental difficulty arises from the field- dependent shift and broadening by the second-order perturbation of quadrupolar interactions under the dominant Zeeman interaction. 2 The second-order quadrupolar effect expressed in ppm is propor- tional to the inverse square of Larmor frequency, and consequently a quadratic gain in both resolution and sensitivity is expected with high magnetic fields. 3 This is of major importance in terms of applicability of solid-state NMR to crystalline and amorphous inorganic materials containing quadrupolar nuclei such as 27 Al, 11 B, 69,71 Ga, 23 Na, 87 Rb, and 25 Mg especially for compounds with severe second-order quadrupolar line broadening that may prevent spectral resolution. 3 Various types of one- and two-dimensional experiments have been developed to overcome the second-order quadrupolar broadening by double rotation (DOR) 4 or spreading spectra in two dimensions with DAS, 5 MQMAS, 6 and STMAS 7 experiments introduced in 1988, 1995, and 2000, respectively. The two- dimensional approach relies on the same principal of averaging out the second-order quadrupolar effect by correlating different transi- tions of quadrupolar spins. The development of these methods has considerably extended the applicability of solid-state NMR to study structure and properties of organic and inorganic materials in crystalline, amorphous, or glassy states. In this contribution we shall demonstrate the possibilities of using very high magnetic fields, 25-T resistive and 40-T hybrid magnets, at the National High Magnetic Field Laboratory (NHMFL) to reduce the second-order quadrupolar line broadening such that high- resolution spectra can be obtained using simple excitation schemes for keeping the quantitative interpretation of obtained spectra. We use the sample aluminoborate 9Al 2 O 3 + 2B 2 O 3 (A 9 B 2 ) for the field- dependent demonstration. The structure and NMR parameters of A 9 B 2 are already known from previous studies. 8 The compound has four different sites: one AlO 4 , two AlO 5 , and double intensity AlO 6 sites. At currently available superconducting magnetic fields, the four sites are not resolved in MAS spectra due to large second- order quadrupolar broadening. The overlap can nevertheless be resolved using DOR, MQMAS, or STMAS experiments, at a cost, however, of more complex experimental settings, data acquisition time, and difficult quantitative interpretation of the resulting spectra especially in the case of MQMAS and STMAS where the excitation and coherence transfer efficiencies have to be known or computed. Figure 1 shows the experimental and modeled spectra of the A 9 B 2 compound obtained with a simple one-pulse experiment at the principal field of 17.6 T. The spectrum shows partial resolution of the AlO 6 line while the AlO 4 and the two AlO 5 lines remain overlapped. Experiments at lower fields (7 and 9.4 T) would show more complex spectra with less resolution. 8 MQMAS using amplitude modulation excitation and STMAS do provide better resolution, allowing the separation of all four different sites. Figure 2 shows the experimental and modeled spectra of MQMAS (9.4 T) and STMAS (19.6 T). The separation of sites in two dimensions allows extraction of their quadrupolar interaction parameters individually. The STMAS spectrum correlates single-quantum satellite and central transitions, and it appears with more efficient excitation and less dependence on the quadrupolar couplings; however, the experiment requires a more stringent setup, especially on the magic-angle setting. 7 With current superconductor technology for NMR magnets, the strength of homogeneous principal fields is limited to 21 T (900 MHz). Higher fields are nevertheless accessible with resistive magnet technology. We used the 25-T resistive magnet and the hybrid 40-T magnet (11 T superconducting plus 29 T resistive) at the NHMFL. With magic-angle spinning and reduced sample volume (2 mm MAS rotor), the effects from magnetic field homogeneity were kept minimal (<0.5 ppm) through careful adjustment of probe position without active shimming. Compara- tively, magnetic field fluctuation and drift cause a much larger (∼3 ppm) additional line broadening. 27 Al spectra at 19.6, 25, and 40 T were acquired using a home-built 2-mm MAS probe, and a Tecmag NMR console was placed in the vicinity of the high field magnets, ∼4 m (25 T) and ∼6 m (40 T). The MAS probe was loaded upside down from the top, and the spinning rate was about 20 kHz. With the increased sensitivity at high fields, data acquisition times were kept short to minimize the field-drift effect. Sixteen scans with a 1-s recycle time and a small flip-angle (<30°) pulse were collected. Figure 3 displays the spectra of A 9 B 2 obtained at 14, 19.6, 25, and 40 T. Until 14 T, severe spectral overlap makes the fitting of all four 27 Al sites difficult. Chemical shift and quadrupolar coupling parameters could not be reliably measured from just one- dimensional spectra. At 19.6 T, a field close to the maximum magnetic field available with superconducting magnets, the AlO 6 * To whom correspondence should be addressed. E-mail: gan@magnet.fsu.edu. † National High Magnetic Field Laboratory. ‡ National Institute of Chemical Physics and Biophysics. § CRMHT-CNRS. Published on Web 04/30/2002 5634 9 J. AM. CHEM. SOC. 2002, 124, 5634-5635 10.1021/ja025849p CCC: $22.00 © 2002 American Chemical Society