Detection of multiple boron sites in glasses by 11B satellite transition nuclear magnetic resonance spectroscopy

Effects of pulse strength, width, and sample spinning speed on the spectral spin diffusion of multiquantum coherences of spin-32 quadrupolar nuclei

The effects of radio-frequency pulse strength, width, and sample spinning speed on the spin-diffusion spectrum of half-integer quadrupolar spins in solids have been studied by theoretical, numerical, and experimental investigations. It is revealed that the line shape of the cross peaks changes nonmonotonically with respect to the change of pulse strength, pulse width, or sample spinning speed. It is also found that the sample spinning speed has much more pronounced influence on the spin diffusion spectral line shape. In many cases of practical importance, the effect of sample spinning must be included in spectral analysis, in contrast to the practice of previous studies. Moreover, this effect can be exploited to further improve the precision in the determination of relative orientation of the electric-field gradient tensors of the exchange partners.

Three-coordinated boron-11 chemical shifts in borates

Despite the importance of (11)B nuclear magnetic resonance (NMR) in structural studies of borate glasses, no clear means of correlating NMR parameters with the number of nonbridging oxygens on three-coordinate boron has been demonstrated. In this work, a series of anhydrous, polycrystalline, binary borates has been examined by (11)B magic-angle spinning (MAS) NMR to obtain precise measurements of their three-coordinate boron isotropic chemical shifts. The shifts generally increase with the replacement of bridging oxygens by nonbridging oxygens, ranging from 14.6 ppm in crystalline B(2)O(3) to 22.5 ppm in magnesium orthoborate. The underlying physical basis for this trend is satisfactorily accounted for by considering second neighbor effects using bond valence sums. These data are supportive of a structural model for B(2)O(3) glass in which 72% of the boron atoms are in rings. High-field MAS NMR experiments (B(0) = 18.8 T) indicate that the boron shielding is anisotropic, with greater anisotropy measured for three-coordinate borons possessing one or two nonbridging oxygens, than for those with zero or three nonbridging oxygens.

Five-quantum coherence of I = 52 nuclei: 27Al in an Al2O3 single crystal

Optimal conditions were calculated for excitation and detection of the five-quantum coherence of quadrupolar nuclei with I = 52 in single crystals, observed by the two-pulse sequence (theta1)x - tau1 - (theta2)alpha - tau2, where alpha is the phase cycling angle. Variations in the pulse lengths, the relative values of the nutation frequency omega1 = gammaB1, and the quadrupolar frequency omegaQ as well as in the resonance offset were taken into account. In addition, the effect of the pulse length on the intensity of spectral lines was considered. Theoretical results were compared with experiments on 27Al nuclei in an Al2O3 single crystal. Copyright 1998 Academic Press.

Five quantum coherence of I=5/2 nuclei: 27Al in polycrystalline AlCl3

Optimal conditions were calculated for the excitation and detection of the five quantum coherence of quadrupolar nuclei with I = 5/2 in powder samples, observed by the two-pulse sequence (theta1)x - tau1 - (theta2)alpha - tau2, where alpha is the phase cycling angle. We varied the pulse lengths and the relative values of the nutation frequency omega1 = gammaB1 and the quadrupolar frequency omegaQ. Also, the effect of the resonance offset was studied under optimal conditions. Besides, the conditions for obtaining the maximal echo amplitude after the two-pulse sequence with alpha = y were found. Theoretical results were compared with experiments on 27Al nuclei in polycrystalline AlCl3.