NMR spectra @ 21.1 Tesla
(Click on a picture to enlarge)
angle adjustment in MAS probes is commonly performed by observing ST spinning
sidebands in 79Br MAS NMR spectra of KBr at low spinning speeds. The magic
angle is set correctly when the sidebands have the highest intensity.
A Hall effect magnetic flux sensor can be used for the same purpose ,
for example in low-gamma MAS probes incapable of 79Br NMR. We have recently
tested this approach at the 900 NMR Facility: The figure shows the normalized
intensity of the 79Br ST spinning sidebands in KBr versus the Hall voltage
measured. This Hall sensor will be used for magic angle setup in a 2.5
mm boron-free MAS probe which is currently under construction.
For more information see
 S. Mamone, A. Dorsch, O.G. Johannessen, M.V. Naik, P.K. Madhu, M.H. Levitt, "A Hall effect angle detector for solid-state NMR," Journal of Magetic Resonace 190 (2008) 135-141. http://dx.doi.org/10.1016/j.jmr.2007.07.012
standard QCPMG NMR pulse sequence consists
of a 90 pulse followed by a train of 180 pulses (more).
Ideally, the resulting spikelet envelope should outline the static lineshape
(middle spectrum, 90-180). If the first pulse deviates from 90 due to incorrect
calibration, the QCPMG spikelet pattern does not change significantly, the
only effect is somewhat lower overall intensity (Figure
While the miscalibrated 90 pulse alone has little impact on the QCPMG lineshape, of course it is often used to calculate the 180 pulse. As can be seen both, experimentally and using SIMPSON calculations, the miscalibrated 180 pulse leads to significantly distorted spikelet patterns (Figure B). The 180 pulse misset by as little as 20-30 degrees could produce considerable oscillations in spikelet intensity across the envelope. This illustrates that the QCPMG NMR experiments are much more sensitive to proper setup of the 180 degree pulse than the Hahn-echo experiments.
QCPMG spectra shown were calculated by Eric Ye (900 Facility) using the SIMPSON software for a central transition of a spin 3/2 nucleus resonating at 295 MHz, CQ=10 MHz, etaQ=0.7, CS anisotropy -200 ppm, coincidental EFG and CSA tensors.
For more information see
Renée Siegel, Thomas T. Nakashima and Roderick E. Wasylishen, "Signal-to-Noise Enhancement of NMR Spectra of Solids Using Multiple-Pulse Spin-Echo Experiments", Conc. Magn. Reson. 26A (2005) 62-77. http://dx.doi.org/10.1002/cmr.a.20038
A WURST-QCPMG NMR technique has recently been introduced by Luke O'Dell and Rob Schurko (Windsor) to achieve uniform excitation of quadrupolar nuclei across very wide bandwidth. Using this approach wideline spectra of stationary samples can now be acquired with no need or just minimal transmitter frequency adjustment. The spectrum shown was acquired in about 1 hour by co-adding 10 individual pieces. Experiments were performed by Luke O'Dell, who also kindly provided the WURST-QCPMG pulse sequence for the 900 MHz NMR instrument. This pulse program is now available to our users. For more information about WURST-QCPMG:
L.A. O'Dell and R.W. Schurko, "QCPMG Using Adiabatic Pulses for Faster Acquisition of Ultra-Wideline NMR Spectra," Chem. Phys. Lett. 464 (2008) 97-102. http://dx.doi.org/10.1016/j.cplett.2008.08.095
Recently developed Polarization Inversion Spin Exchange at the Magic Angle (PISEMA) technique finds applications in correlation experiments and is used for accurate measurements of chemical shift and heteronuclear dipolar interactions (see Annual Reports on NMR Spectroscopy, 2004, volume 52, p.1-52). This 1H-15N PISEMA experiment with 5N-labeled N-acetyl valine was performed on the 900 MHz instrument using the Facility's flat-coil E-free probe (M. Monette, Bruker).
19F-13C CP MAS spectrum recorded for a hydrated Nafion film. In such high-resolution spectra various C-F species can readily be identified. Experiments performed by Shane Pawsey (the 900 Facility) with the 2.5 mm MAS probe.
Similar to MQMAS, satellite transition MAS of half-integer quadrupole nuclei (STMAS) can be used to separate anisotropic from isotropic interactions. Experiments performed by S. Steuernagel (Bruker) with the 2.5 mm MAS probe.
enhancement in MAS spectra of half-integer quadrupole nuclei, DFS (double
frequency sweep), RAPT (rotor assisted population transfer), compared
with a SP (single pulse) experiment. Performed by S. Steuernagel (Bruker)
with the 2.5 mm MAS probe.
NMR of NaIO4 revisited at 21.1 T. Cq=42.3 MHz, CSA is negligible (the span
is <20 ppm). For more see: G.
Wu and S. Dong, SSNMR 20 (2001) 100-107
In materials ultrahigh magnetic fields are beneficial when studying low-gamma (and) quadrupolar nuclei. These 39K NMR spectra of KNO3 were recorded on our 900 instrument at the resonance frequency of 42 MHz (compare with 23 MHz at 11.7 T). Experiments performed by I. Moudrakovski (SIMS-NRC) with the wideline static probe built by J. Bennett (NRC).
Both experiments were performed with the 3.2 mm MAS probe. With very short rf-pulses available on the 900 very broad static lineshapes can be correclty recorded. In this case a solid-state 90-pulse for 55Mn was 0.6 us.
13C CP/MAS spectrum of caffeine. At 21.1 T note absence of the line splitting due to the dipolar coupling with the quadrupolar 14N. At lower magnetic fields such coupling results in very complicated spectra.