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Duer Research Group

 

Solid-state NMR suffers from the problem that the signals from nuclei are anisotropic, as the chemical shift depends to the orientation of the molecule relative to the magnetic field and this is not averaged out as it is in liquid-state NMR by molecular tumbling. This leads to broad, overlapping peaks in the NMR spectrum, from which very little useful information can be extracted. By spinning our sample at high frequency at the magic angle we can average out these anisotropic effects, but it turns out that the size of the chemical shift anisotropy (CSA) can be useful in structural determination.

The isotropic chemical shift is the most ubiquitously used parameter in NMR spectroscopy for molecular structural studies. It arises from the electronic shielding, which depends upon the electron distribution in the vicinity of the observed nucleus. The electronic shielding, and thus also the chemical shift, is an anisotropic property, varying with the orientation of the molecules relative to the magnetic field. Usually this chemical shift anisotropy (CSA) is averaged out using magic-angle spinning (MAS) and only the isotropic shift is recorded. However, more structural information can in principle be obtained from measurement of the CSA as well as the isotropic shift.

Accurate measurement of CSA remains a problem. Ideally we would like to enhance the resolution with MAS whilst retaining structural information from the CSA, hence for this purpose we have developed several multi-dimensional experiments, designed to give the high spinning rate, well-defined spectrum in the direct dimension and anisotropic information in the indirect dimension. With all the peaks resolved according to their isotropic chemical shifts, it is easy to measure the CSA from the indirect dimension and so we uncover all the information that is available from the experiment. The experiments are designed for general applicability on natural abundance materials.

Optimised and constant time 2DCSA

The optimised 2DCSA sequence is a 2D experiment based on a 6 pi pulse CSA recoupling experiment by Tycko and coworkers (1989). Using cogwheel phase cycling and a series of optimised incremental time delays, this experiment gives well-defined and artefact-free powder pattern lineshapes in the indirect dimension.

The original code for calculating time delays (for a Bruker spectrometer) was written by Dr Robin Orr in MuPAD. Since then, Wing Ying Chow has translated the code into GNU octave, which is available for GNU/Linux, MacOS and Windows, and may be run with very minimal modifications with Matlab.

Using rotor cycle (n) = 3, F1 SW (sw) = 15000, MAS rate (wr) = 10000, F1 TD points (nsteps) = 32, Xmax = 0.24, we generate the following:

Recoupling of chemical-shift anisotropy powder patterns in MAS NMR.
RM Orr, MJ Duer – Journal of Magnetic Resonance (2006) 181, 1
doi: 10.1016/j.jmr.2006.03.010

 

Reduction of Apparent Rate (ROAR, also CSA-amplified 2D-PASS)

ROAR, or more technically (but less succinctly), CSA-amplified 2D-PASS, is a two-dimensional experiment that correlates a sideband pattern characteristic of a reduced effective spinning rate (in the indirect dimension) with a sideband pattern characteristic of the actual spinning rate (in the direct dimension).

Experimentally this usually involves rapid MAS to give a well-resolved isotropic spectrum in the direct dimension. Each isotropic peak is then correlated with a sideband pattern characteristic of a spinning rate that is scaled by a factor, N, from the actual rate. The scaling factor can be arbitrarily varied by changing the number and timings of the rotor-synchronised pi-pulses used.

The experiment faithfully reproduces sideband patterns for scaling factors up to N=30. The pulse sequence is fixed length and avoids storage periods and quadrature detection, reducing losses in signal strength.

Such an experiment is required for measuring small CSA, since slow spinning would be required to obtain a sideband pattern directly.

Related Publications 

Applications of the CSA-amplified PASS experiment.
RM Orr, MJ Duer – Solid State Nucl Magn Reson (2006) 30, 1
Correlating fast and slow chemical shift spinning sideband patterns in solid-state NMR.
RM Orr, MJ Duer, SE Ashbrook – Journal of magnetic resonance (San Diego, Calif. : 1997) (2005) 174, 301