Tuesday, October 11, 2016

Pure Shift NMR - PSYCHE

Pure shift NMR spectra are spectra in which the scalar coupling has been removed so that all signals appear as singlets, as in 1D 13C spectra. For 13C spectra the coupling is removed by applying 1H composite pulse decoupling during acquisition. Acquiring a pure shift 1H spectrum, however, is not so easy because applying 1H decoupling during acquisition would saturate the 1H signals and prevent them from being detected. To acquire pure shift 1H NMR spectra two ingenious modifications have been developed to enable removal of the coupling in a 1D 1H spectrum. This technique is known as PSYCHE (Pure Shift Yielded by CHirp Excitation)1.

Shown below is an expansion of a standard 1D 1H NMR spectrum of cholesteryl acetate in red, and the same region of a PSYCHE spectrum in blue. The large signals from the methyl groups near 0.9 ppm have been reduced to singlets, as have the overlapping multiplets between 0.9 and 1.6. The PSYCHE spectrum offers much better resolution and makes it much easier to determine the number of resonances.

The first modification employed by the PSYCHE experiment is slice-selective excitation which excites resonances in isolation from their coupled nuclei. Slice-selective excitation is achieved by applying a selective pulse at the same time as a weak z-gradient pulse. The z-gradient linearly alters the magnetic field along the vertical axis of the sample so that one end of the sample experiences a slightly stronger magnetic field while the other end experiences a slightly reduced magnetic field. Applying a selective pulse during this gradient results in different parts of the spectrum being excited in different parts of the sample volume. The figure below attempts to explain this graphically.

from Zanger 2015

At the left of the figure the gray diagonal crossing the vertical line indicates the modified magnetic field induced by the z-gradient. The tall thin cylinder represents the NMR tube containing the sample. The colored disks indicate the slices of the sample inside the NMR tube that produce the matching colored resonances in the spectra to the right. Each sample slice produces only the resonance of the matching color, but because the selective pulse excites different parts of the spectrum from different parts of the sample the entire spectrum is recorded. Note that since not all of the sample is contributing to the recorded signal the sensitivity is reduced relative to a standard 1D 1H NMR spectrum.

The second modification used by the PSYCHE experiment is chunked data acquisition. During acquisition of an FID scalar coupling evolves and splits the resonances, however, the scalar coupling takes a finite time to evolve. If chunks of the FID shorter than this time are recorded then the spectrum can be recorded without scalar coupling. In practice this is done by recording a series of short FIDs which during processing are stitched together to give a complete FID without coupling.

from Castanar and Parella 2015

The figure above shows a series of FIDs recorded with different starting times to sample different portions of the signal evolution. A chunk from each FID is used to produce the final coupling-free FID which can be processed in the normal fashion.

Chunked data acquisition obviously requires acquisition of multiple FIDs, and slice selective excitation obtains signal from only a portion of the sample in the NMR tube, so the PSYCHE experiment is inherently less sensitive and more time consuming than a standard 1D 1H NMR experiment. The standard spectrum shown in the first figure took 13 seconds to acquire while the PSYCHE spectrum took 330 seconds. PSYCHE also requires an additional processing step to combine the FIDs. Despite these drawbacks there may be situations where PSYCHE can provide useful information and parameters for running it are available on the Facility spectrometers.

1. M. Foroozandeh, R.W. Adams, N.J. Meharry, D. Jeannerat, M. Nilsson, G.A. Morris
"Ultrahigh-Resolution NMR Spectroscopy"
Angew. Chem. Int. Ed., 2014 53(27):6990-6992.

2. K. Zangger
"Pure Shift NMR"
Prog. NMR. Spec. 2015 86-87:1-20.

3. L. Castanar and T. Parella
"Broadband 1H homodecoupled NMR experiments: recent developments, methods and applications"
Magn. Reson. Chem. 2015 53(6):399-426.

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