Interpretation of NMR spectra is often hampered by the overlap of signals. Particularly for compounds with numerous complex multiplets, assigning individual signals can be tricky. However, a recent improvement in selective excitation techniques, GEMSTONE, offers a method to tease apart the overlapping signals.
The GEMSTONE experiment is a 1D pulse sequence that produces a spectrum of a single, targeted multiplet. To test the sequence I used a halogenated steroid with numerous overlapping multiplets. The figure below shows a stackplot with the normal 1D 1H spectrum at the top, and a series of GEMSTONE spectra arranged below. Compare how the GEMSTONE spectra separate overlapped resonances like those at 1.841 and 1.803 ppm, and the group between 1.40 and 1.25 ppm, with the mess of signals in the standard 1D 1H spectrum at the top of the figure.
Peak positions for selective excitation in the GEMSTONE spectra were determined by recording a pure shift PSYCHE spectrum. Each GEMSTONE spectrum takes about the same amount of time as a standard 1D 1H spectrum, and the selective excitation does not cause a loss in sensitivity. Setup is easy. For the spectra above the only parameter I changed was the position of the peak to excite. Some broad multiplets, such as 2.246 and 1.803 ppm, required a wider excitation range (40 Hz vs 25 Hz) to prevent reduced intensity of the edges of the multiplet.
The figure below compares the pulse sequence of a standard selective experiment with the GEMSTONE sequence. Selective excitation in these sequences is produced by the shaped 180o pulse flanked by gradient pulses. The enhanced specificity of the GEMSTONE sequence is achieved by the addition of a pair of adiabatic pulses applied simultaneously with low power gradient pulses. Adiabatic pulses sweep through a range of frequencies exciting different frequencies at different times. In the GEMSTONE sequence the first adiabatic pulse sweeps from low frequency to high while the second goes in the opposite direction.
The net effect of the adiabatic pulse and the simultaneous gradient pulse is a phase difference that depends on both chemical shift and position. Signals on resonance at the center of the spectrum remain in phase, but those further away do not. The phase difference also depends on the position of the molecule in the NMR sample tube. As indicated by the figure below, off resonance peaks from sample near the top of the tube (yellow and purple) acquire a different phase from those due to sample near the bottom (red and blue). However, each scan collects signal from the entire sample column at once so that the off resonance signals, with a mix of phases, are averaged out and do not produce a signal.
The GEMSTONE sequence provides a quick and easy way to tease apart overlapping resonances. It can also be combined with TOCSY or NOESY experiments to produce 1D selective versions of these 2D experiments, and I will cover these in the next post.
References
Single-Scan Selective Excitation of Individual NMR Signals in Overlapping Multiplets.
Kiraly P, Kern N, Plesniak MP, Nilsson M, Procter DJ, Morris GA, Adams RW.
Angew Chem Int Ed Engl. 2021 Jan 11;60(2):666-669. doi: 10.1002/anie.202011642.
Acknowledgments
Thanks again to Prof. Ted Molinski for preparing and providing the sample.
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