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Wednesday, October 31, 2018

Pure-Shift HSQC

Pure-shift NMR has become more and more common over the last few years.  A special issue of Magnetic Resonance in Chemistry has recently been dedicated to developments in these methods.  Pure-shift NMR methods offer simplified proton NMR spectra free of 1H - 1H coupling.  These methods have been extended to proton detected 2D NMR measurements, yielding 2D data sets with higher proton resolution compared to conventional 2D measurements.  The NMR Methodology Group at the University of Manchester has been a primary contributor to this technique and has kindly shared their efforts on-line.  The figure below compares the 600 MHz partial Pure-Shift HSQC spectrum of sucrose in DMSO-d6 to a more conventional HSQC spectrum acquired under similar conditions.  The projections on the spectra are independently collected high resolution 1H NMR spectra.  Clearly, the  Pure-Shift HSQC data have higher 1H resolution than the more conventional HSQC.  What may not be so obvious from the figure is that the sensitivity is also improved in the Pure-Shift HSQC.  The gain in signal-to-noise-ratio depends strongly on the degree of coupling collapsed.  For some signals in this spectrum, an improvement in the signal-to-noise-ratio as high as 72% was observed.
The the top and middle panels of the figure below show the 1D 1H projections of the Pure-Shift HSQC and HSQC data from the above figure, respectively.  The bottom panel is the conventional high resolution 1H spectrum for comparison.
Clearly, the Pure-Shift HSQC proton projection offers much improved resolution.