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Tuesday, May 29, 2018

Decoupling Bandwidth and Distorted Line Shapes

Broadband X nucleus decoupling (X = 13C, 15N, 31P, 11B, 19F etc.....) is frequently used in 1H detected 2D HSQC/HMQC data collection or in standard 1D 1H spectra to aid in structure assignment.  When broadband decoupling schemes are used, one must keep in mind that they are not infinitely broadbanded.   They have finite bandwidths over which they are effective thus limiting the chemical shift range for the decoupled nuclide.  The effective bandwidth depends on the particular decoupling scheme and the decoupling power used.  If multiple peaks are to be decoupled, one must insure that all peaks are within the decoupler bandwidth.  One can determine experimentally the effective decoupling bandwidth by running a series of 1H spectra varying the decoupler offset frequency.  Such a measurement is shown in the figure below for the P-CH3 methyl resonance of dimethyl methylphosphonate. 300 MHz 1H [31P] NMR spectra were collected in a pseudo-2D fashion, incrementing the decoupler offset frequency from 200 ppm to -200 ppm from the 31P resonance frequency in 1 ppm steps.  Broadband GARP decoupling was employed with a power of 3125 Hz  (80 ┬Ásec 90° pulses).  The pseudo-2D contour plot is shown in the left-hand panel and a stacked plot is shown in the right-hand panel.
One can see that the effective decoupling bandwidth is 15.56 kHz or 128 ppm on a 300 MHz instrument.  When the decoupler offset exceeds ±64 ppm from the 31P resonance frequency, one obtains distorted line shapes.  Representative distorted line shapes are shown in the figure below.  The bottom spectrum was collected with no 31P decoupling.  The top, fully decoupled spectrum was collected with on-resonance 31P GARP decoupling.  The middle spectra, highlighted in pink, are representative distorted spectra outside of the effective decoupling bandwidth.
If one is not aware of the decoupling offset and available bandwidth, one may obtain misleading line shapes subject to misinterpretation.

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