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Wednesday, May 6, 2015

Decoupling in 2D HSQC Spectra

HMQC and HSQC NMR data are commonly used to correlate the chemical shifts of protons and 13C (or 15N) across one chemical bond via the J coupling interaction.  The data are 1H detected, with the 1H chemical shift in the horizontal F2 domain and the 13C (or 15N) chemical shift in the vertical F1 domain.  In the case of 1H and 13C, the technique depends on protons bonded to 13C.  1H–12C spin pairs provide no coupling information and are suppressed by the method.  If one is to observe the 1H signal of a 1H-13C spin pair, one expects to observe a doublet with splitting 1JH-C (i.e. the 13C satellites).  Likewise, if one is to observe the 13C signal of a 1H-13C spin pair, one expects to observe a doublet with the same splitting.  2D HSQC spectra are normally presented with both 1H and 13C decoupling yielding a simplified 1H-13C chemical shift correlation map over one chemical bond.  The figure below shows one of the most commonly used gradient HSQC pulse sequences.  The 1H and 13C decoupling elements of the sequence are highlighted in yellow and pink, respectively.
During the evolution time, t1, the 13C chemical shift and 1H-13C coupling evolve.  The 1H 180° pulse (color coded in yellow) in the center of the evolution time refocuses the coupling and as a result decouples protons in the F1 (13C) domain of the spectrum.  13C is broadband decoupled from the F2 (1H) domain by applying a GARP pulse train (color coded in pink) at the 13C frequency during the collection of the FID.  One can turn each of these elements “on” or “off” for data collection.  The figure below shows the 1H-13C gradient HSQC spectrum of benzene with all possible combinations of 1H and/or 13C decoupling.
In the top left panel both 1H and 13C decoupling are turned “on” and one observes a singlet in both the F2 (1H) and F1 (13C) domains.  In the top right panel, the 1H decoupling element is “on” while the 13C decoupling element is “off”.  The result is a 1H-13C doublet in the F2 (1H) domain and a singlet in the F1 (13C) domain.  In the bottom left panel, the 1H decoupling element is “off” while the 13C decoupling element is “on”.  The result is a 13C-1H doublet in the F1 (13C) domain and a singlet in the F2 (1H) domain.  In the bottom right panel, both the 1H and 13C decoupling elements are “off”.  The result is a 1H-13C doublet in both the F2 (1H) and F1 (13C) domains.

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