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 ¹J_H-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.