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Thursday, August 23, 2018

13C Satellite Observation with 1D 1H - 13C HSQC

Two dimensional 1H - 13C or  1H - 15N HSQC spectra are typically used to obtain one-bond heteronuclear correlation information between 1H and 13C or 1H and 15N.  The data are proton detected and typically employ pulsed field gradients for coherence selection.  The technique uses an incremented delay for chemical shift correlation and essentially discards the very intense signals from protons bound to 12C or 14N while enhancing the remaining doublet signals for protons bound to 13C or 15N.  Usually the 1H FIDs are collected with 13C or 15N decoupling to collapse the doublets into singlets in the F2 domain leaving a single correlation between proton signals in F2 at the 13C or 15N chemical shifts in F1 of the nuclei to which the protons are bound.   One can use a 1D version of this experiment (without the incremented delay for chemical shift correlation) to collect 1H NMR spectra exclusively of the protons bound to 13C or 15N.  If used without 13C or 15N decoupling, it allows easy observation of the 13C or 15N satellites in the absence of the very intense signals due to protons bound to 12C or 14N and allows the precise measurement of one-bond  1H - 13C  or  1H - 15N coupling constants.  The top panel of the figure below shows the 300 MHz 1D  1H - 13C HSQC spectrum of ethyl acetate.  Clearly the 13C satellites are observed and the  1H - 12C signals are suppressed.  The 1H spectrum of ethyl acetate is shown in the bottom panel of the figure for comparison.