Isotope Effects and the 19F - 13C HMQC Spectrum of Trifluoroacetic Acid

The ¹⁹F - ¹³C HMQC spectrum of trifluoroacetic acid is shown in the figure below.

The data were collected with a delay appropriate for a ¹⁹F - ¹³C J  coupling constant between the ¹J_F-C coupling constant of 284 Hz and the ²J_F-C coupling constant of 44 Hz.  The top and side traces are the one-pulse ¹⁹F and ¹³C spectra, respectively.  Why are the HMQC responses not at the same ¹⁹F chemical shift and why aren't they correlated to the peak in the ¹⁹F spectrum?  In order to answer these questions one must take into consideration the ¹⁹F - ^{12, 13}C isotope effects.  The chemical shift of the fluorine depends on whether it is bound to a ¹²C or a ¹³C.  The effect is largest across one bond and gets smaller over multiple bonds.  The ¹⁹F NMR spectrum for trifluoroacetic acid is shown in the figure below with and without ¹³C broadband decoupling in the upper and lower traces, respectively.

Approximately 98% of the trifluoroacetic acid is the ¹²CF₃-¹²COOH isotopomer, giving rise to a large singlet plotted off-scale in the figure. Approximately 1% of the signal is from the ¹³CF₃-¹²COOH isotoponer giving rise to a doublet with ¹J_F-C = 284 Hz and approximately 1% of the signal is from the ¹²CF₃-¹³COOH isotoponer giving rise to a doublet with ²J_F-C = 44 Hz.  All of these signals are clearly present in the lower trace of the figure.  When ¹³C broadband decoupling is applied, the doublets collapse into singlets.  The singlets from each of the isotopomers are resolved in the top trace.  The one-bond ¹⁹F - ^{12, 13}C isotope effect is 0.13 ppm and the two-bond effect is 0.02 ppm.  The figure below shows the same HMQC data with the spectrum from the top trace used as a projection.

One can see that the HMQC responses are correlated to their respective isotopomers.  These effects are also present in ¹H - ¹³C HMQC spectra, but the ¹H - ^{12, 13}C isotope effect is much smaller than the ¹⁹F - ^{12, 13}C isotope effect.