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 1JF-C coupling constant of 284 Hz and the 2JF-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, 13C 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 1JF-C = 284 Hz and approximately 1% of the signal is from the 12CF3-13COOH isotoponer giving rise to a doublet with 2JF-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, 13C 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, 13C isotope effect is much smaller than the 19F - 12, 13C isotope effect.