Unsymmetric Carbon Satellites in Fluorine-19 NMR

Recently, we employed 2,4-dichlorobenzotriflouride (DCBTF) as an internal calibrant for some ¹⁹F qNMR experiments. In this blog post let's look at carbon satellites in the ¹⁹F NMR spectra.

In the ¹⁹F NMR spectrum of DCBTF, we can observe the singlet resonance of the trifluoromethyl fragment at δ = −63.07 ppm. While it might not jump into your eye at first glance, please note that the carbon satellites are not centered at the "main signal", which is what we are used to from ¹H NMR spectra.

The short explanation of this phenomenon is that the isotope effect leads to a different chemical shift for carbon-13 bonded fluorine atoms in comparison to carbon-12 bonded ones. In ¹⁹F NMR this effect is so strong that it is even observed over two bonds: R-¹³C-¹²C-F-R’ is shifted in comparison to R-¹²C-¹²C-F-R’.

Figure 1. Zoomed 57 MHz ¹⁹F NMR spectrum of 2,4-dichlorobenzotrifluoride. An unidentified impurity is indicated by an asterisk (*).

For those of you who are not satisfied with this short answer, let’s dig a little deeper starting by recapping carbon satellites in ¹H NMR.

Carbon-13 (6 protons, 7 neutrons) is NMR active (I = 1/2), while carbon-12 (6 protons, 6 neutrons) is not (I = 0). The natural abundance of ¹³C is 1.1%. As ¹³C and ¹²C are the only stable isotopes, 98.9% of all natural carbon atoms are ¹²C.

Let's think of toluene as an analyte for a theoretical ¹H NMR spectrum. The CH₃ group is expected to appear as a singlet in the aliphatic range at ~2 ppm. Well, while this is what probably everybody would agree on, that's actually not very precise. In the carbon coupled ¹H NMR spectrum what is observed as a singlet is the proton resonance for those protons that are attached to a carbon-12 atom, ¹²CH₃. 1.1% of the carbon atoms in the methyl group in toluene consist of carbon-13, ¹³CH₃, which is observed as a doublet (number of neighboring NMR active nuclei + 1) - typical coupling constant values for aliphatic CH groups are ¹J_CH ≈ 150 Hz. This doublet will appear as two low (0.55%) intense peaks left and right of the ¹²CH₃ signal. Therefore, they are called carbon satellites - they surround the "main peak". If you run carbon decoupled, ¹H NMR experiments, these carbon satellites will disappear.

Now, for ¹⁹F NMR due to an isotopic effect, the ¹³CF₃ group in DCBTF is observed as a doublet which is not centered at the same chemical shift as the ¹²CF₃ singlet. We can observe the ¹³CF₃ group signal as a doublet with a coupling constant of ¹J_CF = 272 Hz, centered at −63.19 ppm (doublet A) - shifted by Δ = 0.12 ppm from the carbon-12 trifluoromethyl resonance.

This effect is smaller but still noticeable over two bonds. The ¹³C¹²CF₃. doublet with a coupling constant of ²J_CF = 31 Hz is centered at −63.08 ppm (doublet B) - shifted by Δ = 0.01 ppm from the ¹²C analogue.

In ¹H NMR this isotope effect is also present, but it is so small that it becomes neglectable, which is why the center of the ¹³CH multiplet overlaps with the ¹²CH resonance.

If you have any questions regarding carbon decoupling capabilities or ¹⁹F NMR analysis on Nanalysis benchtop NMR instruments, please don't hesitate to contact us via sales@nanalysis.com