Spin-spin coupling is an important facet of 1H NMR spectroscopy, as crucial details about the structure of a molecule are revealed based on the pattern of multiplets observed. In general, the signal for a group of equivalent protons will be split into a multiplet based on the n+1 rule, where n is the number of equivalent protons that are adjacent to the protons. For example, the signal of the CH2 protons in an ethyl group will be observed as a quartet (adjacent to three equivalent protons; 3 + 1) while the signal for the CH3 protons in the same ethyl group will be a triplet (adjacent to two equivalent protons; 2 + 1).
The same rules also apply to any NMR active nuclei with a spin of ½, even if the spin-spin coupling is happening with different nuclei. The 19F NMR spectrum, recorded on the NMReady-60PRO, of lithium hexafluorophosphate is displayed in Figure 1.
The spectrum consists of a doublet centred around −72.54 ppm with a large coupling constant of 707.5 Hz. The doublet pattern is a result of the equivalent fluorine atoms coupling to the singular phosphorus atom.
On the other hand, the 31P NMR spectrum of lithium hexafluorophosphate revealed a septet at −141.20 ppm with a nearly identical coupling constant of 707.6 Hz, as seen in Figure 2.
In this case, the NMR signal is split into a septet as a result of the phosphorus atom coupling to the six equivalent fluorine atoms.
As you can see, spin-spin coupling extends beyond 1H NMR spectroscopy and the same rules can be applied to analyze and interpret the spectrum. Don’t hesitate to contact us if you have any questions about multinuclear NMR spectroscopy or if you want to see how the NMReady-60PRO can be incorporated into your workflow!