13C satellites

Recently I was asked about the small peaks appearing symmetrically around a large peak in a 1D ¹H spectrum. These peaks are called ¹³C satellites and arise from heteronuclear coupling to the small amount of ¹³C naturally present.

The figure below shows a 1D ¹H spectrum of o-dichlorobenzene in acetone-d₆. The aromatic ODCB signals appear at 7.57 and 7.35 ppm, the residual undeuterated acetone signal at 2.05 ppm, and the chemical shift reference compound tetramethylsilane (TMS) at 0.00 ppm. I'm not sure what causes the peak at 3.59 ppm. Note the small peaks either side of the TMS signal, these are the ¹³C satellites. The insets show expansions of the acetone and TMS peaks, including their ¹³C satellite peaks.

Typically we think of the peak splitting in NMR spectra as being due exclusively to nearby hydrogen atoms because this is the most common case, but coupling from other elements is possible as well. In fact coupling to carbon is present in all 1D ¹H spectra of organic compounds. However, since the natural abundance of the NMR active carbon isotope, ¹³C, is only 1.1% the multiplets due to carbon coupling account for just 1.1% of the total intensity. In the TMS inset above the two small peaks either side of the large central peak are due to the 1.1% of the TMS methyl groups where the carbon atom is ¹³C. These two peaks form a doublet where the ¹J_CH coupling is 118.1 Hz. The large central peak is due to the remaining 98.9% of the TMS methyl groups where the carbon atom is ¹²C, which cannot be detected by NMR. Since the ¹³C satellites combined account for 1.1% of the signal, each peak in the doublet is 0.55% of the intensity of the original signal.

The acetone inset shows the peak from the undeuterated acetone. This is a -CHD₂ peak split into a 1:2:3:2:1 pentet due to ²H coupling, like those of methanol and DMSO discussed previously. The ¹³C satellites of this peak can be observed showing a ¹J_CH coupling of 126.0 Hz. Interestingly, these peaks show the same splitting pattern as the central peak due to the ²H coupling, so these multiplets arise from ¹J_CH coupling and ²J_DH coupling.

Note that the ¹J_CH coupling is different for the acetone and TMS peaks. ¹J_CH couplings cover a range of 120-250 Hz and it is possible to use the size of ¹J_CH coupling to assign peaks and stereochemistry¹. It is also possible to use the intensity of the ¹³C satellites to determine sample concentration².

References

1. R.H. Contreras, J.E. Peralta
Angular dependence of spin–spin coupling constants
Prog. NMR Spec 2000 37(4) 321-425

2.D.S. Dalisay, T.F. Molinski
NMR Quantitation of Natural Products at the Nanomole Scale
J. Nat. Prod. 2009 72(4) 739–744