6.8 ¹³C NMR Spectroscopy

For a carbon element, the most abundant isotope ¹²C (with ~99% natural abundance) does not have a nuclear magnetic moment and thus is NMR-inactive. The C NMR is therefore based on the ¹³C isotope, which accounts for about 1% of carbon atoms in nature and has a magnetic dipole moment just like a proton. The theories we have learned about ¹H NMR spectroscopy also apply to ¹³C NMR, but with several important differences in the spectrum.

The magnetic moment of a ¹³C nucleus is much weaker than that of a proton, meaning that ¹³C NMR signals are inherently much weaker than proton signals. This, combined with the low natural abundance of ¹³C, means that it is much more difficult to observe carbon signals. Usually, samples with a high concentration and large number of scans (thousands or more) are required to bring the signal-to-noise ratio down to acceptable levels for ¹³C NMR spectra.

Chemical Equivalent

For carbons that are chemical equivalent, they only show one signal in ¹³C NMR, similar to protons for ¹HNMR. Considering this, it is very important to be able to identify equivalent carbons in the structure to interpret the ¹³C NMR spectrum correctly. Taking toluene as an example, there are five sets of different carbon atoms (shown in different colors), so there are five signals in the ¹³C NMR spectrum of toluene.

Chemical Shift

¹³C nuclei have a different value of g (the magnetogyric ratio) compared to 1H nuclei, so the resonance frequencies of 13C nuclei are different from those of protons in the same applied field (referring to formula 6.4, in section 6.5). In an instrument with a 7.05 Tesla magnet, protons resonate at about 300 MHz, while carbons resonate at about 75 MHz. This allows us to look at ¹³C signals using a completely separate ‘window’ of radio frequencies. Just like in ¹H NMR, tetramethylsilane (TMS) is also used as the standard compound in ¹³C NMR experiments to define the 0 ppm; however, it is the signal from the four equivalent carbon atoms in TMS that serves as the standard. Chemical shifts for ¹³C nuclei in organic molecules are spread out over a much wider range of about 220 ppm  (see Table 6.3).

The chemical shift of a ¹³C nucleus is influenced by essentially the same factors that influence the chemical shift of a proton: the deshielding effect of electronegative atoms and anisotropy effects tend to shift signals downfield (higher resonance frequency, with higher chemical shifts). In addition, sp² hybridization results in a large downfield shift. The ¹³C NMR signals for carbonyl carbons are generally the furthest downfield (170–220 ppm) due to both sp² hybridization and the double bond to oxygen.

Integration and Coupling in ¹³C NMR

Unlike ¹H NMR, the area under a ¹³C NMR signal cannot easily be used to determine the number of carbons to which it corresponds. The signals for some types of carbons are inherently weaker than for other types; for example, peaks corresponding to carbonyl carbons are much smaller than those for methyl or methylene (CH₂) peaks. For this reason, signal integration is generally not useful in ¹³C NMR spectroscopy.

Because of the low natural abundance of ¹³C nuclei, the spin-spin coupling between two nonequivalent ¹³C atoms is negligible. ¹³C nuclei are coupled to nearby protons, which results in complicated spectra. For clarity, chemists generally use a technique called broadband decoupling, which essentially ‘turns off’  C-H coupling, resulting in a spectrum in which all carbon signals are singlets. Below is the proton-decoupled ¹³C NMR spectrum of ethyl acetate in CDCl₃ (Fig. 6.8a), showing the expected four signals, one for each of the carbons.

For our purposes, ¹³C NMR spectra are usually used as supporting information to confirm the structure of a compound.

Exercises 6.2

Below are ¹³C NMR spectra for methylbenzene (common name toluene) and methyl methacrylate.  Refer to Table 6.3 to match the spectra to the correct structure.