The basics of ¹³C-NMR spectroscopy

The magnetic moment of a ¹³C nucleus is much weaker than that of a proton, meaning that NMR signals from ¹³C nuclei 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: more sample is required, and often the data from hundreds of scans must be averaged in order to bring the signal-to-noise ratio down to acceptable levels. Unlike ¹H-NMR signals, the area under a ¹³C-NMR signal cannot be used to determine the number of carbons to which it corresponds. This is because the signals for some types of carbons are inherently weaker than for other types – peaks corresponding to carbonyl carbons, for example, are much smaller than those for methyl or methylene (CH₂) peaks. Peak integration is generally not useful in ¹³C-NMR spectroscopy, except when investigating molecules that have been enriched with ¹³C isotope (see section 5.6B).

The resonance frequencies of ¹³C nuclei are lower than those of protons in the same applied field - in a 7.05 Tesla instrument, protons resonate at about 300 MHz, while carbons resonate at about 75 MHz. This is fortunate, as it allows us to look at ¹³C signals using a completely separate 'window' of radio frequencies. Just like in ¹H-NMR, the standard used in ¹³C-NMR experiments to define the 0 ppm point is tetramethylsilane (TMS), although of course in ¹³C-NMR it is the signal from the four equivalent carbons in TMS that serves as the standard. Chemical shifts for ¹³C nuclei in organic molecules are spread out over a much wider range than for protons – up to 200 ppm for ¹³C compared to 12 ppm for protons (s. 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 to the double bond to oxygen.

While broadband decoupling results in a much simpler spectrum, useful information about the presence of neighboring protons is lost. However, another modern NMR technique called DEPT (Distortionless Enhancement by Polarization Transfer) allows us to determine how many hydrogens are bound to each carbon. For example, a DEPT experiment tells us that the signal at 171 ppm in the ethyl acetate spectrum is a quaternary carbon (no hydrogens bound, in this case a carbonyl carbon), that the 61 ppm signal is from a methylene (CH₂) carbon, and that the 21 ppm and 14 ppm signals are both methyl (CH₃) carbons. The details of the DEPT experiment are beyond the scope of this text, but DEPT information will often be provided along with ¹³C spectral data in examples and problems.