6.2: C-13 NMR Spectroscopy- Signal Averaging and FT-NMR

Learning Objectives

• Learn how ¹H NMR and ¹³C NMR are similar and different.
• Understand the basics of how ¹³C NMR works.

Why are we talking about ¹³C NMR when the ¹²C isotope of carbon - which accounts for up about 99% of the carbons in organic molecules?  The ¹²C isotope does not have a nuclear magnetic moment, and thus is NMR-inactive (cannot be seen by the NMR). Fortunately for organic chemists, however, the ¹³C isotope, which accounts for most of the remaining 1% of carbon atoms in nature, has a magnetic moment just like protons, which is why we will be discussing ¹³C NMR spectroscopy and not ¹²C NMR spectroscopy. Most of what the theory we have learned about ¹H-NMR spectroscopy also applies to ¹³C-NMR, although there are several important differences.

The basics of ¹³C NMR spectroscopy

The magnetic moment of a ¹³C nucleus is much weaker than that of a proton. This means that NMR signals from ¹³C nuclei are inherently much weaker than proton signals. Combining the weaker magnetic moment 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. 

Chemical Shift

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 (see Table 3 for a list of typical ¹³C NMR chemical shifts). This is also fortunate, because it means that the signal from each carbon in a compound can almost always be seen as a distinct peak, without the overlapping that often plagues ¹H NMR spectra. The chemical shift of a ¹³C nucleus is influenced by essentially the same factors that influence a proton's chemical shift: bonds to electronegative atoms and diamagnetic anisotropy effects tend to shift signals downfield (higher resonance frequency). 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. Symmetry will also play a role in how many signals are observed on a ¹³C NMR spectrum. If there is no symmetry, then each carbon should show up as a signal in the spectrum. If symmetry is present in the molecule, then there will be less than the total number of carbons in the molecule. Only non-equivalent carbons will appear as a signal in ¹³C NMR. 

Example \(\PageIndex{1}\)

How many sets of non-equivalent carbons are there in ethyl benzene?

Solution

There are 5 different carbons in ethyl benzene. There is symmetry in ethyl benzene in the aromatic ring.  On the labeled molecule, there are two carbon 3's and two carbon 4's due to the symmetry of the molecule. 

Integration

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. There are some quantitative ¹³C NMR experiments that when enriched with ¹³C can be integrated, but typically it is not done.

Spin-spin Splitting

Because of the low natural abundance of ¹³C nuclei, it is very unlikely to find two ¹³C atoms near each other in the same molecule, which means that spin-spin coupling is not observed between neighboring carbons in a ¹³C NMR spectrum. However,there is heteronuclear coupling between ¹³C carbons and the hydrogens to which they are bound. Carbon-proton coupling constants are very large, on the order of 100 – 250 Hz. Proton-coupled ¹³C spectra show complex overlapping multiplets, which makes for a very difficult interpretation. For clarity, chemists generally use a technique called broadband decoupling , which essentially 'turns off' C-H coupling, resulting in a spectrum where all carbon signals are singlets. Below is the proton-decoupled ¹³C NMR spectrum of ethyl acetate, showing the expected four signals, one for each of the carbons with no spin-spin splitting. 

While broadband decoupling results in a much simpler spectrum, useful information about the presence of neighboring protons is lost. However, there are other ¹³C NMR experiments that can give more information, such as Distortionless Enhancement by Polarization Transfer (DEPT) allows us to determine how many hydrogens are bound to each carbon. 

Exercise \(\PageIndex{1}\)

How many sets of non-equivalent carbons are there in:

a. 2-pentanone: 

b. para-xylene: 

c. serotonin: 

Answer

a. 5 non-equivalent carbons

b. 3 non-eqivalent carbons

c. 10 non-equivalent carbons

Contributors and Attributions

• Prof. Steven Farmer (Sonoma State University)

• Chris P Schaller, Ph.D., (College of Saint Benedict / Saint John's University)