6.8: Principles of ¹³C NMR Spectroscopy

Most of what we have learned about ¹H-NMR spectroscopy also applies to ¹³C-NMR, although there are several important differences.

Number of signals in ¹³C-NMR spectroscopy

In analogy to ¹H NMR, ¹³C NMR can be used to obtain information about the C-C skeleton in an organic compound. The number of signals in a ¹³C NMR spectrum is related to the number of nonequivalent carbons in the chemical structure of the compound. For example, the ¹³C NMR spectrum of ethyl acetate shows four signals, one for each of the nonequivalent carbons.

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, and thus we do not see spin-spin coupling between neighboring carbons in a ¹³C-NMR spectrum. There is, however, 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. 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. Above is the proton-decoupled¹³C-NMR spectrum of ethyl acetate, showing the expected four signals, one singlet for each of the carbons.

 

Example

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

1. toluene
2. 2-pentanone
3. para-xylene

Solution

 

¹³C NMR Spectral window

One of the greatest advantages of ¹³C-NMR compared to ¹H-NMR is the breadth of the spectrum - recall that carbons resonate from 0-220 ppm relative to the TMS standard, as opposed to only 0-12 ppm for protons. Because of this, ¹³C signals rarely overlap, and we can almost always distinguish separate peaks for each carbon, even in a relatively large compound containing carbons in very similar environments. In the proton spectrum of 1-heptanol, for example, only the signals for the alcohol proton (H_a) and the two protons on the adjacent carbon (H_b) are easily analyzed. The other proton signals overlap, making analysis difficult.

In the ¹³C spectrum of the same molecule, however, we can easily distinguish each carbon signal, and we know from this data that our sample has seven non-equivalent carbons. (Notice also that, as we would expect, the chemical shifts of the carbons get progressively smaller as they get farther away from the deshielding oxygen.)

This property of ¹³C-NMR makes it very helpful in the elucidation of larger, more complex structures.

¹³C NMR Chemical Shifts for functional groups

The Carbon NMR is used for determining functional groups using characteristic shift values. ¹³C chemical shifts are greatly affected by electronegative effects. If a H atom in an alkane is replaced by substituent X, electronegative atoms (O, N, halogen), ¹³C signals for nearby carbons shift downfield (left; increase in ppm) with the effect diminishing with distance from the electron withdrawing group. Figure 13.11.1 shows typical ¹³C chemical shift regions of the major chemical class.

Figure 13.11.1: ¹³C Chemical shift range for organic compound

 

¹³C NMR Chemical Shifts Intensity

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, and therefore, the most useful information provided by the ¹³C-NMR spectrum is the number and chemical shift of the signals, but no integration or multiplicity of signals.

 

Contributors and Attributions

• Prof. Steven Farmer (Sonoma State University)

• Organic Chemistry With a Biological Emphasis by Tim Soderberg (University of Minnesota, Morris)

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

• Layne Morsch (University of Illinois Springfield)