Introduction
As we have seen, NMR spectroscopy is not limited to hydrogen nuclei. Any nucleus with an odd number of protons and/or neutrons is NMR active. Next to hydrogen, the most commonly studied nucleus is C-13. In 1H-NMR we observe signals arising from hydrogen nuclei and we infer the presence of the carbon atoms to which they are attached. Thus when we see a spin-spin coupling pattern that consists of two doublets, we deduce the molecular fragment H-C-C-H, inferring the presence of the carbon atoms. In 13C-NMR we observe carbon atoms directly.
Chemical Shifts
The electromagnetic radiation required to promote 1H nuclei from the a to the b state must have energy that lies in the radio frequency range. While the same is true for 13C nuclei, the frequency range is different. Figure 1 attempts to put the energies associated with the transitions of these two nuclei in perspective. The frequencies specified in the figure are for a magnetic field strength of 9.4 Tesla. (9.4 Tesla is approximately 100,000 times the magnetic field stength of the earth.) At that magnetic field strength the resonance frequency of 1H nuclei is around 400 MHz with a chemical shift range of approximately 10 ppm; 13C nuclei absorb around 100 MHz with a chemical shift range of over 200 ppm.
Figure 1
What's Your Favorite Station?
The chemical shift of a carbon atom depends upon the electronic environment around that carbon in the same way that the chemical shift of a proton does. However, since the chemical shift range is much greater for carbon than hydrogen, assignment of a particular 13C resonance can be more problematic. For example, most aromatic hydrogens resonate between 7 and 8 ppm, while aromatic carbons absorb between 100 and 170 ppm! Table 1 lists the chemical shifts of a select set of carbon atoms in different chemical environments. A more complete summary of chemical shifts is presented in Table 2.
Table 1
A 13C-NMR Correlation Table
Integration
Since the intensity of a 13C resonance depends upon the number of hydrogens attached to the carbon atom, integration of 13C spectra is subject to error. Consequently, it is not normally done.
Spin-Spin Coupling
Spin-spin coupling occurs between13C nuclei in the same way that it does between 1H nuclei. However, since the natural abundance of 13C is only 1%, the probability of finding a 13C- 13C fragment within a molecule is roughly 1 in 10,000 (0.01 x 0.01). Consequently, 1JC-C coupling between is not ordinarily observed. However, 1JC-H coupling is very common. Furthermore, the n+1 rule applies, which is to say the 13C resonance for a 13CH fragment is a doublet, while that for a 13CH2 fragment is a triplet, and a 13CH3 fragment is appears as a quartet. The 13C-NMR spectrum of chloromethane, CH3Cl, consists of a quartet centered at 28.7 ppm with a C-H coupling constant of 147 Hz, i.e. d = 28.7 ppm, 1JC-H = 147 Hz. While C-H coupling may provide useful insights into molecular structure, most 13C spectra are recorded in the broad band decoupling mode. In this mode spin-spin coupling does not occur and each unique carbon atom appears as a singlet. Figure 2 compares the coupled and decoupled 13C-NMR spectra of chloromethane, CH3Cl.
Figure 2
13C-NMR Spectra of Chloromethane
Bottom line Broad-band decoupled 13C-NMR spectra contain 1 signal for each unique carbon atom in a molecule. The chemical shift of the signal is characteristic of the carbon atom's chemical and electronic environment.
Exercise 1 Indicate the number of peaks in the 13C-NMR spectrum of compounds A-E.
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Exercise 3 The acid catalysed dehydration of 2-methylcyclohexanol is a standard experiment in many undergraduate organic chemistry laboratory manuals. It has been the subject of several articles in the Journal of Chemical Education. This yields a mixture of 1-methylcyclohexene and 3-methylcyclohexene along with less than 5% of an isomeric alkene whose 13C-NMR spectrum is shown below.
Which alkene would you expect to be formed in greater amount? 1-methylcyclohexene 3-methylcyclohexene
Suggest a structure for the minor isomer that is consistent with the 13C-NMR data.
The elucidation of molecular structure by NMR spectroscopy is similar to putting a jigsaw puzzle together. The peaks in a spectrum allow the chemist to identify molecular sub-units that are analogous to the pieces of a puzzle. For this reason organic chemists think of molecules in terms of molecular fragments or groups, for example, a methyl group, CH3, a carbonyl group, C=O, or an hydroxyl group, OH. The more common groups that chemists deal with are outlined in the topic functional groups. Alternatively, it may be useful to think about these sub-units as analagous to letters of an alphabet. From this perspective, a word corresponds to a molecular structure that is constructed by putting the molecular letters together in a specific way. Table 2 lists a number of molecular sub-units that are part of the organic chemist's alphabet. It is in your best interests to familiarize yourself with these fragments, i.e memorize them (gasp!), so that you can start to think like an organic chemist (gasp!). Note that some of the molecular fragments in the table are "active", i.e. give a signal, only in 13C-NMR spectra (e.g. C=O), some only in 1H-NMR spectra (e.g. OH), and others in both (e.g. CH3). Part of the fun and challenge of interpreting an NMR spectrum is developing the ability to infer the presence of a sub-unit that is not NMR-active.
Table 2
Common Pieces of the Molecular Puzzle
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Exercise 4 Draw structures of molecules that contain only the molecular fragments indicated. You may use the same fragment more than once in a structure.
a. methine and methyl (2 examples)
b. methine, methylene, and methyl (2 examples)
c. methylene and methyl (three examples)
d. methylene, methyl, and carbonyl (three examples)
e. methylene, phenyl, and carboxyl
f. methine, phenyl, and carbonyl
As an example of the way in which 13C-NMR and 1H-NMR complement each other, consider the molecules methyl chloromethyl ether and chloroacetone:
. The 1H-NMR spectra of these compounds each contains two singlets that integrate in a 3/2 ratio. However, the 13C-NMR spectrum of the ether contains only two peaks, while that of the ketone contains three. While neither the oxy group nor the carbonyl group is active in 1H-NMR, the carbonyl group is active in 13C-NMR . The power of 13C-NMR spectroscopy extends beyond its ability to tell you how many unique carbon atoms there are in a molecule. Using a technique called DEPT spectroscopy, it is possible to distinguish between carbons that have different numbers of hydrogen atoms attached to them, i.e. between C, CH, CH2, and CH3 fragments.
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