Nuclear Magnetic Resonance

¹³C-NMR

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.

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, ¹J_C-C coupling between is not ordinarily observed. However, ¹J_C-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, ¹J_C-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 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 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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