Since organic compounds are largely based on carbon, ¹³C NMR spectroscopy is a pretty important tool for studying organic compounds. The ¹³C isotope is the only isotope of carbon that is "NMR-active"; ¹²C and ¹⁴C atoms do not absorb radio waves in a magnetic field. The ¹³C NMR spectrum of cyclohexane is shown below.

Figure NMR2.1.¹³C NMR spectrum of cyclohexane.¹

Cyclohexane is constructed of a ring of six carbon atoms.  There are two hydrogen atoms attached to each of the carbons.

Notice these features of the spectrum:

• the x axis corresponds to the energy or frequency of the radio waves that are absorbed.
• the scale on the x axis runs from roughly zero to two hundred "parts per million", abbreviated ppm.
• no scale is given on the y axis, which corresponds to the amount of radio wave "light" absorbed at a given frequency. This situation is different from IR spectroscopy, in which transmittance is plotted rather than absorbance.
• in¹³C NMR spectroscopy, a peak looks like a spike that rises from the baseline.

There are some peculiar terms used in NMR spectroscopy that are not used in IR spectroscopy. These terms arose from the use of magnetic fields in measuring these spectra:

• the high frequency end of the spectrum, which is high ppm and is found at the left-hand end of the x axis, is termed "downfield".
• the low frequency end of the spectrum, which is low ppm and is found at the right-hand end of the x axis, is termed "upfield".
• the x-axis value is not referred to as "frequency" but rather as "chemical shift" or just "shift".

In cyclohexane, only one frequency of radio waves is absorbed by the carbon atom, and that is at about 27 ppm. Other frequencies could be absorbed by the hydrogen atoms, but hydrogen atoms absorb at very different frequencies from carbon atoms, so they wouldn't be detected in a ¹³C NMR spectrum.

Problem NMR2.1.

Which peak would show up farther to the right in the NMR spectrum?

a)  10 pmm or 27 ppm        b) 122 ppm or 64 ppm        c) 196 ppm or 158 ppm

Problem NMR2.2.

Which peak would show up farther downfield in the NMR spectrum?

a)  17 pmm or 63 ppm        b) 201 ppm or 155 ppm        c) 71 ppm or 43 ppm

¹³NMR spectra are relatively simple. They just show several vertical lines, each representing a carbon atom in a different environment within a molecule. Obtaining the spectrum is more complicated. The problem is, ¹³C is only a minor isotope of carbon. It only represents about 0.1 percent of carbon on earth. In contrast, ¹²C makes up about 98.9 percent of carbon on earth; that's why the periodic table shows an atomic mass of carbon of almost exactly 12 amu. That's a low enough level of ¹³C that an individual molecule of the size that you will see in this course probably doesn't contain any ¹³C. So, if you take a sample of an organic compound and bounce some radio waves off it to measure its ¹³C NMR spectrum, you are really just hearing from those molecules that happen to contain an atom of that isotope; the rest are silent. The experiment is still possible, because even a drop of an organic liquid contains billions of molecules, and millions of those contain some ¹³C. However, it does make the response a little fainter.

Later, we will look at ¹H NMR spectroscopy. ¹H, or protium, is by far the most common isotope of hydrogen. The atomic mass listed in the periodic table is very close to 1 amu, because the average of all the hydrogen isotopes is dominated by this one major contributor. Every organic molecule is very likely to contain this isotope. As a result, when we shine radio waves of the right frequency to detect ¹H on a sample of an organic compound and listen for a response, it pretty much shouts back at us. With carbon, we have to lean in close and listen very carefully in the hopes that it can muster a tiny little "yop".

There are some technical things that an NMR instrument does to amplify that faint carbon signal. One thing we can do is to actually make use of those abundant protons in the molecule. We can send in a pulse of radio waves that the protons will absorb just before we try to detect the carbon. When we do that, the hydrogens can actually help nearby carbons respond more strongly. There are consequences to that technique. It actually distorts the peaks that we see in the spectrum, because carbons that are bonded to more hydrogens respond much more strongly than carbons that are not. It is sometimes difficult to see carbons that have no hydrogens attached. Also, this distortion turns ¹³C NMR into a qualitative technique, generally. For the most part, you can't use it to measure how many of one carbon you have compared to another. In contrast, in ¹H NMR, we take quantitative measurements all the time.

Reference: 1. SDBSWeb : http://riodb01.ibase.aist.go.jp/sdbs/ (National Institute of Advanced Industrial Science and Technology of Japan, 15 August 2008)

This site is written and maintained by Chris P. Schaller, Ph.D., College of Saint Benedict / Saint John's University (with contributions from other authors as noted).  It is freely available for educational use.

Structure & Reactivity in Organic, Biological and Inorganic Chemistry by Chris Schaller is licensed under a Creative Commons Attribution-NonCommercial 3.0 Unported License. 

Send corrections to cschaller@csbsju.edu

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