13C NMR
Carbon NMR
Hydrogen atoms are not the only atoms that can be observed using an NMR. Carbon atoms can also be seen. But, not all isotopes of carbon can be seen. Sadly, the nucleus of the most common isotope of carbon, carbon-12, does not have a magnetic moment. But, carbon-13, because of its odd number of nucleons, does have a nucleus with a magnetic moment. We call a carbon NMR a 13C NMR (pronounced C-13 NMR).
Only about 1.1% of the carbon in nature is carbon-13. So, for a given organic sample, each carbon atom only has a 1.1% chance of being observed in an NMR. Because of this, it is a little more difficult to obtain a carbon NMR. Therefore, we usually need a more concentrated sample to scan and we usually scan the sample for a longer period of time than for a proton NMR.
Carbon NMR chemical shifts
The chemical shifts for carbon atom peaks are usually 15-20 times greater than their proton counterparts. An aldehyde proton shows up around 9-10 ppm in a 1H NMR spectrum, whereas the carbon atom of an aldehyde carbonyl shows up around 190 ppm in a 13C NMR spectrum. This is because the carbon atoms are one bond closer to deshielding groups than are the protons.
The typical 13C NMR spectrum goes from 0-200 ppm compared to 0-10 ppm for 1H NMR. In general, it is best to view a 13C NMR spectrum as being broken into four different portions, 0-50 ppm, 50-100 ppm, 100-150 ppm, and 150-200 ppm. Become familiar with what types of carbon atoms appear in these different portions of the spectrum. These divisions are not perfect, so you need to have a little flexibility when interpreting a 13C NMR spectrum.
In the 13C NMR spectrum for 1-penten-3-one, we have five different carbon atoms. The carbonyl, C, appears near 200 ppm. The two C=C carbon atoms, A and B, appear between 100-150 ppm, and the two aliphatic ethyl carbon atoms, D and E, appear under 50 ppm.
10. Which region in the 13C NMR spectrum would you find the peak for each carbon atom marked with “*”: 0-50 ppm, 50-100 ppm, 100-150 ppm, or >150 ppm?
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Integrations
Carbon NMR spectra are not usually integrated to tell how many carbon atoms are involved to make each peak. This is because not all carbon atoms are created equally. Some types of carbon atoms make larger peaks than other types of carbon atoms. The more hydrogen atoms attached to a carbon atom, the larger the carbon peak tends to be. CH3 and CH2 carbon peaks are the tallest, CH peaks are sometimes a little smaller, and quaternary carbon peaks are usually noticeably smaller. In the above spectrum for 1-penten-3-one, notice that carbon B, a CH, is a slightly smaller peak than carbon A, a CH2. The quaternary carbonyl carbon, C, is the smallest peak in the spectrum. Also, there is only one C carbon and one B carbon, but carbon B is twice as tall as carbon C. That is why we cannot integrate this spectrum.
Off-Resonance Decouples 13C NMR spectra
So far, the 13C NMR peaks have always been single peaks. This is a common way to do 13C NMR. But, it is possible to get more information out of the peaks by letting them become multiplets.
Since the signals in 13C NMR are so weak, an NMR method is performed to put the entire signal from a 13C nucleus in one spot, to make the peaks as large as possible. This method is called proton-spin decoupling. In this method, broadband radio noise is transmitted to the proton magnets causing them to be in resonance and flip like crazy. The carbon nuclear magnet then sees an average of these proton magnetic spin states. The carbon-hydrogen splitting is eliminated.
To get more information from our peak, this broadband radio noise can be turned off and the proton magnets can then be in either a spin up or a spin down state. Allowing the carbon nucleus to couple with the protons is called off-resonance decoupling.
If a 13C nuclear magnet is next to one proton, and that proton can be either spin up or spin down, the N+1 rule would tell us that 13C nucleus would show two peaks on the NMR, a doublet. So, the peak for a 13C nucleus with zero protons attached is a singlet, with one proton attached is a doublet, two protons attached is a triplet, and three protons attached is a quartet.
Wow! This is wonderful. If we have the off-resonance decoupled 13C NMR spectrum of an organic compound, we can tell how many types of carbon atoms there are in the molecule as well as how many hydrogen atoms are on each carbon atom. With a 1H NMR spectrum, we can do a little thinking and head scratching and figure out which of these carbon atoms are next to each other.
With so much information, why is a proton decoupled 13C NMR spectrum the most common type to run? Notice that when the peaks of the proton decoupled spectrum are allowed to be split into doublets, triplets, or quartets, the size of those peaks gets smaller because the area for the one peak is divided into two, three, or four smaller peaks. This can be a problem because split 13C NMR peaks become very difficult to see. Sometimes, a spectrum would need to be run for hours to get the carbon signals to come out of the baseline noise of a spectrum. If an off-resonance decoupled spectrum is run, it could simply take too long to make it useful. Also, if the multiplets are too close together and they overlap, they can make things more confusing.
Answers
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