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This page takes an introductory look at how you can get useful information from a C-13 NMR spectrum.
Introduction
Looking closely three13C NMR spectra below. O13C NMR spectrum for ethanol
The NMR spectra on this page were produced from graphs taken from the Spectral Data Base System for Organic Compounds (SDBS) no National Institute of Materials and Chemical Research in Japan.
Remember that each peak identifies a carbon atom in a different environment within the molecule. In this case, there are two peaks because there are two different environments for carbons. The carbon in the CH3group is bonded to 3 hydrogens and one carbon. The carbon in the CH2group is bonded to 2 hydrogens, a carbon and an oxygen. So which peak is which?
You may recall from the introductory page that the external magnetic field experienced by carbon nuclei is affected by the electronegativity of the atoms bound to them. The effect of this is that the chemical shift of carbon increases if you bond an atom like oxygen to it. This means that the peak at around 60 (the highest chemical shift) is due to the CH2 group because it has a more electronegative atom attached.
In principle, you should be able to calculate the fact that carbon bonded to oxygen will have the greatest chemical shift. In practice, you always work with tables of chemical shift values for different groups (see below).
What if you needed to resolve this? Electronegative oxygen pulls electrons away from the carbon nucleus, leaving it more exposed to any external magnetic field. That means you'll need a smaller external magnetic field to bring the nucleus into resonant condition than if it's attached to less electronegative things. The smaller the required magnetic field, the greater the chemical shift.
A table of typical chemical changes in C-13 NMR spectra
| carbon environment | chemical shift (ppm) |
|---|---|
| C=O (in ketones) | 205 - 220 |
| C=O (in aldehydes) | 190 - 200 |
| C=O (in acids and esters) | 170 - 185 |
| C in aromatic rings | 125 - 150 |
| C=C (in alkenes) | 115 - 140 |
| RCH2OH | 50 - 65 |
| RCH2Cl | 40 - 45 |
| RCH2NH2 | 37 - 45 |
| R3CH | 25 - 35 |
| CH3CO- | 20 - 30 |
| R2CH2 | 16 - 25 |
| RCH3 | 10 - 15 |
In the table, the "R" groups will not necessarily be simple alkyl groups. In each case there will be a carbon atom attached to the one shown in red, but there may be other things substituted in the "R" group.
If a substituent is very close to the carbon in question, and very electronegative, this may slightly affect the values given in the table. For example, ethanol peaks at around 60 because of the CH2OH group. No problem! Also has a peak due to RCH3group. The "R" group this time is CH2Oh. The electron-attracting effect of the oxygen atom slightly increases the chemical shift from that shown in the table to a value of about 18. A simplification of the table:
| carbon environment | chemical shift (ppm) |
|---|---|
| C-C | 0 - 50 |
| C-O | 50 - 100 |
| C=C | 100 - 150 |
| C=O | 150 - 200 |
This could, of course, change and other programs might want something similar. The only way to find out is to check your resume and recent question books to see if you have received chemical change tables or not.
O13C NMR spectrum for but-3-en-2-one. This is also known as 3-buten-2-one (among many other things!)
Here is the structure for the compound:
You can pick all the peaks in this compound using the simplified table above.
- The peak at just under 200 ppm is due to a carbon-oxygen double bond. The two peaks at 137 ppm and 129 ppm are due to the carbons at either end of the carbon-carbon double bond. And the peak at 26 is the methyl group which, of course, is joined to the rest of the molecule by a single carbon-carbon bond. If you want to use the most accurate table, you should give it some more thought - and especially worry about values not always exactly matching those in the table!
- The peak carbon-oxygen double bond for the ketone group has a slightly lower value than the table suggests for a ketone. There is an interaction between the carbon-oxygen and carbon-carbon double bonds in the molecule that slightly affects the value. This is not something we need to go into in detail for the purposes of this topic.
- You should be prepared to find small discrepancies of this sort in more complicated molecules - but don't worry about this for examination purposes at this level. Your examiners must provide offset values that exactly match the compound you received.
- The two peaks of the carbons in the carbon-carbon double bond are exactly where they should be. Note that they are not exactly in the same environment and therefore do not have the same offset values. The one closest to the carbon-oxygen double bond has the highest value.
- And the methyl group at the end has exactly the kind of value you'd expect for one bonded to C=O. The table gives you a range of 20 to 30, and that's where it is.
One last important thing to note. There are four carbons in the molecule and four peaks because they are all in different environments. But they are not all the same height. On the C-13 NMR, youit cannotdraw simple conclusions from the heights of the various peaks.
1-Methylethyl propanoate is also known as isopropyl propanoate or isopropyl propionate.
Here is the structure of 1-methylethyl propanoate:
two single peaks
There are two very simple peaks in the spectrum that can be easily identified in the second table above.
- The peak at 174 is due to a carbon in a carbon-oxygen double bond. (Looking at the more detailed table, this peak is due to carbon in a carbon-oxygen double bond in an acid or ester.)
- The peak at 67 is due to a different carbon individually bonded to an oxygen. These two peaks are therefore due to:
If you look at the more detailed table of chemical changes, you'll find that a carbon individually bonded to an oxygen has a range of 50 to 65. 67 is obviously a little higher than that.
As before, you should expect these small differences. No table can explain all the tiny differences in the environment of a carbon in a molecule. Different tables will quote slightly different ranges. At that level, you can just ignore this issue!
Before we look at the other peaks, look at the heights of these two peaks we're talking about. Both are due to a single carbon atom in the molecule, yet they have different heights. Again, you cannot read any reliable information directly from peak heights in these spectra.
The three peaks on the right
From the simplified table, all you can tell is that this is due to carbons bonded to other carbon atoms by single bonds. But because there are three peaks, the carbons must be inthreedifferent environments.
The easiest peak to classify is 28. If you look back at the table, it could very well be a carbon bonded to a carbon-oxygen double bond. The table cites the group as \(\ce{CH_3CO-}\), but replacing one of the hydrogens with a simple CH3 group won't make much difference in the displacement value.
The right peak is also quite easy. This is the left methyl group in the molecule. It is attached to an admittedly complicated R group (the rest of the molecule). It is the minimum value given in the detailed table.
The high peak at 22 must be due to the two methyl groups on the right end of the molecule - because that's all that's left. These combine to give a single peak because they are both in exactly the same environment.
If you are looking at the detailed table, then you need to think very carefully which of the rooms you should look at. Without thinking, it's tempting to go to R2CH2 with peaks in the region 16 - 25. But you would be wrong! The carbons we are interested in are the ones in the methyl group, not the ones in the R group. Those carbons are again in the environment: RCH3. The R is the rest of the molecule. The table says these should have peaks in the 10 to 15 range, but our peak is a bit higher. This is due to the presence of the nearby oxygen atom. Their electronegativity is pulling electrons away from the methyl groups - and as we saw above, this tends to slightly increase the chemical shift.
Working with structures from C-13 NMR spectra
So far we have only tried to see the relationship between carbons in particular environments in a molecule and the spectrum produced. We had all the necessary information. Now let's make this a little more difficult - but we'll work with examples much easier! In each example, try to solve it yourself before reading the explanation. How could you tell with just a quick look at a C-13 NMR spectrum (and without worrying about chemical changes) whether you had propanone or propanal (assuming those were the only options)?
Because they are isomers, they each have the same number of carbon atoms, but there is a difference between the environments of the carbons that will have a big impact on the spectra.
In propanone, the two carbons on the methyl groups are in exactly the same environment and therefore will only produce a single peak. This means that the propanone spectrum will only have 2 peaks - one for the methyl groups and one for the carbon in the C=O group. But in propanal, all the carbons are in completely different environments, and the spectrum will have three peaks.
Example \(\PageIndex{3}\): \(C_4H_{10}O\)
There are four alcohols with the molecular formula \(C_4H_{10}O\).
Which one produced the C-13 NMR spectrum below?
You can do this perfectly fine without looking at chemical shift tables.
In the spectrum there are a total of three peaks - this means that there are only three different environments for the carbons, despite there being four carbon atoms.
In A and B, there are four totally different environments. Both would produce four peaks.
In D, there are only two different environments - all methyl groups are exactly equivalent. D would only produce two peaks.
That leaves C. Two of the methyl groups are in exactly the same environment - attached to the rest of the molecule in exactly the same way. They would only produce a peak. With the other two carbon atoms, that would make a total of three. Alcohol is C.
Example \(\PageIndex{4}\):
This follows from Example \(\PageIndex{3}\) and also involves an isomer of \(C_4H_{10}O\), but which is not an alcohol. Its C-13 NMR spectrum is below. Find out what your structure is.
Since we don't know what kind of structure we're looking at, this time it would be a good idea to look at the offset values. The approximations are perfectly fine, and let's work from this table:
| carbon environment | chemical shift (ppm) |
|---|---|
| C-C | 0 - 50 |
| C-O | 50 - 100 |
| C=C | 100 - 150 |
| C=O | 150 - 200 |
There is one peak for carbon(s) in a carbon-oxygen single bond and one for carbon(s) in a carbon-carbon single bond. This would be consistent with C-C-O in structure.
It's not an alcohol (you were told that in the question) and so there must be another carbon on the right side of the oxygen in the structure in the last paragraph. The molecular formula is C4H10O, and there are only two peaks. The only solution to this is to have two identical ethyl groups on either side of the oxygen. The compound is ethoxyethane (diethyl ether), CH3CH2E2CH3.
Example \(\PageIndex{5}\)
Using the simplified table of chemical shifts above, calculate the structure of the compound with the following C-13 NMR spectrum. Its molecular formula is \(C_4H_6O_2\).
Let's sort out what we have.
- There are four peaks and four carbons. No two carbons are in exactly the same environment.
- The peak at just over 50 must be a carbon bonded to an oxygen by a single bond.
- The two peaks around 130 must be the two carbons on either end of a carbon-carbon double bond.
- The peak at just under 170 is carbon in a carbon-oxygen double bond.
Putting it together is a matter of playing around with the structures until you have something reasonable. But you can't be sure you got the structure right using this simplified table. In this particular case, the spectrum was for the compound:
If you consult the more precise table of chemical changes at the top of the page, you will get better confirmation of this. The relatively low peak value of the carbon-oxygen double bond suggests an ester or acid rather than an aldehyde or ketone.
It can't be an acid because there must be a carbon bonded to an oxygen by a single bond somewhere - except the one on the -COOH group. We've already accounted for that peak carbon atom at about 170. If it were an acid, you would have already used the two oxygen atoms in the structure of the -COOH group. Without that information, though, you could probably come up with reasonable alternative structures. If you were working with the simplified table on an exam, your examiners would have to allow any valid alternatives.
Collaborators and Assignments
Jim Clark (Chemguide.co.uk)
FAQs
What does a 13C NMR tell you? ›
The 13C NMR is directly about the carbon skeleton not just the proton attached to it. a. The number of signals tell us how many different carbons or set of equivalent carbons b. The splitting of a signal tells us how many hydrogens are attached to each carbon.
What does the number of peaks in 13C NMR mean? ›In the spectrum there are a total of three peaks - that means that there are only three different environments for the carbons, despite there being four carbon atoms.
What signals are in the 13C NMR spectrum? ›The presence of this chiral center eliminates the possibility of a plane of symmetry that would make any of its carbon atoms equivalent. Therefore, each of the eight carbons in the compound are distinct, producing 1 signal each on a 13C NMR spectrum, totaling to 8 signals.
How do you interpret NMR values? ›It is important to understand trend of chemical shift in terms of NMR interpretation. The proton NMR chemical shift is affect by nearness to electronegative atoms (O, N, halogen.) and unsaturated groups (C=C,C=O, aromatic). Electronegative groups move to the down field (left; increase in ppm).
What is the splitting of C-13 NMR? ›The NMR spectrum from the carbon-13 nucleus will yield one absorption peak in the spectrum. Adding the nuclear spin from one hydrogen will split the carbon-13 peak into two peaks. Adding one more hydrogen will split each of the two carbon-13 peaks into two, giving a 1:2:1 ratio.
How many peaks are in the 13C NMR spectrum? ›In the 13C NMR spectrum of pentane (below), you can see three different peaks, even though pentane just contains methyl carbons and methylene carbons like butane. As far as the NMR spectrometer is concerned, pentane contains three different kinds of carbon, in three different environments.
What mainly influences the position of signals in the 13C NMR spectrum? ›The chemical shift of a 13C nucleus is influenced by essentially the same factors that influence a proton's chemical shift: bonds to electronegative atoms and diamagnetic anisotropy effects tend to shift signals downfield (higher resonance frequency). In addition, sp2 hybridization results in a large downfield shift.
What are the characteristics of carbon 13? ›Carbon-13 (13C) is a natural, stable isotope of carbon with a nucleus containing six protons and seven neutrons. As one of the environmental isotopes, it makes up about 1.1% of all natural carbon on Earth.
What information do we get from carbon NMR? ›The most useful information you can get from a 13C NMR spectrum is the number of non-equivalent carbon atoms. For example, you have the 13C NMR spectrum of an unknown ester.
What are the advantages of carbon 13 NMR over proton NMR? ›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.
What is the difference between c13 and h1 NMR? ›
There are two types of NMR techniques named as 1H NMR and 13C NMR. The main difference between 1H NMR and 13C NMR is that 1H NMR is used to determine the types and number of hydrogen atoms present in a molecule whereas 13C NMR is used to determine the type and number of carbon atoms in a molecule.
What are the advantages of 13C NMR over 1H NMR? ›C NMR offers many advantages for a metabolomics study, either alone or as a complement to 1H NMR: (1) 13C spectral windows are typically 200 ppm, providing much greater chemical shift dispersion than 1H; (2) at natural abundance, 13C resonances of small metabolites are narrow singlets (with 1H decoupling) resulting in ...