Interpreting c Nmr

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INTERPRETING C-13 NMR SPECTRA?

This page takes an introductory look at how you can get usefulinformation from a C-13 NMR spectrum.

Important: If you have come straight to this page via asearch engine, you should be aware that this is the secondof two pages about C-13 NMR. Unless you are familiar withC-13 NMR, you should read theintroduction to C-13 NMRfirst by following this link.

Taking a close look at three C-13 NMR spectra

The C-13 NMR spectrum for ethanol

Note: The nmr spectra on this page have been producedfrom graphs taken from the Spectral Data Base System forOrganic Compounds (SDBS) at the National Institute ofMaterials and Chemical Research in Japan.

It is possible that small errors may have been introducedduring the process of converting them for use on this site,

but these won't affect the argument in any way.

Remember that each peak identifies a carbon atom in a differentenvironment within the molecule. In this case there are twopeaks because there are two different environments for the

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carbons.

The carbon in the CH3 group is attached to 3 hydrogens and acarbon. The carbon in the CH2 group is attached to 2 hydrogens,a carbon and an oxygen.

So which peak is which?

You might remember from the introductory page that theexternal magnetic field experienced by the carbon nuclei isaffected by the electronegativity of the atoms attached to them.The effect of this is that the chemical shift of the carbonincreases if you attach an atom like oxygen to it. That meansthat the peak at about 60 (the larger chemical shift) is due to theCH2 group because it has a more electronegative atomattached.

Note: In principle, you should be able to work out the factthat the carbon attached to the oxygen will have the largerchemical shift. In practice, given the level I am aiming at (16- 18 year old chemistry students), you always work fromtables of chemical shift values for different groups (seebelow).

What if you needed to work it out? The electronegativeoxygen pulls electrons away from the carbon nucleusleaving it more exposed to any external magnetic field. Thatmeans that you will need a smaller external magnetic field to

bring the nucleus into the resonance condition than if it wasattached to less electronegative things. The smaller themagnetic field needed, the higher the chemical shift.

All this is covered in more detail on theintroduction to C-13NMRpage mentioned above.

A table of typical chemical shifts in C-13 NMR spectra

carbon environment chemical shift (ppm)C=O (in ketones) 205 - 220C=O (in aldehydes) 190 - 200C=O (in acids and esters) 170 - 185

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C in aromatic rings 125 - 150C=C (in alkenes) 115 - 140RCH2OH 50 - 65RCH2Cl 40 - 45RCH2NH2 37 - 45R3CH 25 - 35CH3CO- 20 - 30R2CH2 16 - 25RCH3 10 - 15

In the table, the "R" groups won't necessarily be simple alkylgroups. In each case there will be a carbon atom attached to theone shown in red, but there may well be other things substitutedinto the "R" group.

If a substituent is very close to the carbon in question, and veryelectronegative, that might affect the values given in the tableslightly.

For example, ethanol has a peak at about 60 because of the

CH2OH group. No problem!It also has a peak due to the RCH3 group. The "R" group thistime is CH2OH. The electron pulling effect of the oxygen atomincreases the chemical shift slightly from the one shown in thetable to a value of about 18.

A simplification of the table

At the time of writing, a draft UK syllabus (Cambridge pre-U)was expecting their students to learn the following simplification:

carbon environment chemical shift (ppm)C-C 0 - 50

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C-O 50 - 100C=C 100 - 150C=O 150 - 200

This may, of course, change and other syllabuses might wantsomething similar. The only way to find out is to check yoursyllabus, and recent question papers to see whether you aregiven tables of chemical shifts or not.

Note: If you are following a UK-based syllabus, and haven'tgot a copies of yoursyllabus and past papers, follow this linkto find out how to get hold of them.

The C-13 NMR spectrum for but-3-en-2-one

This is also known as 3-buten-2-one (amongst many otherthings!)

Here is the structure for the compound:

You can pick out all the peaks in this compound using thesimplified table above.

The peak at just under 200 is due to a carbon-oxygen double

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bond. The two peaks at 137 and 129 are due to the carbons ateither end of the carbon-carbon double bond. And the peak at26 is the methyl group which, of course, is joined to the rest ofthe molecule by a carbon-carbon single bond.

If you want to use the more accurate table, you have to put a bitmore thought into it - and, in particular, worry about the valueswhich don't always exactly match those in the table!

The carbon-oxygen double bond in the peak for the ketonegroup has a slightly lower value than the table suggests for aketone. There is an interaction between the carbon-oxygen andcarbon-carbon double bonds in the molecule which affects thevalue slightly. This isn't something which we need to look at indetail for the purposes of this topic.

You must be prepared to find small discrepancies of this sort inmore complicated molecules - but don't worry about this forexam purposes at this level. Your examiners should give youshift values which exactly match the compound you are given.

The two peaks for the carbons in the carbon-carbon doublebond are exactly where they would be expected to be. Noticethat they aren't in exactly the same environment, and so don'thave the same shift values. The one closer to the carbon-oxygen double bond has the larger value.

And the methyl group on the end has exactly the sort of valueyou would expect for one attached to C=O. The table gives arange of 20 - 30, and that's where it is.

One final important thing to notice. There are four carbons in themolecule and four peaks because they are all in differentenvironments. But they aren't all the same height. In C-13 NMR,you can't draw any simple conclusions from the heights of thevarious peaks.

The C-13 NMR spectrum for 1-methylethyl propanoate

1-methylethyl propanoate is also known as isopropyl propanoateor isopropyl propionate.

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Here is the structure for 1-methylethyl propanoate:

Two simple peaks

There are two very simple peaks in the spectrum which could beidentified easily from the second table above.

The peak at 174 is due to a carbon in a carbon-oxygen doublebond. (Looking at the more detailed table, this peak is due to thecarbon in a carbon-oxygen double bond in an acid or ester.)

The peak at 67 is due to a different carbon singly bonded to anoxygen. Those two peaks are therefore due to:

If you look back at the more detailed table of chemical shifts,you will find that a carbon singly bonded to an oxygen has a

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range of 50 - 65. 67 is, of course, a little bit higher than that.

As before, you must expect these small differences. No tablecan account for all the fine differences in environment of acarbon in a molecule. Different tables will quote slightly different

ranges. At this level, you can just ignore that problem!

Before we go on to look at the other peaks, notice the heights ofthese two peaks we've been talking about. They are both due toa single carbon atom in the molecule, and yet they havedifferent heights. Again, you can't read any reliable informationdirectly from peak heights in these spectra.

The three right-hand peaks

From the simplified table, all you can say is that these are due tocarbons attached to other carbon atoms by single bonds. Butbecause there are three peaks, the carbons must be in threedifferent environments.

The more detailed table is more helpful.

Here are the structure and the spectrum again:

The easiest peak to sort out is the one at 28. If you look back atthe table, that could well be a carbon attached to a carbon-oxygen double bond. The table quotes the group as CH3CO-,

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but replacing one of the hydrogens by a simple CH3 group won'tmake much difference to the shift value.The right-hand peak is also fairly easy. This is the left-handmethyl group in the molecule. It is attached to an admittedly

complicated R group (the rest of the molecule). It is the bottomvalue given in the detailed table.

The tall peak at 22 must be due to the two methyl groups at theright-hand end of the molecule - because that's all that's left.These combine to give a single peak because they are both inexactly the same environment.

If you are looking at the detailed table, you need to think verycarefully which of the environments you should be looking at.Without thinking, it is tempting to go for the R2CH2 with peaks in

the 16 - 25 region. But you would be wrong!The carbons we are interested in are the ones in the methylgroup, not in the R groups. These carbons are again in theenvironment: RCH3. The R is the rest of the molecule.The table says that these should have peaks in the range 10 -15, but our peak is a bit higher. This is because of the presenceof the nearby oxygen atom. Its electronegativity is pullingelectrons away from the methyl groups - and, as we've seenabove, this tends to increase the chemical shift slightly.

Once again, don't worry about the discrepancies. In an exam,perhaps your examiners will just want you to have learnt thesimple table above - in which case, they can't expect you towork out which peak is which in a complicated spectrum of thissort. Or they will give you tables of chemical shifts - in whichcase, they will give you values which match the peaks in thespectra.

Remember that you are only doing an introduction to C-13 NMRat this level. It isn't going to be that hard in an exam!

Working out structures from C-13 NMR spectra

So far, we've just been trying to see the relationship betweencarbons in particular environments in a molecule and thespectrum produced. We've had all the information necessary.

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Now let's make it a little more difficult - but we'll work from mucheasier examples!

In each example, try to work it out for yourself before you readthe explanation.

Example 1

How could you tell from just a quick look at a C-13 NMRspectrum (and without worrying about chemical shifts) whetheryou had propanone or propanal (assuming those were the onlyoptions)?

Because these are isomers, each has the same number ofcarbon atoms, but there is a difference between theenvironments of the carbons which will make a big impact on thespectra.

In propanone, the two carbons in the methyl groups are inexactly the same environment, and so will produce only a single

peak. That means that the propanone spectrum will have only 2peaks - one for the methyl groups and one for the carbon in theC=O group.

However, in propanal, all the carbons are in completely differentenvironments, and the spectrum will have three peaks.

Example 2

Thare are four alcohols with the molecular formula C4H10O.

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Which one produced the C-13 NMR spectrum below?

You can do this perfectly well without referring to chemical shifttables at all.

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.

In A and B, there are four totally different environments. Both ofthese would produce four peaks.

In D, there are only two different environments - all the methylgroups are exactly equivalent. D would only produce two peaks.

That leaves C. Two of the methyl groups are in exactly the sameenvironment - attached to the rest of the molecule in exactly the

same way. They would only produce one peak. With the othertwo carbon atoms, that would make a total of three. The alcoholis C.

Example 3

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This follows on from Example 2, and also involves an isomer ofC4H10O but which isn't an alcohol. Its C-13 NMR spectrum isbelow. Work out what its structure is.

Because we don't know what sort of structure we are looking at,this time it would be a good idea to look at the shift values. Theapproximations are perfectly good, and we will work from thistable:

carbon environment chemical shift (ppm)C-C 0 - 50C-O 50 - 100C=C 100 - 150C=O 150 - 200

There is a peak for carbon(s) in a carbon-oxygen single bondand one for carbon(s) in a carbon-carbon single bond. Thatwould be consistent with C-C-O in the structure.

It isn't an alcohol (you are told that in the question), and so theremust be another carbon on the right-hand side of the oxygen inthe structure in the last paragraph.

The molecular formula is C4H10O, and there are only two peaks.The only solution to that is to have two identical ethyl groupseither side of the oxygen.

The compound is ethoxyethane (diethyl ether),

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CH3CH2OCH2CH3.

Example 4

Using the simplified table of chemical shifts above, work out thestructure of the compound with the following C-13 NMRspectrum. Its molecular formula is C4H6O2.

Let's sort out what we've got.

There are four peaks and four carbons. No two carbonsare in exactly the same environment.

The peak at just over 50 must be a carbon attached to anoxygen by a single bond.

The two peaks around 130 must be the two carbons ateither end of a carbon-carbon double bond.

The peak at just less than 170 is the carbon in a carbon-oxygen double bond.

Putting this together is a matter of playing around with thestructures until you have come up with something reasonable.But you can't be sure that you have got the right structure usingthis simplified table.

In this particular case, the spectrum was for the compound:

If you refer back to the more accurate table of chemical shifts

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towards the top of the page, you will get some betterconfirmation of this. The relatively low value of the carbon-oxygen double bond peak suggests an ester or acid rather thanan aldehyde or ketone.

It can't be an acid because there has to be a carbon attached toan oxygen by a single bond somewhere - apart from the one inthe -COOH group. We've already accounted for that carbonatom from the peak at about 170. If it wasan acid, you wouldalready have used up both oxygens in the structure in the -COOH group.

Without this information, though, you could probably come upwith reasonable alternative structures. If you were working fromthe simplified table in an exam, your examiners would have toallow any valid alternatives.

Note: It is worth explaining again why I am using thissimplified table of chemical shifts. This site is aimedprimarily at students doing UK-based chemistry exams for16 - 18 year olds. I know it is widely used beyond that, but Ican't risk making life more difficult than it need be for mytarget audience by, for example, approaching C-13 NMR inthe sort of detail needed by university students.

At the time of writing, no current UK-based syllabuses evenmentioned C-13 NMR, but two draft syllabuses for teachingfrom September 2008 do mention it. The one going into

most detail is the Cambridge pre-U syllabus, and they usethe simplified shift table. When things become clearer whenthe syllabus is finalised and specimen exam papers areproduced, I may need to rewrite some of this page if it turnsout that they (or AQA - the other syllabus mentioning it) arelooking for something more sophisticated. I doubt that theywill be though!

http://www.chemguide.co.uk/analysis/nmr/interpretc13.html Jim Clark 2007

WHAT IS C-13 NMR?

This page describes what a C-13 NMR spectrum is and how ittells you useful things about the carbon atoms in organicmolecules.

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The background to C-13 NMR spectroscopy

Nuclear magnetic resonance is concerned with the magnetic

properties of certain nuclei. On this page we are focussing onthe magnetic behaviour of carbon-13 nuclei.

Carbon-13 nuclei as little magnets

About 1% of all carbon atoms are the C-13 isotope; the rest(apart from tiny amounts of the radioactive C-14) is C-12. C-13NMR relies on the magnetic properties of the C-13 nuclei.

Carbon-13 nuclei fall into a class known as "spin " nuclei forreasons which don't really need to concern us at the introductorylevel this page is aimed at (UK A level and its equivalents).

The effect of this is that a C-13 nucleus can behave as a littlemagnet. C-12 nuclei don't have this property.

If you have a compass needle, it normally lines up with theEarth's magnetic field with the north-seeking end pointing north.Provided it isn't sealed in some sort of container, you could twistthe needle around with your fingers so that it pointed south -

lining it up opposed to the Earth's magnetic field.

It is very unstable opposed to the Earth's field, and as soon asyou let it go again, it will flip back to its more stable state.

Because a C-13 nucleus behaves like a little magnet, it meansthat it can also be aligned with an external magnetic field oropposed to it.

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Again, the alignment where it is opposed to the field is lessstable (at a higher energy). It is possible to make it flip from themore stable alignment to the less stable one by supplyingexactly the right amount of energy.

The energy needed to make this flip depends on the strength ofthe external magnetic field used, but is usually in the range ofenergies found in radio waves - at frequencies of about 25 - 100MHz. (BBC Radio 4 is found between 92 - 95 MHz!) If you havealso looked at proton-NMR, the frequency is about a quarter ofthat used to flip a hydrogen nucleus for a given magnetic fieldstrength.

It's possible to detect this interaction between the radio waves ofjust the right frequency and the carbon-13 nucleus as it flips

from one orientation to the other as a peak on a graph. Thisflipping of the carbon-13 nucleus from one magnetic alignmentto the other by the radio waves is known as the resonancecondition.

The importance of the carbon's environment

What we've said so far would apply to an isolated carbon-13nucleus, but real carbon atoms in real bonds have other things

around them - especially electrons. The effect of the electrons isto cut down the size of the external magnetic field felt by thecarbon-13 nucleus.

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Suppose you were using a radio frequency of 25 MHz, and youadjusted the size of the magnetic field so that an isolatedcarbon-13 atom was in the resonance condition.

If you replaced the isolated carbon with the more realistic caseof it being surrounded by bonding electrons, it wouldn't befeeling the full effect of the external field any more and so wouldstop resonating (flipping from one magnetic alignment to theother). The resonance condition depends on having exactly theright combination of external magnetic field and radio frequency.

How would you bring it back into the resonance condition again?You would have to increase the external magnetic field slightlyto compensate for the shielding effect of the electrons.

Now suppose that you attached the carbon to something moreelectronegative. The electrons in the bond would be furtheraway from the carbon nucleus, and so would have less of alowering effect on the magnetic field around the carbon nucleus.

Note: Electronegativity is a measure of the ability of anatom to attract a bonding pair of electrons. If you aren'thappy aboutelectronegativity, you could follow this link atsome point in the future, but it probably isn't worth doing itnow!

The external magnetic field needed to bring the carbon intoresonance will be smaller if it is attached to a moreelectronegative element, because the C-13 nucleus feels moreof the field. Even small differences in the electronegativities of

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the attached atoms will make a difference to the magnetic fieldneeded to achieve resonance.

Summary

For a given radio frequency (say, 25 MHz) each carbon-13 atomwill need a slightly different magnetic field applied to it to bring itinto the resonance condition depending on what exactly it isattached to - in other words the magnetic field needed is auseful guide to the carbon atom's environment in the molecule.

Features of a C-13 NMR spectrum

The C-13 NMR spectrum for ethanol

This is a simple example of a C-13 NMR spectrum. Don't worryabout the scale for now - we'll look at that in a minute.

Note: The nmr spectra on this page have been producedfrom graphs taken from the Spectral Data Base System forOrganic Compounds (SDBS) at the National Institute ofMaterials and Chemical Research in Japan.

It is possible that small errors may have been introducedduring the process of converting them for use on this site,but these won't affect the argument in any way.

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There are two peaks because there are two differentenvironments for the carbons.

The carbon in the CH3 group is attached to 3 hydrogens and acarbon. The carbon in the CH2 group is attached to 2 hydrogens,a carbon and an oxygen.

The two lines are in different places in the NMR spectrumbecause they need different external magnetic fields to bringthem in to resonance at a particular radio frequency.

The C-13 NMR spectrum for a more complicated compound

This is the C-13 NMR spectrum for 1-methylethyl propanoate(also known as isopropyl propanoate or isopropyl propionate).

This time there are 5 lines in the spectrum. That means thatthere must be 5 different environments for the carbon atoms inthe compound. Is that reasonable from the structure?

Well - if you count the carbon atoms, there are 6 of them. Sowhy only 5 lines? In this case, two of the carbons are in exactly

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the same environment. They are attached to exactly the samethings. Look at the two CH3 groups on the right-hand side of themolecule.

You might reasonably ask why the carbon in the CH3 on the left

isn't also in the same environment. Just like the ones on theright, the carbon is attached to 3 hydrogens and another carbon.But the similarity isn't exact - you have to chase the similarityalong the rest of the molecule as well to be sure.

The carbon in the left-hand CH3 group is attached to a carbonatom which in turn is attached to a carbon with two oxygens on it- and so on down the molecule.

That's not exactly the same environment as the carbons in theright-hand CH3 groups. They are attached to a carbon which is

attached to a single oxygen - and so on down the molecule.

We'll look at this spectrum again in detail on the next page - andlook at some more similar examples as well. This all gets easierthe more examples you look at.

For now, all you need to realise is that each line in a C-13 NMRspectrum recognises a carbon atom in one particularenvironment in the compound. If two (or more) carbon atoms ina compound have exactly the same environment, they will berepresented by a single line.

Note: If you are fairly wide-awake, you might wonder whyall this works, since only about 1% of carbon atoms are C-13. These are the only ones picked up by this form of NMR.If you had a single molecule of ethanol, then the chancesare only about 1 in 50 of there being one C-13 atom in it,and only about 1 in 10,000 of both being C-13.

But you have got to remember that you will be working witha sample containing huge numbers of molecules. Theinstrument can pick up the magnetic effect of the C-13nuclei in the carbon of the CH3 group and the carbon of theCH

2group even if they are in separate molecules. There's

no need for them to be in the same one.

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The need for a standard for comparison - TMS

Before we can explain what the horizontal scale means, weneed to explain the fact that it has a zero point - at the right-

hand end of the scale. The zero is where you would find a peakdue to the carbon-13 atoms in tetramethylsilane - usuallycalledTMS. Everything else is compared with this.

You will find that some NMR spectra show the peak due to TMS(at zero), and others leave it out. Essentially, if you have toanalyse a spectrum which has a peak at zero, you can ignore itbecause that's the TMS peak.

TMS is chosen as the standard for several reasons. The mostimportant are:

It has 4 carbon atoms all of which are in exactly the sameenvironment. They are joined to exactly the same thingsin exactly the same way. That produces a single peak,but it's also a strong peak (because there are lots ofcarbon atoms all doing the same thing).

The electrons in the C-Si bonds are closer to the carbonsin this compound than in almost any other one. Thatmeans that these carbon nuclei are the most shieldedfrom the external magnetic field, and so you would haveto increase the magnetic field by the greatest amount tobring the carbons back into resonance.

The net effect of this is that TMS produces a peak on thespectrum at the extreme right-hand side. Almosteverything else produces peaks to the left of it.

The chemical shift

The horizontal scale is shown as (ppm). is called the

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chemical shift and is measured in parts per million - ppm.

A peak at a chemical shift of, say, 60 means that the carbonatoms which caused that peak need a magnetic field 60millionths less than the field needed by TMS to produce

resonance.

A peak at a chemical shift of 60 is said to be downfield of TMS.The further to the left a peak is, the more downfield it is.

Note: If you are familiar with proton-NMR, you will noticethat the chemical shifts for C-13 NMR are much bigger thanfor proton-NMR. In C-13 NMR, they range up to about 200ppm. In proton-NMR they only go up to about 12 ppm. Youdon't need to worry about the reasons for this at this level.

Solvents for NMR spectroscopy

NMR spectra are usually measured using solutions of thesubstance being investigated. A commonly used solvent isCDCl3. This is a trichloromethane (chloroform) molecule in whichthe hydrogen has been replaced by its isotope, deuterium.

CDCl3 is also commonly used as the solvent in proton-NMR

because it doesn't have any ordinary hydrogen nuclei (protons)which would give a line in a proton-NMR spectrum. It does, ofcourse, have a carbon atom - so why doesn't it give a potentiallyconfusing line in a C-13 NMR spectrum?

In fact it does give a line, but the line has an easily recognisablechemical shift and so can be removed from the final spectrum.All of the spectra from the SDBS have this line removed to avoidany confusion.

http://www.chemguide.co.uk/analysis/nmr/backgroundc13.html#top