WHAT IS NUCLEAR MAGNETIC RESONANCE (NMR)?

This page describes what a proton NMR spectrum is and how it tells you useful things about the hydrogen atoms in organic molecules.

The background to NMR spectroscopyNuclear magnetic resonance is concerned with the magnetic properties of certain nuclei. On this page we are focussing on the magnetic behaviour of hydrogen nuclei - hence the termproton NMRor1H-NMR.Hydrogen atoms as little magnetsIf you have a compass needle, it normally lines up with the Earth's magnetic field with the north-seeking end pointing north. Provided it isn't sealed in some sort of container, you could twist the 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 as you let it go again, it will flip back to its more stable state.

Hydrogen nuclei also behave as little magnets and a hydrogen nucleus can also be aligned with an external magnetic field or opposed to it. Again, the alignment where it is opposed to the field is less stable (at a higher energy). It is possible to make it flip from the more stable alignment to the less stable one by supplying exactly the right amount of energy.

The energy needed to make this flip depends on the strength of the external magnetic field used, but is usually in the range of energies found in radio waves - at frequencies of about 60 - 100 MHz. (BBC Radio 4 is found between 92 - 95 MHz!)It's possible to detect this interaction between the radio waves of just the right frequency and the proton as it flips from one orientation to the other as a peak on a graph. This flipping of the proton from one magnetic alignment to the other by the radio waves is known as theresonance condition.

The importance of the hydrogen atom's environmentWhat we've said so far would apply to an isolated proton, but real protons have other things around them - especially electrons. The effect of the electrons is to cut down the size of the external magnetic field felt by the hydrogen nucleus.

Suppose you were using a radio frequency of 90 MHz, and you adjusted the size of the magnetic field so that an isolated proton was in the resonance condition.If you replaced the isolated proton with one that was attached to something, it wouldn't be feeling the full effect of the external field any more and so would stop resonating (flipping from one magnetic alignment to the other). The resonance condition depends on having exactly the right 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 slightly to compensate for the effect of the electrons.Now suppose that you attached the hydrogen to something more electronegative. The electrons in the bond would be further away from the hydrogen nucleus, and so would have less effect on the magnetic field around the hydrogen.

Note: Electronegativity is a measure of the ability of an atom to attract a bonding pair of electrons. If you aren't happy aboutelectronegativity, you could follow this link at some point in the future, but it probably isn't worth doing it now!

The external magnetic field needed to bring the hydrogen into resonance will be smaller if it is attached to a more electronegative element, because the hydrogen nucleus feels more of the field. Even small differences in the electronegativities of the attached atom or groups of atoms will make a difference to the magnetic field needed to achieve resonance.

SummaryFor a given radio frequency (say, 90 MHz) each hydrogen atom will need a slightly different magnetic field applied to it to bring it into the resonance condition depending on what exactly it is attached to - in other words the magnetic field needed is a useful guide to the hydrogen atom's environment in the molecule.

Features of an NMR spectrumA simple NMR spectrum looks like this:

Note: The nmr spectra on this page have been produced from graphs taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan.It is possible that small errors may have been introduced during the process of converting them for use on this site, but these won't affect the argument in any way.

The peaksThere are two peaks because there are two different environments for the hydrogens - in the CH3group and attached to the oxygen in the COOH group. They are in different places in the spectrum because they need slightly different external magnetic fields to bring them in to resonance at a particular radio frequency.The sizes of the two peaks gives important information about the numbers of hydrogen atoms in each environment. It isn't the height of the peaks that matters, but the ratio of the areas under the peaks. If you could measure the areas under the peaks in the diagram above, you would find that they were in the ratio of 3 (for the larger peak) to 1 (for the smaller one).That shows a ratio of 3:1 in the number of hydrogen atoms in the two environments - which is exactly what you would expect for CH3COOH.

The need for a standard for comparison - TMSBefore we can explain what the horizontal scale means, we need 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 peak due to the hydrogen atoms intetramethylsilane- usually calledTMS.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 to analyse a spectrum which has a peak at zero, you can ignore it because that's the TMS peak.TMS is chosen as the standard for several reasons. The most important are: It has 12 hydrogen atoms all of which are in exactly the same environment. They are joined to exactly the same things in exactly the same way. That produces a single peak, but it's also a strong peak (because there are lots of hydrogen atoms). The electrons in the C-H bonds are closer to the hydrogens in this compound than in almost any other one. That means that these hydrogen nuclei are the most shielded from the external magnetic field, and so you would have to increase the magnetic field by the greatest amount to bring the hydrogens back into resonance.The net effect of this is that TMS produces a peak on the spectrum at the extreme right-hand side. Almost everything else produces peaks to the left of it.The chemical shiftThe horizontal scale is shown as(ppm).is called thechemical shiftand is measured inparts per million- ppm.A peak at a chemical shift of, say, 2.0 means that the hydrogen atoms which caused that peak need a magnetic fieldtwo millionths lessthan the field needed by TMS to produce resonance.A peak at a chemical shift of 2.0 is said to bedownfieldof TMS. The further to the left a peak is, the more downfield it is.

Solvents for NMR spectroscopyNMR spectra are usually measured using solutions of the substance being investigated. It is important that the solvent itself doesn't contain any simple hydrogen atoms, because they would produce confusing peaks in the spectrum.There are two ways of avoiding this. You can use a solvent such as tetrachloromethane, CCl4, which doesn't contain any hydrogen, or you can use a solvent in which any ordinary hydrogen atoms are replaced by its isotope, deuterium - for example, CDCl3instead of CHCl3. All the NMR spectra used on this site involve CDCl3as the solvent.Deuterium atoms have sufficiently different magnetic properties from ordinary hydrogen that they don't produce peaks in the area of the spectrum that we are looking at.

Note: Several text books say that deuterium atoms don't have a magnetic field. It isn't true - they do have a field but it is less than an ordinary hydrogen atom.

LOW RESOLUTION NMR SPECTRA

This page describes how you interpret simple low resolution nuclear magnetic resonance (NMR) spectra. It assumes that you have already read the background page on NMR so that you understand what an NMR spectrum looks like and the use of the term "chemical shift".

The difference between high and low resolution spectra

Note: This high resolution nmr spectrum has been produced from a graph taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan.

A low resolution spectrum looks much simpler because it can't distinguish between the individual peaks in the various groups of peaks.

Note: I haven't been able to find a reliable source of low resolution NMR spectra. The low resolution spectra on this page are my best guess at what the low resolution spectra would look like, based on the high resolution ones.

The numbers against the peaks represent the relative areas under each peak. That information is extremely important in interpreting the spectra.

Interpreting a low resolution spectrumUsing the total number of peaksEach peak represents a different environment for hydrogen atoms in the molecule. In the methyl propanoate spectrum above, there are three peaks because there are three different environments for the hydrogens.Remember that methyl propanoate is CH3CH2COOCH3. The hydrogens in the CH2group are obviously in a different environment from those in the CH3groups. The two CH3groups aren't in the same environment either. One is attached to a CH2group, the other to an oxygen.Using the areas under the peaksThe ratio of the areas under the peaks tell you the ratio of the numbers of hydrogens in the various environments. In the methyl propanoate case, the areas were in the ratio of 3:2:3, which is exactly what you want for the two differently placed CH3groups and the CH2group.You will probably be told the relative areas under the peaks - especially if you are only looking at low resolution spectra, but it is just possible that you might have to work them out. NMR spectrometers have a device which draws another line on the spectrum called anintegrator trace(or integration trace). You can measure the relative areas from this trace.

Note: You need to find out whether your examiners expect you to know how to interpret an integrator trace. Check your syllabus and, particularly, past papers to see whether they ask it. If you are doing a UK-based exam and haven't got copies of yoursyllabus and past papers, follow this link to find out how to get them.If you do need to be able to interpretintegrator traces, you can find out how by following this link. You can also find it from the NMR menu.

Using chemical shiftsThe position of the peaks tells you useful things about what groups the various hydrogen atoms are in. In any exam, you will be given a table of chemical shifts if you need them. The important shifts for the groups present in methyl propanoate are:

Notes: "R" represents an alkyl group (like methyl, ethyl, etc) which in this case may have other things substituted in it.The shifts are shown as ranges of values. The exact position varies depending on what else is near that particular group in the molecule.

Showing these groups on the low resolution spectrum gives:

Some sample questionsExample 1An organic compound was known to be one of the following. Use its low resolution NMR spectrum to decide which it is.

Notice that there are three peaks showing three different environments for the hydrogens. That eliminates methyl ethanoate as a possibility because that would only give two peaks - due to the two differently situated CH3group hydrogens.Does the ratio of the areas under the peaks help? Not in this case - both the other compounds would have three peaks in the ratio of 1:2:3.Now you need to look at the chemical shifts:

Checking the positions of the various hydrogens in the two possible compounds against the chemical shift table gives you this pattern of shifts:

Comparing these with the actual spectrum means that the substance was propanoic acid, CH3CH2COOH.

Example 2How would you use low resolution NMR to distinguish between the isomers propanone and propanal?

The propanone would only give one peak in its NMR spectrum because both CH3groups are in an identical environment - both are attached to -COCH3.The propanal would give three peaks with the areas underneath in the ratio 3:2:1.You could refer to the chemical shift table above to decide where the peaks are likely to be found, but it isn't really necessary.

Example 3How many peaks would there be in the low resolution NMR spectrum of the following compound, and what would be the ratio of the areas under the peaks?

All the CH3groups are exactly equivalent so would only produce 1 peak. There would also be peaks for the hydrogens in the CH2group and the COOH group.There would be three peaks in total with areas in the ratio 9:2:1.

HIGH RESOLUTION NMR SPECTRA

This page describes how you interpret simple high resolution nuclear magnetic resonance (NMR) spectra. It assumes that you have already read the background page on NMR so that you understand what an NMR spectrum looks like and the use of the term "chemical shift". It also assumes that you know how to interpret simple low resolution spectra.

The difference between high and low resolution spectraWhat a low resolution NMR spectrum tells youRemember: The number of peaks tells you the numbers of different environments the hydrogen atoms are in. The ratio of the areas under the peaks tells you the ratio of the numbers of hydrogen atoms in each of these environments. The chemical shifts give you important information about the sort of environment the hydrogen atoms are in.High resolution NMR spectraIn a high resolution spectrum, you find that many of what looked like single peaks in the low resolution spectrum is split into clusters of peaks.For A'level purposes, you will only need to consider these possibilities:1 peaka singlet

2 peaks in the clustera doublet

3 peaks in the clustera triplet

4 peaks in the clustera quartet

You can get exactly the same information from a high resolution spectrum as from a low resolution one - you simply treat eachcluster of peaksas if it were a single one in a low resolution spectrum.But in addition, the amount of splitting of the peaks gives you important extra information.

Interpreting a high resolution spectrumThe n+1 ruleThe amount of splitting tells you about the number of hydrogens attached to the carbon atom or atomsnext doorto the one you are currently interested in.The number of sub-peaks in a cluster isone morethan the number of hydrogens attached to the next door carbon(s).So - on the assumption that there is only one carbon atom with hydrogens on next door to the carbon we're interested in (usually true at A'level!):singletnext door to carbon with no hydrogens attached

doubletnext door to a CH group

tripletnext door to a CH2group

quartetnext door to a CH3group

Using the n+1 ruleWhat information can you get from this NMR spectrum?

Note: The nmr spectra on this page have been produced from data taken from the Spectral Data Base System for Organic Compounds (SDBS) at the National Institute of Materials and Chemical Research in Japan.Any small errors that I've introduced during the process of converting them for use on this site won't affect the argument in any way.

Assume that you know that the compound above has the molecular formula C4H8O2.Treating this as a low resolution spectrum to start with, there are three clusters of peaks and so three different environments for the hydrogens. The hydrogens in those three environments are in the ratio 2:3:3. Since there are 8 hydrogens altogether, this represents a CH2group and two CH3groups.What about the splitting?The CH2group at about 4.1 ppm is a quartet. That tells you that it is next door to a carbon with three hydrogens attached - a CH3group.The CH3group at about 1.3 ppm is a triplet. That must be next door to a CH2group.This combination of these two clusters of peaks - one a quartet and the other a triplet - is typical of an ethyl group, CH3CH2. It is very common. Get to recognise it!Finally, the CH3group at about 2.0 ppm is a singlet. That means that the carbon next door doesn't have any hydrogens attached.So what is this compound? You would also use chemical shift data to help to identify the environment each group was in, and eventually you would come up with:

Note: You now know how to get the information you need from NMR spectra, but it often isn't easy to fit all that information together into a final formula. You simply need to practise! Go through all the examples in past papers from your Exam Board. How complicated they are will vary markedly from Board to Board. Some of the compounds you will come across may be very unfamiliar. Don't forget to use the information in chemical shift tables - if your examiners include some obscure group, it's almost certain you will need to use it. Take all the hints that are going!

Two special casesAlcoholsWhere is the -O-H peak?This is very confusing! Different sources quote totally different chemical shifts for the hydrogen atom in the -OH group in alcohols - often inconsistently. For example: The Nuffield Data Book quotes 2.0 - 4.0, but the Nuffield text book shows a peak at about 5.4. The OCR Data Sheet for use in their exams quotes 3.5 - 5.5. A reliable degree level organic chemistry text book quotes1.0 - 5.0, but then shows an NMR spectrum for ethanol with a peak at about 6.1. The SDBS database (used throughout this site) gives the -OH peak in ethanol at about 2.6.

The problem seems to be that the position of the -OH peak varies dramatically depending on the conditions - for example, what solvent is used, the concentration, and the purity of the alcohol - especially on whether or not it is totally dry.

Help! Do you need to worry about this? Not really - you can assume that in an exam question, any NMR spectrum will be consistent with the chemical shift data you are given.

A clever way of picking out the -OH peakIf you measure an NMR spectrum for an alcohol like ethanol, and then add a few drops of deuterium oxide, D2O, to the solution, allow it to settle and then re-measure the spectrum, the -OH peak disappears! By comparing the two spectra, you can tell immediately which peak was due to the -OH group.

Note: Deuterium oxide (sometimes called "heavy water") is simply water in which all the normal hydrogen-1 atoms are replaced by its isotope, hydrogen-2 (or deuterium).

The reason for the loss of the peak lies in the interaction between the deuterium oxide and the alcohol. All alcohols, such as ethanol, are very, very slightly acidic. The hydrogen on the -OH group transfers to one of the lone pairs on the oxygen of the water molecule. The fact that here we've got "heavy water" makes no difference to that.

The negative ion formed is most likely to bump into a simple deuterium oxide molecule to regenerate the alcohol - except that now the -OH group has turned into an -OD group.

Deuterium atoms don't produce peaks in the same region of an NMR spectrum as ordinary hydrogen atoms, and so the peak disappears.You might wonder what happens to the positive ion in the first equation and the OD-in the second one. These get lost into the normal equilibrium which exists wherever you have water molecules - heavy or otherwise.

The lack of splitting with -OH groupsUnless the alcohol is absolutely free of any water, the hydrogen on the -OH group and any hydrogens on the next door carbon don't interact to produce any splitting. The -OH peak is a singlet and you don't have to worry about its effect on the next door hydrogens.

The left-hand cluster of peaks is due to the CH2group. It is a quartet because of the 3 hydrogens on the next door CH3group. You can ignore the effect of the -OH hydrogen.Similarly, the -OH peak in the middle of the spectrum is a singlet. It hasn't turned into a triplet because of the influence of the CH2group.

Note: The reason for this is quite complex, and certainly goes beyond A'level. It lies in the very rapid interchange that occurs between the hydrogen atoms on the -OH group and either water molecules or other alcohol molecules. To find out about it you will have to read either a degree level organic chemistry book or one specifically about NMR.For A'level purposes just accept the fact that -OH produces a singlet and has no effect on neighbouring groups!

Equivalent hydrogen atomsHydrogen atoms attached to the same carbon atom are said to beequivalent.Equivalent hydrogen atoms have no effect on each other - so that one hydrogen atom in a CH2group doesn't cause any splitting in the spectrum of the other one.But hydrogen atoms on neighbouring carbon atoms can also be equivalent if they are in exactly the same environment. For example:

These four hydrogens are all exactly equivalent. You would get a single peak with no splitting at all.You only have to change the molecule very slightly for this no longer to be true.

Because the molecule now contains different atoms at each end, the hydrogens are no longer all in the same environment. This compound would give two separate peaks on a low resolution NMR spectrum. The high resolution spectrum would show that both peaks subdivided into triplets - because each is next door to a differently placed CH2group.

MD. KAMRUL ALAM KHAN, B.Sc Honors in Chemistry (SUST), M.Sc in Chemistry (SUST), CCNAE (All through first class), CELL: 01557704046 Kamrulclassroom.blogspot.com