Dif ference between NMR and MS MS is destructive, whereas NMR is not. However, a much

smaller amount of material is needed for MS techniques. NMR and Mass Spectrometry (MS) are complementary techniques:

while MS can tell the weight (and thus the molecular formula)

of a molecule, NMR can differentiate between structural isomers, and provide information about connectivities

between atoms within a molecule.

Teacher Notes on:

NMR Spectroscopy

What is i t?

Nuclear Magnetic Resonance (NMR) spectroscopy is (arguably) the most powerful tool available for

determining the structure of organic compounds. This technique relies on the ability of atomic nuclei to

behave like a small magnet and align themselves with an external magnetic field. When irradiated with a

radio frequency signal the nuclei in a molecule can change from being aligned with the magnetic field to

being opposed to it. Therefore, it is called “nuclear”

for the instrument works on stimulating the “nuclei” of the atoms to absorb radio waves. The energy

frequency at which this occurs can be measured and is displayed as an NMR spectrum. The most

common nuclei observed using this technique are 1H and 13C, but also 31P, 19F, 29Si and 77Se NMR are

available.

What is i t used for?

To identify and/or elucidate detailed structural information about chemical compounds. For

example:

• Determining the purity of medicines before they leave the

factory

• Identifying contaminants in food, cosmetics, or medications

• Helping research chemists discover whether a chemical

reaction has occurred at the correct site on a molecule

• Identifying drugs seized by police and customs agents

• Checking the structure of plastics, to ensure they will have the desired properties

How does i t work? To get the nuclei in a molecule to all align in the same direction, a very strong magnetic field is generated using a

superconducting electromagnet, which requires very low temperatures to function. The coils of the magnet are surrounded by liquid helium (4K, or -269°C), which is prevented from boiling off too quickly by a surrounding layer

of liquid nitrogen (-77°C). These coolants are all contained in double-layer steel with a vacuum between the

layers, to provide insulation just like a thermos. There is a narrow hole through the middle of the magnet, and the

sample tube and radio frequency coils ("probe”) are located there.

Further In format ion on NMR Spectroscopy: http://tinyurl.com/yje6t3

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The 13C and 1H NMR spectra for aspirin are shown in figures 3 and 4,

respectively. Aspirin, with nine different carbons produces a 13C NMR spectrum with nine individual signals. Again, the positions of the signals indicate the

individual structural environments of each carbon. Six signals are clustered around the 120-150ppm region, typical for carbons in an aromatic benzene

ring. The two carbonyl carbons (C=O) appear characteristically towards the left of the spectrum (170 ppm) whilst the carbon of the CH3 group, not being

attached to an electronegative element or part of an aromatic ring, appears at the right of the spectrum.

The 1H NMR spectrum of aspirin (figure 4) shows 6 signals, due to six different

hydrogen environments. The signals in the 7-8 ppm range are typical for hydrogens attached to an aromatic (benzene) ring. The hydrogen of the

carboxylic acid (COOH) produces a broad signal at 11.2 ppm and the CH3 group is at 2.2 ppm.

How to read the spectrum An NMR spectrum appears as a series of vertical peaks/signals distributed along the x-axis of the spectrum

(Figures 1-4). Each of these signals corresponds to an atom within the molecule being observed. The position of each signal in the spectrum gives information about the local structural environment of the atom producing the

signal.

A similar effect is seen in the 1H NMR spectrum of ethanol. The two protons of the CH2 group neighbouring the oxygen

are further to the left in the spectrum, whilst the hydrogens of the CH3 group that is most remote from the oxygen

produce a signal towards the right of the spectrum. The signals in the 1H NMR spectrum do not necessarily appear as

a single line, as can be seen in f igure 2. The ‘splitting pattern’ seen in these signals gives information as to how

many hydrogens are present on the neighbouring carbon.

Also, integration of the 1H NMR signals allows the number of hydrogens in each environment to be determined.

For example, the 13C NMR spectrum of ethanol (CH3CH2OH) is shown in f igure 1 . The two carbons

in ethanol are in different structural environments and hence each produces a signal in the NMR spectrum.

The carbon attached to oxygen is ‘deshielded’ due to

the electronegative nature of oxygen and this shifts its signal towards the left in the spectrum. Whereas the

carbon bonded only to hydrogens and carbon appears at the right of the spectrum.

Fig . 1: 13C NMR spectrum o f ethano l

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Table1: Peaks for ethanol

H3C

CH2OH

Aspirin

Aspi

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Figure 4: 1H NMR spectrum o f asp ir in

Figure 3: 13C NMR spectrum o f asp ir in

13C NMR ppm

CO2H 170.2

COCH3 169.8

CH3 21.0

6 aromatic C 122-151

1H NMR

CO2H 11.2

CH3 2.35

4 aromatic CH 7.14-8.13

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Maree is undertaking a PhD in Free Radical Chemistry at The University of Melbourne. She is working

on developing radical methodology to synthesis selenium containing anti-oxidants. Selenium is useful in

the body as an anti-oxidant to mop up free radical damage and Maree’s project is looking to more

effectively synthesis such compounds.

The main reaction she is researching is illustrated below.

From her starting compound C17H18OS2Se [1] she adds an alkene [2], then irradiates it with a mercury

lamp. This cleaves the Carbon-Sulfur bond to generate a benzyl radical that adds to the alkene to give the

intermediate [3]. The intermediate radical then undergoes homolytic substitution at selenium to afford the

product [4] and a by-product [5].

As the methodology which she is working on has never been tested before, she uses NMR to calculate the

yield and identify the products of this free radical cyclisation reaction.

As she knows what the spectra of all key compounds in the reaction [1-5], she is able to calculate the yield

of the product [4] under different conditions allowing for the optimisation of the reaction. She does this

through comparing the integration of certain peaks in the product with an internal standard. Each NMR

analysis takes around 3 minutes, and Maree will probably complete hundreds for her PhD.

Spotlight on Science- Maree and the Free Radicals

For some profiles of chemists see www.freeradical.org.au then follow the links to

the community pages, then to teachers page

More complex NMR The spectra shown above are described as being one-

dimensional (1D), because we are looking at the individual resonance frequencies for the different nuclei

in a molecule. As we move towards bigger molecules with more and more atoms, the 1D spectra become very

complex, and two-dimensional (2D) spectroscopy becomes important in understanding the relationships and interactions between different atoms in the molecule.

F igure 5: 2D 1H,13C-correlation spectrum of a

neuraminic acid derivative. (Source: “Basic One- and Two-Dimensional NMR Spectroscopy”, 3rd Ed., Wiley-

VCH)

There are many different types of 2D NMR experiments, which allow scientists to determine the chains of

connectivity between atoms, bond angles, and sometimes even “through-space” distances for atoms not closely connected. In this way, the structures of large molecules, such as proteins, can be solved. Proteins form the

molecular machinery in our bodies, and so understanding their structure and function is of great use to medical science, and for the chemistry of drug design.

F igure 6: The structure of the protein SSB-2, which was elucidated by NMR at Bio21 in 2005.

(Source: http://tinyurl.com/yf5aw9 )

(Pictures\ sourced from http://tinyurl.com/ynfc3q ; http://tinyurl.com/wro2j )

NMR resonance technology is used by doctors, too, when they do

an MRI scan of a patient. MRI stands for Magnetic Resonanace Imaging – they leave the “nuclear” off the name so as not to

scare the patients!

ith the above examples as reference, try to solve the following exercises related

to NMR spectroscopy.

Exercise 2: Assign the carbons and protons that belong to each signal in the 1H and 13C NMR spectra of ethyl

acetate.

H3C O

CH2CH3

O

Ethy l acetate

13C NMR spectrum of ethyl acetate 1H NMR spectrum of ethyl acetate

W

Exercise 1: An unknown

compound has been shown to

possess the molecular formula C3H8O. There are 3 possible

structures (isomers) for this compound. Based on the 13C

NMR spectrum below, which is the likely structure.

Exercise 3: An absent-minded chemistry professor has accidentally mixed up his unlabelled medication

bottles. Fortunately he is able to analyse each of his bottles using 13C NMR spectroscopy. Which one of the bottles

(A or B) contains his adrenaline medication that is urgently needed to enable him to get through his next lecture?

HO

HO

N

CH3

OH H

Adrenaline

Bottle A Bottle B

www.freeradical.org.au

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