nuclear magnetic resonance spectroscopy workshop objectives: by the end of this workshop you will be...

Nuclear Magnetic Resonance Nuclear Magnetic Resonance Spectroscopy WorkshopSpectroscopy WorkshopObjectives: By the end of this workshop you will be able to ...

1. ... describe how n.m.r. spectra are obtained and how they can

be used to tell us about the structure of an organic molecule.

2. ... use low-resolution proton n.m.r. spectra to identify different

chemical environments within a molecule (including use of the integration trace and chemical shift values).

3. ... explain how the fine structure in high-res spectra arises

anduse it to find the identity of unknown samples (using

mass and i.r. spectra as additional evidence where

necessary).

Throughout this workshop:

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View NMR section in Edexcel A2 SpecificationView NMR section in Edexcel A2 Specification

NMR Chemical Shift dataNMR Chemical Shift dataNMR IR

IR dataIR data

Introduction to Nuclear Magnetic ResonanceIntroduction to Nuclear Magnetic Resonance

Interpreting NMR spectraInterpreting NMR spectra

Using spectra to identify moleculesUsing spectra to identify molecules

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Introduction to NuclearIntroduction to NuclearMagnetic ResonanceMagnetic Resonance

What is resonance?What is resonance?

A model of magnetic resonanceA model of magnetic resonance

How an NMR spectrometer worksHow an NMR spectrometer works

MagMagnetnetic moments!ic moments!

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ExamplesExamples ofofresonanceresonance

(so

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it?)

(so

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So what is So what is nuclearnuclearmagneticmagnetic resonance? resonance?

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Some nuclei can act like little magnets. They will line up with an external field but can be flipped to oppose the field if the correct amount of energy is applied. They will thenrelax back to their ground statewhen the energy input ceases.

This flipping occurs because atomic nuclei are so small that the alignment is quantised.

The energy required to resonate the nuclei depends on the element and on the size of the external field, but is usually around 60 – 100 MHz for hydrogen when we carry out NMR. (Radio 1 is 98.8MHz so that would do!) We can investigate what affects this resonant frequency using this equipment ...

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A model of magnetic resonance A model of magnetic resonance Magnetic stirrer

(oscillator)Permanent magnet Compass needle

Switch on the magnetic stirrer and very slowly increase the frequencyof oscillations (i.e. turn its speed up) until you hit the resonant frequency of the compass ... you will see it swing wildly!

Try achieving the resonance again by fixing the speed (frequency) and moving the permanent magnet closer to increase the field strength.

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Investigating magnetic resonance Investigating magnetic resonance

If you are very careful, you can

slowly increase the frequency and get

each compass needle to resonate

individually.

What effect does shielding the compass needle (e.g. with iron) have on the resonance?

Are the other resonances

affected?

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How an NMR spectrometer works How an NMR spectrometer works

A typical machine for proton NMR will need to operate at 60–100 MHz requiring a field strength of 1.4 – 2.3 Tesla. The high fields at the top end of this range require superconducting magnets which must be cooled by liquid helium. Many modern research NMR machines can run at 300-600 MHz and there are even 800MHz machines in use with fields of more than 23 Tesla. A 1GHz machine is undergoing tests!

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NMR spectrometer gallery 1NMR spectrometer gallery 1

A 60MHz NMR machine...

... and its superconducting 600MHz cousin

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NMR spectrometer gallery 2NMR spectrometer gallery 2

A medical NMR scanner (MRI machine)

An NMR machine for probing proteins

and an 800MHz behemoth!

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MagMagnetnetic moments! ic moments!

Only nuclei with a magnetic moment will be able to undergo nuclear magnetic resonance 1HThis means nuclei with an odd number of

protons, or an odd number of neutrons ... or both!

But what effect does the environment within a molecule have on the resonance?

Interpreting Nuclear Magnetic Interpreting Nuclear Magnetic Resonance SpectraResonance Spectra

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PN1 and PN2 (Low-Resolution)PN1 and PN2 (Low-Resolution)

PN2 (Low-Res) and PN3 (High-Res)PN2 (Low-Res) and PN3 (High-Res)

PN4PN4

PN6PN6

PN8 PN8

PN5PN5

PN7PN7

PN9 PN9

NMR Chemical Shift dataNMR Chemical Shift dataNMR Main

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NMR

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NMR

Using spectra to identifyUsing spectra to identifyorganic moleculesorganic molecules

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Task 1Task 1

Task 3Task 3

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Task 4Task 4

NMR Chemical Shift dataNMR Chemical Shift dataNMR IR

IR dataIR data

For each task you will be given the mass, proton nmr and infrared spectrum of an unknown organic molecule. Discuss the data with your partner and attempt to determine the molecule. Be prepared to give evidence for your structure.

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The mass spectrum will give you the relative molecular mass. I have marked the parent (molecule) ion on each spectrum with *. You should identify possible fragments (all 1+ ions) for the major peaks.

s = singlet d = doublet t = triplet q = quartet sept = septet m = complex (unresolved) multiplet

The i.r. spectrum can be used to deduce of confirm the presence of particular bonds (such as carbonyls and hydroxyls) in your molecule. Remember the effect of H-bonding on the infrared spectrum.

i.e. 1H 2H etc gives ratio of areas under peaks

The proton n.m.r. spectrum will indicate the number of different kinds (environments) of hydrogens in your molecule. I have provided the ratio of areas under the peaks and an indication of the splitting pattern in case it is not clearly visible.

t/d = triplet of doublets etc

NMR

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NMR SpectrumNMR Spectrum IR SpectrumIR Spectrum

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Edexcel A2 SpecificationEdexcel A2 Specification

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