Advanced Higher Unit 3 Nuclear Magnetic Resonance Spectroscopy.

Advanced HigherAdvanced HigherUnit 3Unit 3

Nuclear Magnetic ResonanceNuclear Magnetic ResonanceSpectroscopySpectroscopy

NMR SpectroscopyNMR Spectroscopy

NMR is one of the most important NMR is one of the most important analytical techniques available to an analytical techniques available to an organic chemist.organic chemist.

It uses a small sample size and is non-It uses a small sample size and is non-destructive.destructive.

A single spectrum can be used to derive a A single spectrum can be used to derive a huge amount of information concerning huge amount of information concerning the environment of the carbon and the environment of the carbon and hydrogen atoms in an organic molecule.hydrogen atoms in an organic molecule.

Any spinning charged particle produces Any spinning charged particle produces a magnetic field i.e. it will behave like a a magnetic field i.e. it will behave like a tiny magnet.tiny magnet.

Any Any 11H nucleus will have either of two H nucleus will have either of two possible spin states.possible spin states.

The The 11H nucleus is simply a single proton, H nucleus is simply a single proton, therefore it behaves like a spinning therefore it behaves like a spinning proton and so produces a magnetic field.proton and so produces a magnetic field.

The nuclei with one spin state will align The nuclei with one spin state will align themselves in the direction of the themselves in the direction of the magnetic field while the nuclei with the magnetic field while the nuclei with the other spin state will oppose the other spin state will oppose the magnetic field.magnetic field.

Under normal conditions these magnetic Under normal conditions these magnetic fields will not interact with one another, fields will not interact with one another, i.e.i.e.

When a strong magnetic field is applied, When a strong magnetic field is applied, these nuclei will line up along the these nuclei will line up along the applied field.applied field.

However for reasons best left to However for reasons best left to quantum mechanics, some of the nuclei quantum mechanics, some of the nuclei magnetic field line up with the applied magnetic field line up with the applied field and others line up against it.field and others line up against it.

AppliedAppliedMagneticMagneticFieldField

These nuclei can be describes as being These nuclei can be describes as being in a low energy state (in a low energy state ( i.e. those i.e. those aligned with the field) or a high energy aligned with the field) or a high energy state (state ( i.e. those aligned against the i.e. those aligned against the energy field)energy field)

The energy difference between the The energy difference between the low and high energy states low and high energy states corresponds to the energy of corresponds to the energy of electromagnetic radiation in the electromagnetic radiation in the radio frequency (rf) range (60 MHz radio frequency (rf) range (60 MHz - 1000 MHz).- 1000 MHz).

An rf pulse applied to the nuclei in An rf pulse applied to the nuclei in the magnetic field will supply the magnetic field will supply enough energy to excite (or ‘flip’) enough energy to excite (or ‘flip’) some nuclei from the low to the some nuclei from the low to the high energy state.high energy state.

En

ergy High energy

Low energy

rf pulse Energy emitted as nuclei fall back to lower state

Emitted rf radiation detected and measured

• When the pulse ends the excited nuclei will flip back to the lower energy state. When this happens radiation will be emitted.

• The frequency of the rf pulse must be in ‘resonance’ with the nuclei before it can be excited.

The frequency of the radiation emitted as The frequency of the radiation emitted as the nuclei flip from the high energy state the nuclei flip from the high energy state to the low energy state is then measured.to the low energy state is then measured.

The intensity of the radiation emitted is The intensity of the radiation emitted is very weak therefore the pulse must be very weak therefore the pulse must be applied many times and the applied many times and the measurements repeated and added measurements repeated and added together to build up useful results.together to build up useful results.

Why is NMR useful?Why is NMR useful?

The energy difference between the low and high The energy difference between the low and high energy states will not be the same for the different energy states will not be the same for the different hydrogen nuclei within a molecule.hydrogen nuclei within a molecule.

The energy difference will depend on two factors -The energy difference will depend on two factors -1) Strength of the applied field.1) Strength of the applied field.

The stronger the field the bigger the energy The stronger the field the bigger the energy difference. difference.

This is normally kept constant during an NMR This is normally kept constant during an NMR experiment.experiment.

2) The ‘environment’ of the hydrogen nuclei.2) The ‘environment’ of the hydrogen nuclei. Protons aren’t the only moving charges in a Protons aren’t the only moving charges in a

molecule.molecule. Electrons will also generate a magnetic field.Electrons will also generate a magnetic field. These magnetic fields ‘shield’ the hydrogen These magnetic fields ‘shield’ the hydrogen

nuclei from the full effect of the applied nuclei from the full effect of the applied magnetic field.magnetic field.

The degree of shielding, and hence the The degree of shielding, and hence the

magnetic field, and therefore the energy magnetic field, and therefore the energy

difference, experienced by each hydrogen difference, experienced by each hydrogen

nuclei in a molecule will depend on the nuclei in a molecule will depend on the

density of electron clouds of the surrounding density of electron clouds of the surrounding

atoms, i.e. the ‘environment’ of the atoms, i.e. the ‘environment’ of the hydrogen nuclei. hydrogen nuclei.

This means that hydrogen nuclei in This means that hydrogen nuclei in different environments will emit rf different environments will emit rf radiation with differing frequencies.radiation with differing frequencies.

By measuring the emitted frequencies By measuring the emitted frequencies and comparing them with a correlation and comparing them with a correlation chart (p 15 of Data Book) you can chart (p 15 of Data Book) you can discover how many different hydrogen discover how many different hydrogen environments there are in a particular environments there are in a particular compound.compound.

The area underneath the peak in an NMR The area underneath the peak in an NMR spectra will also tell you how many spectra will also tell you how many hydrogen atoms share a particular hydrogen atoms share a particular environment.environment.

Different NMR spectrometers apply different Different NMR spectrometers apply different strengths of magnetic field and therefore give strengths of magnetic field and therefore give different results.different results.

A reference standard is used in all NMR A reference standard is used in all NMR experiments so that results from one experiments so that results from one spectrometer can be compared with another.spectrometer can be compared with another.

That reference standard is Tetramethylsilane That reference standard is Tetramethylsilane (TMS)(TMS)

Any solvents used must not contain any Any solvents used must not contain any 11H H atoms. So ‘deuterated’ solvents are used, e.g. atoms. So ‘deuterated’ solvents are used, e.g. CDClCDCl3.3.

N.B. Deuterium (D) = N.B. Deuterium (D) = 22HH

along with TMS standard

A Low Resolution NMR SpectrometerA Low Resolution NMR Spectrometer

The applied rf pulse contains a wide The applied rf pulse contains a wide enough range of frequencies to flip nuclei enough range of frequencies to flip nuclei in all the different environments.in all the different environments.

The scale used to measure the emitted The scale used to measure the emitted radiation is called the chemical shift (radiation is called the chemical shift () ) and has the units parts per million (ppm).and has the units parts per million (ppm).

The peak associated with TMS is allocated The peak associated with TMS is allocated a value of 0 (zero) on the scale.a value of 0 (zero) on the scale.

Why TMS?Why TMS?

TMS has 12 equivalent hydrogen atoms TMS has 12 equivalent hydrogen atoms (they all have the same environment) and (they all have the same environment) and therefore produces a sharp signal.therefore produces a sharp signal.

The signal it produces is well away from The signal it produces is well away from the region of signals produced by other the region of signals produced by other organic hydrogen atoms.organic hydrogen atoms.

Interpreting NMRInterpreting NMR The number of peaks will give the The number of peaks will give the

number of different environments of the number of different environments of the hydrogen atoms.hydrogen atoms.

NOTENOTE

The number of peaks The number of peaks does notdoes not give give you the number of hydrogen atoms.you the number of hydrogen atoms.

e.g. e.g. The NMR for benzene, CThe NMR for benzene, C66HH66, will , will only show one peak as all the only show one peak as all the hydrogen atoms are equivalent.hydrogen atoms are equivalent.

The ratio of the areas of each peak The ratio of the areas of each peak will give you the number of hydrogen will give you the number of hydrogen atoms in each environment. atoms in each environment.

The areas may be given by an The areas may be given by an integration curve, that you will have integration curve, that you will have to measure using a ruler.to measure using a ruler.

An NMR SpectraAn NMR Spectra

1 : 3 peak ratio

Methanol

The large peak can be assigned to the The large peak can be assigned to the hydrogen atoms of the methyl (CHhydrogen atoms of the methyl (CH33-) -) group.group.

The small peak can be assigned to the The small peak can be assigned to the hydrogen on the hydroxyl (-OH) group.hydrogen on the hydroxyl (-OH) group.

The hydroxyl hydrogen is ‘shifted’ more The hydroxyl hydrogen is ‘shifted’ more than the methyl hydrogen atoms.than the methyl hydrogen atoms.

ExerciseExercise

Now try the exercise on pages 16 Now try the exercise on pages 16 and 17 of your Unit 3(d) notes.and 17 of your Unit 3(d) notes.