NMR SpectroscopyNMR Spectroscopy

NMRNMR

NMR uses energy in the radio frequency NMR uses energy in the radio frequency range.range.

This energy is too low to cause changes in This energy is too low to cause changes in electron energy levels electron energy levels oror in the vibrations in the vibrations of molecules.of molecules.

NMR can cause changes in the spin of NMR can cause changes in the spin of particles in the nucleus of some atoms.particles in the nucleus of some atoms.

Protons, neutrons and electrons spin on their Protons, neutrons and electrons spin on their axes in either an up or down direction. For this axes in either an up or down direction. For this technique, the movement of electrons is not technique, the movement of electrons is not relevant. relevant.

In many nuclei, the number of nucleons is even; In many nuclei, the number of nucleons is even; the spins are paired and cancel each other out.the spins are paired and cancel each other out.

In atoms like In atoms like 11H and H and 1313C, there is an overall spin. C, there is an overall spin.

In the presence of a strong magnetic field, In the presence of a strong magnetic field, the tiny magnetic field due to spinning the tiny magnetic field due to spinning charged particles aligns to be either charged particles aligns to be either withwith or or againstagainst the magnetic field. the magnetic field.

More nucleons will be in the lower energy More nucleons will be in the lower energy state aligned with the magnetic field.state aligned with the magnetic field.

A nucleon can absorb a quantum of A nucleon can absorb a quantum of energy in the radio frequency range and energy in the radio frequency range and align against the magnetic field.align against the magnetic field.

It emits a radio frequency when it drops It emits a radio frequency when it drops back to its original position.back to its original position.

Proton NMRProton NMR

The most common for of NMR is based on The most common for of NMR is based on the hydrogen-1 (the hydrogen-1 (11H), nucleus or proton.H), nucleus or proton.

It can give information about the structure It can give information about the structure of any molecule containing hydrogen of any molecule containing hydrogen atoms. atoms.

Complex biochemical molecules have a Complex biochemical molecules have a large number of carbon atoms so NMR large number of carbon atoms so NMR using the using the 1313C isotope is often also used.C isotope is often also used.

The difference in energy of the two spin The difference in energy of the two spin states depends on :states depends on :

The nucleus being screenedThe nucleus being screened ( ( 11H or H or 1313C)C)The other atoms around the nucleus. The other atoms around the nucleus.

These can shield the nucleus and change These can shield the nucleus and change the amount of energy needed to change the amount of energy needed to change its spin.its spin. (H in CH (H in CH33 will absorb a different will absorb a different

frequency from H in CHfrequency from H in CH22))

To standardise measurements on different NMR To standardise measurements on different NMR instruments, a standard reference sample is instruments, a standard reference sample is used in each experiment. This is used in each experiment. This is tetramethylsilanetetramethylsilane (TMS). (TMS).

This is a symmetrical and inert molecule. All H atoms have the same chemical environment and a single peak is produced from this molecule.

The difference in energy needed to The difference in energy needed to change the spin state in the sample is change the spin state in the sample is compared to TMS and is called the compared to TMS and is called the CHEMICAL SHIFTCHEMICAL SHIFT..

The chemical shift of TMS is defined as The chemical shift of TMS is defined as zerozero

The symbol The symbol represents chemical shift represents chemical shift and is measured in and is measured in ppmppm. The chemical . The chemical shift scale is measured from right to left on shift scale is measured from right to left on the spectrum.the spectrum.

The NMR SpectrophotometerThe NMR Spectrophotometer

InstrumentationInstrumentation

Main features of a basic NMR include:Main features of a basic NMR include: A radio transmitter coil that produces a short powerful A radio transmitter coil that produces a short powerful

pulse of radio wavespulse of radio waves A powerful magnet that produces strong magnetic A powerful magnet that produces strong magnetic

fieldsfields The sample is placed in a glass tube that spins so the The sample is placed in a glass tube that spins so the

test material is subject to uniform magnetic field. test material is subject to uniform magnetic field. Solid samples are dissolved in a solvent that will not give a Solid samples are dissolved in a solvent that will not give a

signalsignal A radio receiver coil that detects radio frequencies A radio receiver coil that detects radio frequencies

emitted as nuclei relax to a lower energy levelemitted as nuclei relax to a lower energy level A computer that analyses and record the dataA computer that analyses and record the data

Proton NMR is used to identify the number Proton NMR is used to identify the number of chemically distinct hydrogen of chemically distinct hydrogen ‘environments’ there are in a molecule.‘environments’ there are in a molecule.

In low resolution proton NMR, the number In low resolution proton NMR, the number of peaks is equal to the number of different of peaks is equal to the number of different bonding environments experience by the bonding environments experience by the hydrogen nuclei in the molecule.hydrogen nuclei in the molecule.

Proton NMRProton NMRLow resolution spectraLow resolution spectra

Proton NMR spectraProton NMR spectraLow resolution spectrum of ethanol

Proton NMRProton NMRHigh Resolution SpectraHigh Resolution Spectra

The NMR spectrum shows more detail. The NMR spectrum shows more detail. High resolution spectra’s show the J splitting of High resolution spectra’s show the J splitting of

the peaks.the peaks. The number of peaks caused by splitting equals The number of peaks caused by splitting equals

n + 1, where n is the number of H atoms on the n + 1, where n is the number of H atoms on the neighbouring atom i.e.:neighbouring atom i.e.: CH splits the signal from hydrogens attached to CH splits the signal from hydrogens attached to

adjacent atoms into two peaksadjacent atoms into two peaks CHCH22 splits the signal from hydrogens attached to splits the signal from hydrogens attached to

adjacent atoms into three peaksadjacent atoms into three peaks CHCH33 splits the signal from hydrogens attached to splits the signal from hydrogens attached to

adjacent atoms into four peaksadjacent atoms into four peaks

High Resolution NMR spectrum of ethanol

What the NMR spectrum tells usWhat the NMR spectrum tells us The number of peaks tell how many different The number of peaks tell how many different

proton environments are in the molecule.proton environments are in the molecule. The peak area ratio shows the relative numbers The peak area ratio shows the relative numbers

of protons in each environment.of protons in each environment. The chemical shift (measured in ppm) helps to The chemical shift (measured in ppm) helps to

identify each of the different environments and identify each of the different environments and provides information about the functional groups provides information about the functional groups to which the hydrogen is attached.to which the hydrogen is attached.

J splitting tells us how many H atoms are on the J splitting tells us how many H atoms are on the neighbouring neighbouring atom according to the rule atom according to the rule n+1.n+1. This supports the chemical shift data.This supports the chemical shift data.

11H Proton NMR Spectroscopy - Sample Spectra; EthanolH Proton NMR Spectroscopy - Sample Spectra; Ethanol

3J Coupling; n+1 = triplet

3J Coupling;n+1 = quartet

Understanding & Identifying Molecular Understanding & Identifying Molecular Structure Structure

NMR SpectroscopyNMR Spectroscopy11H NMR - Sample Spectra; CHH NMR - Sample Spectra; CH33CHClCOOHCHClCOOH

3J Coupling; n+1 = doublet

3J Coupling; n+1 = quartet

Worked Example 7.6Worked Example 7.6

Page 101Page 101

Understanding & Identifying Molecular Understanding & Identifying Molecular Structure Structure

NMR SpectroscopyNMR SpectroscopySample QuestionSample Question

Q. How could 1H NMR be used to distinguish between the two following isomers?

H3C

H2C

CH2

NO2

H3C

C

CH3

H NO2

1-nitropropane 2-nitropropane

Understanding & Identifying Molecular Understanding & Identifying Molecular Structure Structure

NMR SpectroscopyNMR SpectroscopySample QuestionSample Question

Q. How could 1H NMR be used to distinguish between the two following isomers?

H3C

H2C

CH2

NO2

1-nitropropane

1.

1. Triplet.2.

2. Sextet.

3. 3. Triplet.

Understanding & Identifying Molecular Understanding & Identifying Molecular Structure Structure

NMR SpectroscopyNMR SpectroscopySample QuestionSample Question

Q. How could 1H NMR be used to distinguish between the two following isomers?

H3C

C

CH3

H NO2

2-nitropropane

1.

1. Doublet.

2.

2. Septet.3.

3. Doublet.

1313C NMR SpectroscopyC NMR Spectroscopy

Carbon-13 is a naturally occurring isotope of Carbon-13 is a naturally occurring isotope of carbon that has nuclear spin. It is used in NMR carbon that has nuclear spin. It is used in NMR spectroscopy to identify different carbon atoms spectroscopy to identify different carbon atoms environments within a molecule.environments within a molecule.

Chemical shifts range from 0ppm to 200ppmChemical shifts range from 0ppm to 200ppm The peaks in the spectrum are a single line The peaks in the spectrum are a single line

produced for each different carbon atom produced for each different carbon atom environment.environment.

Compare the two spectra for ethanol.Compare the two spectra for ethanol.

1313C NMR spectroscopyC NMR spectroscopy

Steps for analysing NMR spectraSteps for analysing NMR spectra

1.1. Look at the number of peak sets and hence Look at the number of peak sets and hence the number of different environments the number of different environments

2.2. The chemical shift for each peak setThe chemical shift for each peak set3.3. The relative number of protons in each peak The relative number of protons in each peak

set (from the relative peak area)set (from the relative peak area)4.4. The number of fine peaks each major peak set The number of fine peaks each major peak set

is split intois split into5.5. Determine the relative number of hydrogens in Determine the relative number of hydrogens in

each environmenteach environment6.6. The protons responsible for each peak set and The protons responsible for each peak set and

the carbon to which they are bondedthe carbon to which they are bonded

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