what is nmr? nmr is a technique used to probe the structure of molecules. paired with other...
TRANSCRIPT
What is NMR?
NMR is a technique used to probe the structure of molecules.
Paired with other techniques such as MS and elemental analysis it can be used to successfully predict the structure of organic molecules.
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H1 NMR of Ethanol C13 NMR of Ethanol
How it works?
Atoms behave a little bit like magnets
Atoms have a property called spin angular momentum (I).
This can assume values of 0,, 1, , 2,
It is determined by the number of unpaired protons and neutrons in the nucleus of an atom.
Protons and nucleons (nucleons) have a spin number of
I Nuclide
0 12C, 16O1H, 13C, 15N, 19F, 29Si, 31P
1 2H, 14N11B, 23Na, 35Cl, 37Cl
17O, 27Al
3 10B
Nuclei with spin can either:-Line up with the magnetic fieldor oppose the magnetic field
Nuclei which oppose the field have a higher energy than those aligned. This gap in energy, ∆E, can be measured.
Nuclei in a low energy state can be promoted to a higher energy state if energy, equal to ∆E is provided. This is called excitation. This will soon drop back to the lower energy state, called relaxation. This process is called resonance.
What affects the magnetic field?
The strength of the field felt by each nucleus in a molecule is affected by two factors:-a) The strength of the fieldb) Shielding generated by a weak magnetic field generated by
the electrons surrounding the nucleus.
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EnergyShielding
OH
O
HO
H H
H H
The amount of shielding and
absorbed energy is called a chemical
shift, δ
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OH
O
HO
EnergyShielding
The δ Scale
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Measured against a standard, TMS.
Why TMS?• It is chemically unreactive• It has 12 equivalent protons and one carbon environment which give one sharp
and easily measured peak on both 13C and 1H spectra.• It’s nuclei are heavily shielded so there are rarely peaks below it• It is volatile and easily removed from a sample after
Solvents
NMR is usually carried out in solution.Most solvents are organic and therefore contain both carbon and hydrogen which would give peaks.Deuterated solvents are used instead. Deuterium produces no 1H NMR signal.It’s 13C is easily recognised and removed by computer software.
CCl
Cl
Cl
2H
Carbon-13 NMR
13C nuclei can be excited using a magnetic field of the energy corresponding to its energy gap.
The size of the energy gap differs depending on the electron density around each nuclei.
Carbon-13 NMR
• The number of peaks in the NMR spectra is equal to the number of carbons in chemically different environments.
Step 1: Predict the number of environments
HO O
OH
(E)-but-2-ene
Step 1: Predict the number of environments
HO O
OH
(E)-but-2-ene
3
2
Predict the number of carbon environments in each of these examples
butane2-chlorobutane
Cl
benzene
OH
phenol
Predict the number of carbon environments in each of these examples
butane2-chlorobutane
Cl
benzene
OH
phenol
4
4
1
2
Step 2: Predict Each Carbon’s Chemical Shift
e.g. HO O
OHCarbon Functional Group δ / ppm
1
2
3
1
23
Step 2: Predict Each Carbon’s Chemical Shift
e.g. HO O
OHCarbon Functional Group δ / ppm
1 C-O 60ppm
2 C-C 30ppm
3 C=O (carboxylic acid) 180ppm
1
23
Did we get it right?
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HO O
OH
1
23
Predict the chemical shift for each carbon in
O
but-3-en-2-one
Carbon Functional Group δ / ppm
1
2
3
4
1
23
4
Predict the chemical shift for each carbon in
O
but-3-en-2-one
Carbon Functional Group δ / ppm
1 C-C 40ppm
2 C=O (ketone) 200ppm
3 C=C 130ppm
4 C=C 120ppm
1
23
4
Did you get it right?
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Final NoteOH
benzene
phenol1 aromatic environment
4 aromatic environments
Proton NMR
• The number of peaks in the NMR spectra is equal to the number of hydrogens in chemically different environments.
Step 1: Predict the number of different environments and therefore the number of peaks
O O
O
H H
H
H
H
H 3-hydroxypropanoic acid
CC
CCH
HH
H
H
H
HH
(2E)-but-2-ene
Step 1: Predict the number of different environments and therefore the number of peaks
O O
O
H H
H
H
H
H 3-hydroxypropanoic acid
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CC
CCH
HH
H
H
H
HH
(2E)-but-2-ene
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4
2
Predict the number of proton environments in each of these examples
H
HH
H
H
H
H
H
HH
OH
HH
H
H
H
HH
H
H
H
H
H
H
H
H
OH
CH3
H
H
Predict the number of proton environments in each of these examples
H
HH
H
H
H
H
H
HH
OH
HH
H
H
H
HH
H
H
H
H
H
H
H
H
OH
CH3
H
H
2
1
4
4
Step 2: Predict Each Protons’ Chemical Shift
e.g.Proton Relative Area Functional
Groupδ / ppm
1
2
3
4
O O
O
H H
H
H
H
H
1
2
3
4
3
2
Step 2: Predict Each Protons’ Chemical Shift
e.g.Proton Relative Area Functional
Groupδ / ppm
1 1 O-H 1-12
2 2 O-CH 3-4.5
3 2 O=CCH 2-3
4 1 COOH 11-12
O O
O
H H
H
H
H
H
1
2
3
4
3
2
Did we get it right?
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Predict the chemical shift for each proton inProton Relative Area Functional
Groupδ / ppm
1
2
H
HH
H
H
H
H
H
HH
Predict the chemical shift for each proton inProton Relative Area Functional
Groupδ / ppm
1 6 R-CH 1-22
2 4 R-CH 1-2
H
HH
H
H
H
H
H
HH
Did you get it right?
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Spin-spin coupling
On high resolution proton NMR you may see little clusters rather than a single peak. These ‘clusters’ can help you identify the number of protons in the environment next to it.
The N+1 RuleN N+1 Multiplicity
0 0+1 = 1 Singlet
1 1+1 = 2 Doublet
2 2+1 = 3 Triplet
3 3+1 = 4 Quartet
The relative intensities of the lines can be determined by Pascal’s triangle.i.e. a doublet has a relative intensity of 1:1 a triplet has an intensity of 1:2:1 a quartet has an intensity of 1:3:3:1
The gap between the peaks also stays the same and is determined by the adjacent proton environment, this is called the coupling constant.
The N+1 rule
We can use the N+1 rule to identify how many protons are in the environment next to it.Example.
O
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The N+1 rule
We can use the N+1 rule to identify how many protons are in the environment next to it.Example.
O
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Look at this CH2 group, it has a CH3 group next to it, with 3 protons.
3+1 = 4,Therefore we expect to see 4 mini peaks next
to it, or a quartet.
The N+1 rule
We can use the N+1 rule to identify how many protons are in the environment next to it.Example.
O
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Look at this CH3 group. It has no protons next to it.
0+1 = 1, therefore we expect to see one peak, or a singlet.
The N+1 rule
We can use the N+1 rule to identify how many protons are in the environment next to it.Example.
O
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Look at this CH3 group. It has a CH2 group next
to it. The CH2 group contains 2 protons.
2+1 = 3Therefore we expect to see 3 miniature peaks,
or a triplet.
The N+1 Rule, 2nd Example
O
HH
HH
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This aldehyde proton has a CH3 group next to it.
3 + 1 = 4, Therefore we see a quartet
group.
The N+1 Rule, 2nd Example
O
HH
HH
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This CH3 group has a group next to it with one proton.
1 + 1 = 2, therefore we see a doublet.