Nuclear Magnetic
Resonance (NMR)
spectroscopy
What is NMR spectroscopy?
• Nuclear Magnetic Resonance (NMR) spectroscopy is an analytical technique that is
used to determine the structure of organic compounds.
• NMR is a non-destructive approach that only requires a small amount of sample.
• NMR observes the behaviours of nuclei and how they spin when placed in a magnetic
field. However not all nuclei are suitable for NMR.
•
1H
and 13C are the most important NMR active Nuclei.
• The two types of NMR are 1H NMR (proton NMR) and
13C
NMR (carbon NMR).
NMR Instrument Composition
How does NMR work?
• The principle behind NMR is that many nuclei have spin and all nuclei are electrically
charged.
• If an external magnetic field is applied, an energy transfer is possible between the base
energy to a higher energy level (generally a single energy gap).
• The energy transfer takes place at a wavelength that corresponds to radio frequencies
and when the spin returns to its base level, energy is emitted at the same frequency.
• The signal that matches this transfer is measured in many ways and processed in order
to yield an NMR spectrum for the nucleus concerned.
NMR Spectra
To determine a molecules structure, an
NMR spectra provides two pieces of
information:
• Chemical shift – information on the type
of bond
• Spin-spin coupling constant –
information on the atom’s environment
• Integration under the peaks – number of
hydrogens or carbons
Chemical Shifts
• The different local chemical environments surrounding any particular nuclei causes them
to resonate at slightly different frequencies. This is a result of a nucleus being more or
less shielded than another. This is called the chemical shift (δ).
• Using chemical shifts, we are able to predict the expected environments of a peak.
Functional Group
Chemical Shift Range (ppm)
Alkyl (e.g. methyl-CH3)
~1
Alkyl adjacent to oxygen (-CH2-O)
3-4
Alkene (=CH2)
~6
Alkyne (C-H)
~3
Aromatic
7-8
Hydrogen and carbon environments
• A chemical environment is made up of atoms and electrons that surround a specific
atom.
• Inside a molecule, atoms can be said to have the same chemical environment if they are
attached in the same way to the same atoms.
Proton NMR Spectroscopy
• Proton NMR is used to identify the different hydrogen atoms in a molecule. This involves
finding the number of chemically district hydrogen environments there are in a molecule.
Reference Peak
• Each NMR spectra will have a peak at zero. This is the zero point. It is used to scale the horizontal
axis,
• The molecule usually used for the zero peak is tetramethylsilane (TMS). Everything is compared to
TMS.
• TMS is used, because:
•
It has 12 hydrogen atoms all of which are in exactly the same environment. They are joined to exactly the
same things in exactly the same way. That produces a single peak, but it's also a strong peak.
•
The electrons in the C-H bonds are closer to the hydrogens in this compound than in almost any other one.
That means that these hydrogen nuclei are the most shielded from the external magnetic field.
Signal Splitting
• In high resolution NMR spectra, signals may become split. This is
caused by the effect of neighbouring protons.
• The number of lines in a signal are related to the number of protons
connected adjacent atoms. Protons on adjacent carbons will interact
and “split” each others resonances into multiple peaks.
• The splitting of peak follows this rule:
• n + 1 rule – equivalent protons that have n equivalent protons on the
adjacent carbon will be split into n + 1 peaks.