NMR is a very useful technique. Why? Because it allows you to find information about the structure of your organic molecule.
What is nuclear spin?
Nuclear spin is all about the mass number of the atom.
Mass number = protons (+) and neutrons (+/-)
When the mass number is ODD then there is spin present because how will you evenly divide the subatomic particles? Imagine placing a basketball on the table … if the basketball is heavier on one side compared to the other, then there will be ‘spin’ …. it will roll on the table! This is the same will subatomic particles.
What happens when nucleus spins?
When the nucleus spins, because it contains charge , it will also make a magnetic field. These are physics laws…
So every atom in your structure that is able to spin will cause its own ‘mini’ magnetic field depending on the direction it spins. This will happen naturally and will not need any external energy to take place.
What does NMR find?
NMR will use an external magnet on the molecules it identifies. These molecules will align/against themselves with the external magnetic field.
Alignment with the magnetic field; +1/2 state (or beta)
Alignment against the magnetic field; -1/2 state (or alpha)
Some ‘mini’ magnetic field will go with the external magnet of the NMR machine – this is a LOWER energy state…. think of going with the river current. Whereas some atoms will go against the external magnetic field (swimming upstream) and so these go into alpha state.
The energy difference between alpha and beta state.
There is a difference between alpha and beta state because as was mentioned before, one state is moving with the magnetic field – the other against. However the difference between alpha and beta state is not very big. The difference in energy is radio-waves! Remember that alpha and beta will be separated radio-waves…. this is important.
Will all the parts of a molecule, that are capable of spin, have the same difference of energy between alpha and beta state?
NO! That is why NMR works. If each spinning atom received the same amount of external magnetic field then yes the difference should be the same. However this is not the case! Why?
Basically electrons are able to reduce the ‘power’ of the external magnetic field. So therefore any atom that is covered with electrons will feel less magnetic field strength. This will cause a decrease between the energy of alpha and beta state.
On the other hand when an atom is DEPRIVED of electrons (possibly close to electronegative atoms) then there will be nothing shielding the atoms – and therefore the amount of magnetic power felt will be more.
Therefore due to surrounding conditions – the alpha and beta states change in energy difference. So if you send radio waves through the molecules different regions will absorb different energies of radio waves.
What happens when radio waves go through the sample?
If you are analysing a sample – the radio waves of all possible energies will go through your sample. Spinning molecules that are in close contact with electronegative atoms will be less shielded and there will be a large difference between beta and alpha state. Therefore radio waves of high difference in energy will be absorbed in this area … because there is a bigger step to turn beta state into alpha state.
On the other hand spinning atoms that are covered with electrons are more shielded and the difference between alpha and beta state is smaller and thus lower energy radiowaves are required to convert beta state into alpha.
So why is this called NMR spectroscopy?
Nuclear – analysis of the nuclear spin
Magnetic – addition of external
Resonance – the radio waves allow beta –> alpha state movement (and back to alpha) thus resonance.
