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Basics of NMR
In IB chemistry, NMR is an analytical technique used for examining molecular structure; the isotopes that are most commonly investigated are 1H, 13C and 31P.
A strong magnetic field, applied using an electromagnet, and low-energy radio frequency radiation are required for NMR spectroscopy.
Nuclear spin (non-examinable)
- Protons and neutrons are collectively known as nucleons.
- Nucleons can ‘spin’ in one of two directions in IB chemistry
- Opposite spins pair up, but if there is an odd number of nucleons then a small residual nuclear spin is produced – which generates a magnetic field and a signal on an NMR spectrum.
- The nuclei, now tiny magnets, can either line up with the field or be opposed to the field
- Nuclei that oppose the field have a higher energy and if the external magnetic field is stronger there will be a larger energy gap.
Resonance (non-examinable)
- A nucleus can be promoted to its upper-energy spin state by providing energy equal to the gap – known as excitation.
- In NMR the excitation is provided by low-energy radio-frequency radiation.
- When the nucleus emits the energy it will drop back to its low-energy spin state – known as relaxation.
- This cycle of excitation and relaxation is known as resonance. It continues so long as there is a supply of energy (radio radiation) that exactly matches the energy gap – it is the basis for nuclear magnetic resonance (NMR).
Nuclear shielding (non-examinable)
- The strong external magnetic field and weaker magnetic field generated from the atoms electrons and surrounding atoms determine how much of the magnetic field is felt by the nucleus.
- An atom’s nucleus can be shield from the magnetic field by its electrons – nuclear shielding. The extent of the shielding depends on the density of nearby electrons.
- Therefore atoms in different environments have different resonance frequencies – from this we can calculate chemical shift.
Chemical shift
- Chemical shift (δ) – a scale that compares the frequency of an NMR absorption with the frequency of the reference peak of TMS at δ=0 ppm.
- In IB chemistry, TMS (tetramethylsilane) – a standard compound used as the reference signal for chemical shift because it has 12 protons and gives rise to a single sharp peak. (Chemically unreactive and volatile so it can be easily removed from the system after calibration)
- Chemical shift is the place on an NMR spectrum where the nucleus absorbs energy – it’s measured in ppm (parts per million) and calibrated using the delta scale.
End of this topic!
Drafted by Gina (Chemistry)