Summary EDEXCEL A-LEVEL CHEMISTRY REVISION NOTES: MODERN ANALYTICAL TECHNIQUES

,
, -Modern Analytical Techniques 2


Nuclear Magnetic Resonance (NMR)
·
Nucleiofcertain atoms are considered to spin

· Protons and Neutrons Spin in opposite directions
· Nucleus Spins pair up
· When a nucleus has an odd pair of nucleons, there is a nucleon

with an unpaired spin

· This generates a magnetic field

· ifthe nucleus is placed in an external magnetic field can
it




↳line up with the external magnetic field
* line up against the external magnetic field
·
Alignmentwith external magnetic field is more stable. Energy mustbe

absorbed to flip nuclear magnetic field
· A
nucleus lined up againstthe field is regarded as less stable higher
energy state


o to flip the nucleus energy mustbe supplied equal to be

· When itflips from low to high, this is called exitation

· it will shortly flip back -
relaxation


o Process ofFlipping is called resonance



Magnetic field feltby a nucleus depends on

· Applied magnetic force
o weaker magnetic force generated electrons
by surrounding the nucleus

In nearby atoms -
nuclear shielding
· Extentofnuclear shielding depends on electron density in nearby atoms
groups ofatoms

o
changing environmentofthe nucleus
changes the energy gap

·
Spectrum shows many absorbtion peaks

· Relative positions reflectenvironments of nuclei


Two Types of NMR

· Proton NMR

· Carbon NMR


Data interpretation
· Number ofSignals -

no of environments
·
Position ofsignal-chemical shift-type ofenvironment
·
Intensity ofSignal- no. environment
ofnucleiin that

·
Splitting -
tells aboutenvironmentofa proton with to
respect other

nearby proton/Proton NMR only