6.3.2 NMR/Mass Spec Flashcards
Nucleus spin
Only in atoms with an odd mass number.
Gives the nuclei magnetic properties (13C, 1H)
When sample placed in a magnetic field
The nuclei align themselves with the direction of the applied field.
Or can align in the opposite direction.
The spin state with the magnetic field is a lower energy than the spin against the field.
Difference of energy varies with the strength of the magnetic field.
Nuclei with the magnetic field can change their spin to the opposite state if they absorb energy that matches the energy required for the transition.
Energy absorbed corresponds to radio frequencies.
Resonance condition
Flipping of the proton from one magnetic field alignment to the other by radio waves
What is the effect of electrons around the resonator nucleus
The magnetic field needed to achieve resonance.
To cut down the the size of the external magnetic field felt by the Hydrogen nucleus.
Provides a shield between the resonator nucleus and the magnetic field.
Effect of electron density
The greater the electron density the higher the magnetic field strength needed for resonance.
Hydrogen’s environment in different bonds
In C-H bond:
C and H have similar electronegativities.
Electrons in the bond are shared fairly equally, electron density close to the H is quite high.
E- act as a shield and cut down the effect of the external magnetic field around the H nucleus.
In O-H bond:
Oxygen is more electronegative than H, attracts electrons more in the bond, reducing the E-density around the H atom.
H is de-shielded so reaches resonance at a lower external magnetic field.
Electrons are further away from H nucleus so do not affect the magnetic field around it as much.
Counting carbon environments
Number of signals in a 13C NMR spectrum is equal to the number of different environments for carbon in the molecule.
Carbons bonded to different atoms/groups are in different environments because electrons in these affect the exposure of that carbon to the magnetic field
