3.3.15 Nuclear Magnetic Resonance Spectroscopy Flashcards
Which elements flip under radio waves?
Any which has an odd number of nucleons e.g. hydrogen
What happens when a nuclei flip from the beta position to the alpha position?
(Can only happen once the radio waves have been removed)
The energy is released
What happens when atoms with an odd number of nucleons are exposed to radio waves?
They will line up and the nuclei align with the magnetic field
If you supply enough radio energy, the atom in the sample will flip and stay in the wrong orientation (beta)
How can nuclear magnetic resonance be used?
The amount of energy given out is proportional to the number of hydrogen atoms
The larger the chemical shift, the more energy is given out
How does electronegativity affect shielding?
The stronger the electronegativity, the more deshielded the nucleus is due to a decreased electron density
What effect does increased deshielding have on nuclear magnetic spectroscopy?
Once deshielded, the atom is less electronegative so requires more energy to be flipped by the magnets
How is an NMR machine structured?
See card
Are nuclei with more electrons around them more or less shielded?
More
What is a proton also known as?
Hydrogen
What is a hydrogen atom also known as?
Proton
How does the electron density relate to the resonating frequency of an atom?
The greater the electron density
The smaller the magnetic field felt by the nucleus
The lower the frequency at which it resonates
How does a more deshielded nucleus affect the chemical shift?
The nucleus feels a greater magnetic field
Therefore there is a higher chemical shift
What is being examined in proton NMR?
The H+ nucleus that is being examined
What is the chemical shift?
It tells us the strength of the magnetic field that the nucleus feels
Related to the difference between the resonating nucleus to zero
What occurs to the chemical shift when a hydrogen atom is further from an electronegative atom?
The smaller the shift
What does the area under the proton NMR relate to?
The number of hydrogen atoms
What does the integration trace give?
The area of the peaks
What is spin-spin splitting?
When the magnetic field felt by a hydrogen atom is affected by magnetic fields of neighbouring hydrogen atoms
What is shielding?
Where electrons in an atom can shield each other from the pull of the nucleus
The more electron shells there are, the greater the shielding effect experienced by the outermost electron
What do the peaks look like when there is one hydrogen atom on the adjacent atom?
1 : 1
What do the peaks look like when there are two hydrogen atoms on the adjacent atom?
1 : 2 : 1
What do the peaks look like when there are three hydrogen atoms on the adjacent atom?
1 : 3 : 3 : 1
If there are n hydrogen atoms on an adjacent carbon atom, how many peaks will that be?
n + 1
Why is Deuterium used as an NMR solvent?
It is an isotope of hydrogen which doesn’t produce an NMR signal in the same range as hydrogen
What are the two types of NMR?
13C NMR and proton NMR
Why is no splitting seen in 13C NMR?
Because there is a low concentration of 13C compared to 12C, it is unlikely that two 13C atoms would be next to eachother
Therefore the signal is not changed
How do you work out the number of peaks in H NMR?
Add the number of hydrogens on adjacent carbons then add one
What are the first five ratios for splitting peaks?
1 1 : 1 1 : 2 : 1 1 : 3 : 3 : 1 1 : 4 : 6 : 4 : 1
Why does carbon 12 not respond to NMR?
It’s nucleus don’t have a spin
How is shielding linked to chemical shift?
Shielding decreases the nucleus’ pull on other electrons
This causes a lower electronegativity
Therefore the nucleus feels a weaker magnetic field
Therefore there is a reduced chemical shift
