NMR Flashcards
What is NMR particularly good for?
organic compounds
What part of a molecule does NMR detect?
The positively charges nucleus
Why can you not use nuclei with an even number of protons and neutrons in NMR?
bc they will have 0 spin
General premise of NMR
Magnetic field applied
Protons will align or oppose the field
Will give different signals depending on what environment they are in.
The NMR will measure the resonant frequency that causes the spin flip
Basic principles of NMR
Sample in solution
put in magnetic field
Absorption of energy causes occurs with spin flip
Measurement of signal
3 things to look at in the signals of NMR
Position = depends on chemical environment (what its next to) integration = how many relative protons experiencing that environment. Splitting = how many neighbouring protons
What does splitting tell you in NMR?
How many neighbouring protons there are.
N + 1 rule
What does the integration of signals on NMR spectra show?
Ratio of how many protons are experiencing that environment
What does the position of a signal on NMR spectra tell you?
What groups are near it, eg whether electronegative etc
Why does the nucleus have a magnetic moment?
It is a charged moving object so has a magnetic dipole and magnetic moment
What frequency does NMR use?
Radio waves
How can you increase the sensitivity of NMR?
By increasing the magnetic field strength
Why are quartz sample tubes used instead of glass ones?
Glass would absorb radiation bc it has things like iron in it which would interfere with the magnetic field.
What does the position of the signal in NMR depend on?
Chemical environment - what is it next to?
Are the energy levels of spin quantised?
Yes
What does the number of spin energy levels depend on?
l bc number of energy levels is given by 2I+1
What are spin energy levels before a magnetic field is applied?
Degenerate
What can increase the sensitivity of NMR?
The size of the magnet
also lower temp can enhance sensitivity
What is the frequency of applied radiation proportional to? (NMR)
the magnetic field strength.
reminder (NMR) frequency is directly proportional to…
Magnetic field
The bigger the magnet the…
NMR
Stronger the field
What does Boltzman distribution determine (NMR)
Whether protons align or oppose the applied magnetic field.
Why is NMR a low sensitivity technique?
Bc were only measuring the spin transitions
What is spin-lattice relaxation? (NMR)
When excess energy is dissipated into surrounding atoms and molecules
What is spin-spin relaxation? (NMR)
Transfer of excess energy to other magnetic nucleii in sample
If a NMR spectrometer has a higher MHz value is it more or less sensitive?
More
Why do you need to use specific solvents in NMR?
Because hydrogens from this will also be picked up in the spectra.
Why are deuterated solvents used in NMR?
Bc deuterium will not be be picked up at those resonance frequencies. Normal Hydrogen would overshadow sample
3 types of magnet…
permanent magnet
Electromagnet
Superconducting magnet
