Topic 9: NMR Flashcards
(9 cards)
1
Q
Define NMR spectroscopy
A
- Some nuclei = finite spin
- Positively charged = create electric/magnetic field
- Magnetic field = interacts with external magnetic field
- Either 13C/1H nuclei = have finite spin
- Mainly proton NMR = low abundance of C13
2
Q
Explain the magnetic field
A
- In presence of external = spins align
- Parallel/antiparallel spin
- Antiparallel = higher energy
- Transition can take place = energy separation small = radiowaves
- Strength of external increase = energy separation increases
3
Q
Describe the NMR spectrometer
A
- Sample placed in magnet connected to radio transmitter
- RF = constant
- Higher RF = better resolution
- Magnet strength = change
4
Q
Why do nuclei absorb energy at different RF?
A
- Due to e- = shielding effect
5
Q
Describe shielding effect
A
- Thicker e- cloud = strength of external magnet increase = penetrate = resonance
- Surrounding atoms = effect shielding e.g. O2 = high electronegativity = lowers sheilding
6
Q
Describe the NMR axis
A
- X-axis = chemical shift = PPM
- More sheilding = more right on axis = higher external magnetic field
- Less sheilding = to the left
7
Q
Describe TMS
A
- Signals 0 point of chemical shift
- Tetramethylsilane = Si(CH4)4
- Difference in electronegativity of Si + C
- Methyl groups = highly shielded = e- flow towards more electronegative C
8
Q
Define proton equivalence
A
- H atoms can be equal = similar chemical environments = due to C-C rotation
- C=C have no rotation= each branch may be different
9
Q
What does the area under the curve give?
A
- Number of protons = H atoms
- Will correspond to expected ratio values
