NMR Flashcards
What does NMR Stand For?
Nuclear Magnetic Resonance
Explain The Principles of How NMR Works (5)
Nuclei with Spin produce weak magnetic fields
NMR fires out EM (radio) waves at a specific frequencies
The Nuclei with spin that are parallel with the larger magnetic Field absorb the energy
and flip to the higher energy level and are now anti-parallel with the larger magnetic field
This Energy Change can be monitored and recorded
An Nucleus has spin when ……
The atomic nucleus has a odd number of nucleons
-NMR only works with nucleons with uneven nucleons
How would you carry out NMR spectroscopy? (7)
Dissolve liquid sample in a suitable solvent
place dissolved sample along with a small amount of TMS inside a tube
put the tube into the NMR machine
zero the spectrometer against the value read for TMS
The sample is spun to even out ant imperfections in the magnetic field
Radiation with different radio frequencies but a constant magnetic field is applied to the sample
any absorbtions due to resonance are detected and plotted
Name a use of NMR
MRI scans
What kind of Nuclei does NMR work with?
Provide Examples
(protium) H-1 & C-13
Those Nuclei with an uneven number of nucleons
What defines the resonant frequency of a C-13 atom
(What affects the chemical shift of an NMR)
- The chemical environment it is in
-The amount of electron shielding it has
What type of graph is produced by NMR
Energy absorbed against Chemical shift
What is Chemical Shift?
What are its units?
The Resonant frequency of the nuclei compared to that of a H-1 atom in TMS
Parts per million (ppm)
What is the range of Chemical shift for C-13 NMR?
0-200 ppm
What does it mean when C-13 atoms show a different chemical shift value?
Having different chemical environments
What kind of environment leads to a greater chemical shift? (C-13)
When a carbon atom is adjacent to a more electronegative atom/portion (such as C=C, or -Br)
Why is it easier to get a spectrum of H-1 NMR then C-13 NMR
Most atoms that are Hydrogen are protium (H-1) so most have Nuclear spin that can be recognised by NMR
Much more abundant then C-13 relative to C-12
What is the Range of Chemical Shift for H-1 NMR
0-10 ppm
What leads to a smaller chemical shift value for H-1 NMR
H-1 with a greater amount of electrons surrounding them
- Further away from electronegative groups/atoms
Hydrogens resonate more like a hydrogen atom in TMS
On a low resolution spectrum
What peaks you would expect to see for H-1 NMR
One peak for each set of inequivalent H-1 Atoms
(each chemical shift environment shows 1 peak)
What does the area below a peak representation in H-1 NMR
The area below the peak is proportional to the number of H-1 atoms causing the peak
Integration Trace
What is the Integration Trace
A Line located above a graph of H-1 NMR
That is equal to the area below the graph at every value of Chemical shift
Easier to measure the relative abundance of H-1 atoms within each environment
What is TMS
Name?
Structure?
What state is TMS at room Temperature
Liquid
Why is TMS Used?
Inert, non toxic and easy to remove form sample
Provides a peak at 0 ppm
Used as a reference point so that every chemical shift caused by an environment is measured comparatively to it
When does Splitting coupling occur?
Neighbouring hydrogen atoms (3 or fewer bonds away/ on an adjacent carbon atom) affect the magnetic field of H-1 atoms.
Causing their Peaks to split
What is the N+1 rule?
If there are n inequivalent H-1 Atoms on the adjacent carbon
then the peaks will be split into n+1 peaks of varying sizes
What is a requirement for the solvent used to dissolve samples for H-1 Nmr
No Hydrogen atoms/ protium
Would cause major interference with results, so that they become unreadable
Which solvents are used for NMR?
Deuterated Solvents - CDCl3 , D2O ,C6D6
CCl4 - Tetrachloromethane
Deuuterated solvents contain Hydrogen atoms
Why does this have no effect on NMR values
Has even number of nucleons
No Nuclear Spin
Isn’t affect by the applied external magnetic field in an NMR machine , so isn’t detected
Explain 1 problem with using D2O as a solvent
- If molecules contain at O-H bond
O-D bond will form in its place
due to labile protons that move from 1 molecule to another
so the chemical shift peak caused by O-H will disappear
Explain why CCl4 and CDCl3 are used in Proton NMR
Why are their Properties suitable for this?
CCl4 & CDCl3 are inert solvents
ClCl4:
- Non-polar solvent that is ideal for preforming proton NMR with non-polar molecules
-Dipoles exhibit symmetry making it non-polar
-Doesn’t interfere with proton NMR spectra as there are no hydrogen atoms present
CDCl3:
-Polar solvent ideal for proton NMR of polar organic molecules
-Doesn’t interfere with proton NMR spectra as a deuterated hydrogen as no nuclear spin due to an even number of nucleons —> hence no weak magnetic field that is detected by the NMR machine
Explain why TMS is used in NMR
Why is it suitible?
Tetramethylsilicone is used as a standard that all values of chemical shift are measured against
Produces single Peak at 0 PPM
TMS has 12 equivalent hydrogen environments due to symmetry of the molecule —> only produces a single peak that is easily identifiable.
TMS is volatile , low BP so easily Removed from sample once spectra is completed
(just the first one)
H attached to both C-Cl and adjacent to C=O which isn’t on the Table
R has 4 Carbons next to a C=O
S has 2 Carbons next to a C=O
R has 2 peaks in the range of 20-50 ppm while S will only have 1 peak in this range
S has 1 peak in the range 5-40 ppm
S has the peak with the lowest chemical shift
Both contain 2 hydrogen environments each producing 2 peaks
R produces 2 singlet peaks, As no hydrogens attached to a carbon, are adjacent to another molecule with hydrogens attached in a different chemical environment
S produces a triplet peak and a Quartet peak
