ORGANIC CHEMISTRY - NMR Flashcards
What does NMR stand for?
Nuclear Magnetic resonance
What are the basic principles of nmr ?
- find structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
How will you carry out NMR spectroscopy ?
- dissolve the liquid sample in solvent
- put in a tube along with a small amount of TMS and out the tube into a NMR machine
- sample is spun to even out any imperfections
- the magnetic field and the spectrometer is zeroed against the TMS
- radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions (due to resonance) are detected
Give one use of NMR
MRI scans
What kind of nuclei does NMR work with?
1H and 13C = those with uneven numbers of nucleons meaning they will spin
What percentage of carbon atoms are 13C?
1% = but modern instruments are sensitive enough to detect this
What defines the resonant frequency of a 13C atom?
The chemical environment that it is in; the amount of electron shielding it has
What graph is produced by the NMR spectroscopy?
Energy absorbed against chemical shift
What is the chemical shift? What is its symbol? What are its units?
Resonant frequency of nuclei compared to that of a 1H atom in TMS
Ppm ( parts per million )
What is the range of chemical shift for 13C NMR?
0-200ppm
What does it mean when 13C atoms show a different chemical shift value?
Having different chemical environments
What kind of environment leads to a greater chemical shift ?
A C atom next to a more electronegative atom has a greater chemical shift
Summarise what these mean for 13C NMR :
Number of signals:
Chemical shift:
Area under peak:
Splitting :
Number of signals : one signal for each carbon environment
Chemical shift: greater shift from atoms closer to electronegative atoms or C=C
Area under peak = no meaning
Splitting = there is no splitting for CNMR
Why is it easier to get a spectrum of HNMR than CNMR?
Most H atoms are more abundance than C. This means all H atoms have a spin so show up
What is the range of chemical shift for HNMR?
0-10ppm
What leads to a lower chemical shift value for HNMR ?
1H with more electrons around them
I.E further from electronegative groups/atoms
On a low resolution spectrum, what peaks would you expect to see for HNMR?
One peak for each set of inequivalent H atoms = each chemical environment shows 1 peak
What does the area under the peak represent ( for HNMR)?
the area under the peak is proportional to the number of H atoms represented by the peak
What is TMR?
Tetramethylsilane
C4H12Si
What state is TMS at room temperature?
Liquid
Why is TMS used?
Can be added to sample to calibrate the NMR equipment
Provides peak at = 0ppm
Reference point against which all shifts are measured
What are other advantages of using TMS?
inert
Non toxic
Easy to remove form the sample as it’s volatile
When does splitting coupling occur?
Neighbouring hydrogen atoms affect the magnetic field of H atoms and causes their peaks to split
what is the n+1 rule?
If there are n inequivalent H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks
