NMR Spectroscopy Flashcards
What does NMR stand for?
Nuclear Magnetic Resonance
Give one use of NMR?
MRI scans
What kind of nuclei does NMR work with (and examples)?
Those with an uneven number of nucleons, meaning they will spin e.g. 1H, 13C
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 NMR spectroscopy?
Energy absorbed against chemical shift
What is chemical shift?
The resonant frequency of the nuclei, compared to that of a 1H atom in TMS.
What is the range of chemical shift for 13C NMR?
0-200ppm
What means 13C atoms show a different chemical
shift value?
Having different chemical environments (but equivalent atoms show the same peak)
What kind of environment leads to a greater chemical shift?
A C atom next to more electronegative atom
What does the number of signals mean? (13C NMR)
One signal for each carbon environment (each set of inequivalent 13C atoms)
What does the chemical shift mean? (13C NMR)
Greater shift from atoms closer to electronegative atoms or C=C
What does the area under the peak mean? (13C NMR)
no meaning
What does splitting mean? (13C NMR)
There is no splitting for 13C NMR
Why is it easier to get a spectrum of 1H NMR than
13C NMR?
Most H atoms are 1H- it is much more abundant than 13C.
This means almost all H atoms have spin so show up
What is the range of chemical shift for 1H NMR?
0-10ppm
What leads to a lower chemical shift value for H
NMR?
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 H NMR?
One peak for each set of inequivalent H atoms (each chemical environment shows 1 peak)
What does the area under the peak represent (for H
NMR)?
The area under the peak is proportional to the number of 1H atoms represented by the peak
What is the integration trace?
A stepped line that makes it easier to measure the area under the curve (height of line = area under that peak)
What is TMS?
Tetramethylsilane
What state is TMS at room temperature?
liquid
Why is TMS used?
Can be added to sample to calibrate the NMR equipment.
It provides a peak at exactly δ=0ppm.
It is the reference point against which all δ are measured
What are other advantages of using TMS?
Inert, non-toxic, easy to remove from the sample
as relatively volatile