3.15 NMR Spectroscopy Flashcards
What does NMR stand for
Nuclear magnetic resonance
How to carry out NMR
Dissolve the liquid sample in suitable solvent, put into a tube along with a small amount of TMS and put the tube into an NMR machine
The sample is spun to even out any imperfections in 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 are detected
Give one use of NMR
MRI scans
What kind of nuclei does NMR work with and examples
Those with uneven number of nucleons, meaning they will spin
eg 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
What is its symbol
What are its units
The resonant frequency of the nuclei, compared to that of a 1H atom in TMS
Parts per million (ppm)
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 has a greater chemical shift
Summarise what these mean for 13C NMR
Number of peaks
Chemical shift
Area under peak
Spliting
Peaks: 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: 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 s much more abundant than 13C. This means almost all H atoms have spin so show up
What leads to a lower shift value for H NMR
1H with more electrons around them i.e. further from electronegative groups/atoms
On a low res spec, 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 (H NMR)
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 peak
What is TMS (name and structure)
Tetrtamethylsilane
Si with 4 methyl groups / google
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 0ppm
Is is the reference point against which all shifts are measured
What are other advantages of using TMS
Inert, non toxic, easy to remove from the sample (as relatively volatile)
When does splitting/spin-spin coupling occur
Neighboring hydrogen atoms (3 or fewer bonds away, or on the adjacent atom) affect the magnetic field of 1H atoms and causes their peaks to split
What is the n+1 rule
If there are n equivalent 1H atoms on the neighboring carbon then the peak will split into (n+1) smaller peaks
Draw the splitting patterns for 0,1,2 and 3 inequivalent H atoms 3 bonds or less away
No coupled protons - singlet - google
One coupled proton - doublet - google
Two coupled proton - triplet
Three coupled proton - quartet
Why must solvents used for 1H NMR not contain any hydrogen atoms
Signals from the solvent would swamp signals from the sample as there is much more solvent than sample
Which solvents are used
Deuterated solvents: CDCL3, D2O, C6D6
CCL4 - tetrachloromethane
Summarise what these indicate for 1H NMR
Number of signals
Chemical shift
Splitting
Area under peak
Number of signals: One main signal for each set of inequivalent 1H atoms (for each hydrogen environment)
Shift: Larger ppm for 1H atoms closer to electronegative atoms or C=C
Splitting: Number of smaller peaks = 1+ number of inequivalent H atoms 3 bonds away
Area under peak: Proportional to the number of atoms represented by that peak
Why does the peak from O-H bonds disappear if D2O is used as a solvent
O-D bond is formed in preference to O-H due to labile protons that move/swap from one molecule to another
