3.3.15 Nuclear magnetic resonance spectroscopy (A2)
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what are the basic principles of NMR?
you can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them if radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel this energy change can be monitored and recorded uses the resonance of nuclei with spin
how would you carry out NMR spectroscopy?
dissolve the liquid sample in a suitable solvent, put in a tube along with a small amount of TMS and put the tube into an NMR machine 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 (due to resonance) are detected
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 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 is in; the amount of electron shielding it has.
what graph is produces 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 symbol: δ units: parts per million (ppm)
what is the range of the 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 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 (each set of inequivalent 13C atoms) chemical shift: greater δ from atoms closer to electronegative atoms or C=C area under peak: no meaning splitting: 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 HMR?
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 (name and structure)?
tetramethylsilane
what state is TMS at room temperature?
liquid
why is TMS used?
can be added to sample to calibrate NMR equipment provides a peak at exactly δ = 0ppm it is the reference point against which all δ are measured
what are the advantages of using TMS?
inert, non-toxic, easy to remove from the sample (as relatively volatile)
when does splitting/spin-spin coupling occur?
neighbouring hydrogen atoms (3 or fewer bonds away, or on the adjacent carbon) affect the magnetic field of 1H atoms and causes their peaks to split
what is the n+1 rule?
if there are n inequivalent 1H atoms on the neighbouring carbon then the peak will split into (n+1) smaller peaks