NMR
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Why is carbon 13 used rather than carbon 12?
Carbon 12 has no nuclear spin
What is chemical shift measured in?
Parts per million (ppm)
What is being examined in proton NMR?
The ^1H nucleus
What is the reference in NMR?
Tetramethylsilane (TMS) Si(CH4)4
Why is TMS used as a reference?
Inert Non-toxic Easy to remove from the sample
What is the n+1 rule?
n hydrogens on an adjacent carbon atom will split a peak into n+1 smaller peaks.
What is NMR?
An analaytical technique which allows the structure of a molecule to be determined by analysing the energy of each bond environment.
What are the two types on NMR?
Proton NMR Carbon NMR
Why are there different peaks?
Different bond environments absorb different amounts of energy
Why is TMS seen as a standard molecule?
It contains four identical carbon and hydrogen environments. It has a peak of 0 on the x axis.
Why do carbon environments near to an oxygen have chemical shift values that are shifted to the left?
Oxygen is very electronegative and changes the bond environment and how it absorbs energy.
What must be done to samples being analysed with proton NMR?
Must be dissolved in a non-hydrogen containing solvent so that it doesn’t produce any peaks on the spectrum. CCl4 is thus a common solvent used along with deuterated solvents (e.g. CDCl3) containing deuterium (an isotope of hydrogen)
What is a singlet?
No hydrogen on the adjacent carbon
What is a doublet?
1 hydrogen on the adjacent carbon
What is a triplet?
2 hydrogens on the adjacent carbon
What is a quaret?
3 hydrogens on the adjacent carbon
What does NMR stand for?
Nuclear magnetic resonance
Give one use of NMR?
MRI scans
What kind of nuclei does NMR work with?
Those with an uneven number of nucleons meaning they’ll spin e.g. 1H and 13C
What percentage of atoms are 13C?
1% but modern instruments are sensitive enough to detect this.
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?
0-200ppm
What kind of environment leads to a greater chemical shift?
A carbon atom next to a more electronegative atom has a greater chemical shift.