13C NMR Flashcards
Give 2 reasons why tetramethylsilane (TMS) is used as a reference compound in 13C NMR.
Only gives one peak, as all 4 methyl groups are equivalent.
Silicon is less electronegative than carbon, so more electrons density is on the carbons, shielding them and shifting the peak to the right out of the way of the others.
What is the shift of an sp3 carbon that is NOT attached to any electronegative atoms?
0-50ppm
What is the shift of an sp3 carbon that IS attached to an electronegative atom?
50-100ppm.
What is the shift of an sp2 carbon that is NOT attached to any electronegative atoms?
100-150ppm.
What is the shift of an sp2 carbon that IS attached to any electronegative atoms?
150-200ppm.
What is the shift of a carbon in a triple bond in an alkyne?
about 70-80ppm.
What is the shift of a carbon in a benzene ring?
100-150ppm
What is the shift of a carbon in a nitrile group?
about 120ppm.
Why do double bonds have higher resonance than single bonds and triple bonds?
There is a nodal plane in the double bond that results in low electron density, so less shielding.
What is the shift of a carbon in an aldehyde?
Just under 200ppm.
What is the shift of a carbon in a ketone?
Just over 200ppm.
What is the shift of a carbon in an acid derivative?
160-170ppm.
What is a feature of the peaks given by carbons that are not attached to any hydrogen atoms?
They are small.
How can coupling be depicted for nuclei with spin 1/2?
Tree diagrams.
What letter is assigned to the coupling constant, and what are its units?
J
It is measured in Hertz.
What is meant by saying that coupling is a ‘through-bond’ interaction?
The coupling constant decreases the more bonds there are between nuclei.
Why is no C-H coupling usually seen?
Broadband proton decoupling.
Why is no C-C coupling usually seen? When might it be seen?
Carbon-12 has no spin, and carbon-13 has a very low natural abundance, so most carbon-13 nuclei are not next to another carbon-13 to couple with. It may be seen if the sample is enriched with carbon-13.
Why is coupling between equivalent nuclei never seen?
Equivalent spins do interact, but the result of this interaction is never seen as splitting.
What is the general rule for the number of lines a signal is split into when a nucleus with spin I couples with n equivalent nuclei?
2nI + 1
What are satellites in an NMR spectrum?
They are small peaks equidistant from a large single peak. They are formed by an isotope of low abundance having spin, while the other isotopes have no spin, so only a small percentage of carbon-13 atoms are next to the isotope with spin, and only these signals are split.
What is a doublet superimposed on a singlet?
A single peak with a satellite on either side.
How is broadband proton decoupling carried out?
Sample is irradiated over a range of frequencies so that all protons are rapidly interconverting between different spin states, so there is no net effect on the local magnetic environment of the carbons.
Advantages of broadband proton decoupling.
Strength of signals in proton coupled spectra is weaker so signals can easily get lost in background.
Multiplets in coupled spectra often overlap so it can be hard to distinguish which peaks are due to which nucleus.
