1H NMR Flashcards
In what key way do the peaks in proton NMR differ from those in 13C NMR?
The area under the peaks in proton NMR spectra is proportional to the number of protons in that environment.
What is three-bond coupling called?
Vicinal coupling.
What is two-bond coupling called?
Geminal coupling.
What is the typical vicinal coupling constant between hydrogens on adjacent sp3 carbons?
7 Hz.
What is the coupling constant between two hydrogens on the same sp2 carbon in a double bond?
Negligible.
What is the coupling constant between two hydrogens cis to one another in a double bond?
0-12Hz.
What is the coupling constant between protons that are trans to one another over a double bond?
12-18Hz.
What is the typical geminal coupling constant between two protons on the same sp3 carbon?
8-18Hz (usually about 13Hz).
What is roofing?
When two signals are coupling, the outermost peaks decrease in intensity, while the innermost peaks increase in intensity.
What happens to the roofing effect when the signals are closer together?
The roofing effect increases. If they are very close, they can form a single signal.
What is the shift of protons in methyl groups, relative to those in alkyl chains?
About 0.5ppm less than those in alkyl chains.
What is unusual about the shift of protons in three-membered rings?
It is unusually low.
What is generally the shift of protons on carbons that are attached to sp2 carbons?
2.5ppm
What is the shift of protons attached to a carbon attached to oxygen, relative to those attached to a carbon attached to nitrogen?
About 1ppm higher.
Why are the shift of protons on aromatic rings higher than those attached to double bonds in alkenes, and why is the shift of protons attached to alkyne groups lower?
In aromatic rings, the magnetic field set up by the electrons in the pi-system means that the local magnetic field experienced by the protons is greater, while in the alkyne group the local magnetic field experienced is less.
Why do the shifts of protons attached to electronegative atoms give broad peaks?
There is hydrogen bonding between the molecules and the exact shift for each molecule depends on amount of hydrogen bonding affecting the proton.
What alters the amount of hydrogen bonding experienced by a proton attached to an electronegative atom?
Structure of the molecule, concentration of sample, solvent used and temperature.
How can protons attached to electronegative atoms be identified?
D2O shake
How does a D2O shake work?
Shake sample with a solvent like CDCl3 and a little D2O. D2O hydrogen bonds with groups containing a proton attached to an electronegative atom, and the protons are exchanged for deuterons. Deuterons have a completely different shift to protons, so the signal seems to completely disappear.
Why is no coupling seen with protons that are involved in hydrogen bonding?
They are exchangeable, so they rapidly swap in solution and those they swap for are equally as likely to be spin up as spin down, so the other protons experience an average of 0 spin.
Why is coupling not usually seen for nuclei greater than 1/2?
A process called ‘relaxation’ occurs, in which the spin states rapidly interconvert. It occurs for all nuclei with spin, but is much quicker in nuclei with spin greater than 1/2. This mean the spin states average to zero and no coupling is seen.
