1H NMR

Question 1

In what key way do the peaks in proton NMR differ from those in 13C NMR?

Answer 1

The area under the peaks in proton NMR spectra is proportional to the number of protons in that environment.

Question 2

What is three-bond coupling called?

Answer 2

Vicinal coupling.

Question 3

What is two-bond coupling called?

Answer 3

Geminal coupling.

Question 4

What is the typical vicinal coupling constant between hydrogens on adjacent sp3 carbons?

Answer 4

7 Hz.

Question 5

What is the coupling constant between two hydrogens on the same sp2 carbon in a double bond?

Answer 5

Negligible.

Question 6

What is the coupling constant between two hydrogens cis to one another in a double bond?

Answer 6

0-12Hz.

Question 7

What is the coupling constant between protons that are trans to one another over a double bond?

Answer 7

12-18Hz.

Question 8

What is the typical geminal coupling constant between two protons on the same sp3 carbon?

Answer 8

8-18Hz (usually about 13Hz).

Question 9

What is roofing?

Answer 9

When two signals are coupling, the outermost peaks decrease in intensity, while the innermost peaks increase in intensity.

Question 10

What happens to the roofing effect when the signals are closer together?

Answer 10

The roofing effect increases. If they are very close, they can form a single signal.

Question 11

What is the shift of protons in methyl groups, relative to those in alkyl chains?

Answer 11

About 0.5ppm less than those in alkyl chains.

Question 12

What is unusual about the shift of protons in three-membered rings?

Answer 12

It is unusually low.

Question 13

What is generally the shift of protons on carbons that are attached to sp2 carbons?

Answer 13

2.5ppm

Question 14

What is the shift of protons attached to a carbon attached to oxygen, relative to those attached to a carbon attached to nitrogen?

Answer 14

About 1ppm higher.

Question 15

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?

Answer 15

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.

Question 16

Why do the shifts of protons attached to electronegative atoms give broad peaks?

Answer 16

There is hydrogen bonding between the molecules and the exact shift for each molecule depends on amount of hydrogen bonding affecting the proton.

Question 17

What alters the amount of hydrogen bonding experienced by a proton attached to an electronegative atom?

Answer 17

Structure of the molecule, concentration of sample, solvent used and temperature.

Question 18

How can protons attached to electronegative atoms be identified?

Answer 18

D2O shake

Question 19

How does a D2O shake work?

Answer 19

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.

Question 20

Why is no coupling seen with protons that are involved in hydrogen bonding?

Answer 20

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.

Question 21

Why is coupling not usually seen for nuclei greater than 1/2?

Answer 21

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.