13C NMR

Question 1

Give 2 reasons why tetramethylsilane (TMS) is used as a reference compound in 13C NMR.

Answer 1

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.

Question 2

What is the shift of an sp3 carbon that is NOT attached to any electronegative atoms?

Answer 2

0-50ppm

Question 3

What is the shift of an sp3 carbon that IS attached to an electronegative atom?

Answer 3

50-100ppm.

Question 4

What is the shift of an sp2 carbon that is NOT attached to any electronegative atoms?

Answer 4

100-150ppm.

Question 5

What is the shift of an sp2 carbon that IS attached to any electronegative atoms?

Answer 5

150-200ppm.

Question 6

What is the shift of a carbon in a triple bond in an alkyne?

Answer 6

about 70-80ppm.

Question 7

What is the shift of a carbon in a benzene ring?

Answer 7

100-150ppm

Question 8

What is the shift of a carbon in a nitrile group?

Answer 8

about 120ppm.

Question 9

Why do double bonds have higher resonance than single bonds and triple bonds?

Answer 9

There is a nodal plane in the double bond that results in low electron density, so less shielding.

Question 10

What is the shift of a carbon in an aldehyde?

Answer 10

Just under 200ppm.

Question 11

What is the shift of a carbon in a ketone?

Answer 11

Just over 200ppm.

Question 12

What is the shift of a carbon in an acid derivative?

Answer 12

160-170ppm.

Question 13

What is a feature of the peaks given by carbons that are not attached to any hydrogen atoms?

Answer 13

They are small.

Question 14

How can coupling be depicted for nuclei with spin 1/2?

Answer 14

Tree diagrams.

Question 15

What letter is assigned to the coupling constant, and what are its units?

Answer 15

J

It is measured in Hertz.

Question 16

What is meant by saying that coupling is a ‘through-bond’ interaction?

Answer 16

The coupling constant decreases the more bonds there are between nuclei.

Question 17

Why is no C-H coupling usually seen?

Answer 17

Broadband proton decoupling.

Question 18

Why is no C-C coupling usually seen? When might it be seen?

Answer 18

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.

Question 19

Why is coupling between equivalent nuclei never seen?

Answer 19

Equivalent spins do interact, but the result of this interaction is never seen as splitting.

Question 20

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?

Answer 20

2nI + 1

Question 21

What are satellites in an NMR spectrum?

Answer 21

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.

Question 22

What is a doublet superimposed on a singlet?

Answer 22

A single peak with a satellite on either side.

Question 23

How is broadband proton decoupling carried out?

Answer 23

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.

Question 24

Advantages of broadband proton decoupling.

Answer 24

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.

Question 25

Disadvantages of broadband proton decoupling.

Answer 25

Shows less information because you can’t see how many hydrogens are attached to each carbon.

Question 26

What test can you use to work out the number of hydrogens attached to each carbon in a broadband proton decoupled spectrum?

Answer 26

The Attached Proton Test.

Question 27

Which direction, relative to the solvent, do the peaks for carbons attached to an even number of protons point?

Answer 27

The same direction as the solvent.

Question 28

Which direction, relative to the solvent, do the peaks for carbons attached to an odd number of protons point?

Answer 28

Opposite direction to the protons.