3.3.15-16 NMR and chromatography

Question 1

Describe what resonant frequency is.

Answer 1

The energy required to flip the spin of a nuclei sample at a specific frequency.
Unique resonant frequency for each type of nuclei.

Question 2

Why are 1H and 13C NMR used?

Answer 2

Both have nuclei with spins of 1/2 which means they can spin both directions in a magnetic field

Question 3

Describe how you would write notation for 13C NMR.

Answer 3

1. State chemical shift of carbon environment
2. Identify the carbon in a snip of the structural formula
3. Repeat for each carbon environment

Question 4

Describe the relationship between chemical shift, shielding and e- density.

Answer 4

Chemical shift is larger when electrons are less shielded and there is less e- density

Question 5

What makes 13C NMR more difficult to see than 1H NMR?

Answer 5

13C has a much smaller natural abundance

Question 6

What is the integration trace?

Answer 6

The number/proportion of protons in a specific environment.

Question 7

Describe spin-spin splitting and coupling.

Answer 7

Number of protons up to 3 bonds away.
n + 1
singlet 0
doublet 1
triplet 2
quartet 3

Question 8

Describe the two requirements for an NMR solvent.

Answer 8

1. Does not show up in a NMR experiment
2. Good at dissolving samples

Question 9

State two examples of NMR solvents, explain the advantages of each one.

Answer 9

CDCL3 - good solvent, not NMR active

D2O (deuterated water) - dissolves polar compounds, not NMR active

Question 10

What standard is used in NMR?

Answer 10

TMS = tetramethylsilane

Si(CH3)4

Question 11

What is the positives of using TMS as a standard in NMR?

Answer 11

• Produces a much lower chemical shift than any 1H and 13C peaks, able to differentiate between the standard and your sample
• Produces a single sharp peak at 0ppm which you can compare to any other peaks for 1H and 13C
• Inert and not toxic
• Can be easily removed from a sample

Question 12

How do you calculate the Rf value in chromatography?

Answer 12

(origin to centre of sample) divided by (origin to solvent front)

Question 13

What is an example of a stationary phase you can use in TLC and what are the risks?

Answer 13

solid silicon dioxide (SiO2)
- dangerous to inhale fine particles

Question 14

State in general terms what determines the distance travelled by a spot in TLC.

Answer 14

The relative affinity for the stationary phase and mobile phase.