Unit 3: Section 9 - Further Synthesis and Analysis

Question 1

What does NMR stand for?

Answer 1

Nuclear Magnetic Resonance

Question 2

What are the basic principles of NMR?

Answer 2

• You can find the structures of complex molecules by placing them in a magnetic field and applying EM waves of radio frequency to them
• If radio waves of the right frequency are absorbed, the nuclei flips from parallel to applied magnetic to field to anti-parallel
• This energy change can be monitored and recorded
• Uses the the resonance of nuclei with spin

Question 3

How would you carry out NMR spectroscopy?

Answer 3

1. Dissolve the liquid sample in a suitable solvent
2. Put in a tube along with a small amount of TMS and put the tube into an NMR machine
3. The sample is spun to even out any imperfections in the magnetic field and the spectrometer is zeroed against the TMS
4. Radiation with different radio frequencies but a constant magnetic field is applied to the sample and any absorptions (due to resonance) are detected

Question 4

Give a use of NMR?

Answer 4

MRI scans

Question 5

What kind of nuclei does NMR work with (and examples)?

Answer 5

Those with an uneven number of nucleons meaning they will spin

Question 6

What percentage of carbon atoms are C13?

Answer 6

1% - modern instruments are sensitive enough to detect this

Question 7

What defines the resonant frequency of a C13 atom?

Answer 7

The chemical environment that it is in; the amount of electron shielding it has

Question 8

What graph is produced by NMR spectroscopy?

Answer 8

Energy absorbed against chemical shift

Question 9

What is chemical shift? What are its units?

Answer 9

The resonant frequency of a nuclei, compared to that of a H1 atom in TMS
ppm (parts per million)

Question 10

What is the range of chemical shift for C13 NMR?

Answer 10

0-200ppm

Question 11

What means C13 atoms show a different chemical shift value?

Answer 11

Having different chemical environments (but equivalent atoms show the same peak)

Question 12

What kind of envrionment leads to a greater chemical shift?

Answer 12

A C atom next to a more electronegative atom has a greater chemical shift

Question 13

What does the number of signals mean for C13 NMR?

Answer 13

One signal for each carbon environment

Question 14

What does the chemical shift mean for C13 NMR?

Answer 14

Greater δ from atoms closer to electronegative atoms or C=C

Question 15

What does the area under the peak mean for C13 NMR?

Answer 15

No meaning

Question 16

Is there splitting in C13 NMR?

Answer 16

No

Question 17

Why is it easier to get a spectrum of H1 NMR than C13 NMR?

Answer 17

Most H atoms are 1H- - it is much more abundant than C13
This means almost all H atoms have spin so show up

Question 18

What is the range of chemical shift for H1 NMR?

Answer 18

0-10ppm

Question 19

What leads to a lower chemical shift value for H NMR?

Answer 19

H1 with more electrons around them - further from electronegative groups/atoms

Question 20

On a low resolution spectrum, what peaks would you expect to see for H NMR?

Answer 20

One peak for each set of inequivalent H atoms (each chemical environment shows 1 peak)

Question 21

What does the area under the peak represent for H NMR?

Answer 21

The area under the peak is proportional to the number of H1 atoms represented by the peak

Question 22

What is the integration trace?

Answer 22

A stepped line that makes it easier to measure the area under the curve (height of line = area under that peak)

Question 23

What is TMS?

Answer 23

Tetramethylsilane
Silicon bonded to 4 methyl gorups

Question 24

What state is TMS at room temperature?

Answer 24

Liquid