3.15 NMR spectroscopy

Question 1

What are the two main types of NMR

Answer 1

13C NMR - tells us information on how carbon atoms are arranged
1H NMR - tells us how hydrogen atoms are arranged

Question 2

How does a nucleus get a spin and what does this spin do

Answer 2

• Atom has nuclear spin if it has odd nucleon number
• nuclear spin creates a weak magentic field. NMR detects how these magnetic fields are affected by a larger external magnetic field

Question 3

Describe the motion of nuclei spin

Answer 3

• spin in random direction, unless an external magnetic force is applied
• nuclei either spin in the direction of the magnetic field or against it
• Those that spin in the direction of the magnetic field have a lower energy

Question 4

How can nuclei jump up to a higher energy level
b) can nuclei jump to a lower energy level

Answer 4

NMR fires out radio waves
At specific frequency the nuclei that are aligned with the magnetic field absorb the energy and flip to another level
b) Yes and they emit radio waves

Question 5

What does NMR measure

Answer 5

Amount of energy absorbed

Question 6

How is shielding of a nucleus from a magnetic field affected

Answer 6

• can be shielded from electrons
• electronegative element such as oxygen near a carbon atom, would reduce the shielding of the carbon atom

Question 7

What does the effect of the magnetic field on the nuclei depend on

Answer 7

• will be felt differently depending on the environment, as they absorb different energies
• NMR measures the difference

Question 8

What does the environment depend on

Answer 8

environment determined by group of atoms that exist near nuclei, we look along full chain

Question 9

How can atoms be in the same environment

Answer 9

bonded to an atom or group of atoms that are identical

Question 10

For the first two, identify number of carbon environments
For the last two, identify number of hydrogen environments

Answer 10

Question 11

What chemical do we use as a standard when looking at chemical shift in NMR spectra and why

Answer 11

Question 12

What is the chemical shift

Answer 12

Question 13

What do the two peaks represent

Answer 13

Question 14

State how many different carbon environments there are. Label three carbon atom with the peak on the graph

Question 15

Label peaks with hydrogen atom

Answer 15

Question 16

What does it mean to have a
singlet peak
doublet peak
triplet peak
quarlet peak

Answer 16

singlet - 0 hydrogens on adjacent carbon
doublet - 1 hydrogen on adjacent carbon
triplet - 2 hydrogens on adjacent carbon
quarlet peak - 3 hydrogens on adjacent carbon

Question 18

State split number of peaks
Number of neighboring hydrogens
Relative size

Answer 18

Question 19

a) draw structure of K which has only one peak in its proton n.m.r spectrum
b) draw structure of an isomer of L which has only only two peaks in the proton n.m.r spectrum

Answer 19

A) all hydrogens must be the same as there is only one environment
B) there must be two environments

Question 20

Answer 20

Question 21

Answer 21

A = singlet
B = triplet

Question 22

Q is C4H7CLO

Answer 22

Tetramethylsilane is the second answer

Question 23

Answer 23

5

Question 24

Answer 24

Question 25

State number of peaks in each molecule in carbon NMR spectrum

Answer 25

Question 26

Answer 26

Compound is tetramethylsilane Si(CH3)4 Reasons : all 4 methyl groups are chemically equivalent, givign 1 sharp signal. It doesn't react with the sample and interfere the their peaks

Question 27

How do you know if carbon environments is the same

Answer 27

If two carbons don't have an adjacent carbon bonded to everything the same, then not same environment

Question 28

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Question 29

Answer 29

Question 30

Draw structure of isomer of C6H16N2 that contains two primary amine groups and has only two peaks in carbon NMR Spectrum

Answer 30

Question 31

Answer 31

Question 32

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Question 33

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Question 34

Describe the properties of Si(CH3)4 which makes it suitable for Hydrogen NMR spectrum

Answer 34

*Gives a single, sharp peak: All 12 hydrogen atoms are in identical environments, so TMS produces one sharp singlet in the spectrum. This makes it easy to identify and use as a reference point (assigned 0 ppm). * because of the 12 hydrogens that are bonded to methyl groups is bonded to silicon, it produces a large peak away from all other peaks * It is chemically inert, so it doesn't react with sample or interfere with results * it is volatile, so it can be easily removed * it has a low boiling point so it is easy to remove * it is non-toxic

Question 35

Describe the properties of CDCl3 and CCl4 which makes it suitable for Hydrogen NMR spectrum

Answer 35

* none have hydrogen so they give no peaks in Hydrogen NMR * Both inert, so they don't react with the sample * CDCl3 is polar so it is good for dissolving polar molecules * CCl4 is a non-polar molecule so it is good for dissolving non-polar molecules

Question 36

Answer 36

Question 37

What does the integration mean in NMR spectrum

Answer 37

The δ is a measure in parts per million (ppm) is a relative scale of how far the frequency of the proton signal has shifted away from that for TMS.

Question 38

Is CDCl3 and CCl4 used in Carbon NMR spectrum

Answer 38

yes, they produce one peak in carbon NMR spectrum

Question 39

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Question 40

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Question 41

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Question 42

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Question 43

Molecular formula is C5H10O2

Answer 43