NMR and chromatography

Question 1

Why does NMR only work with nuclei with odd mass number?
4
Odd vs even?
Magnetic field?

Answer 1

• Nuclei with odd mass numbers (1H, C13) have spin
• Nuclei with even mass numbers do not have magnetic properties
• Nuclear spin induces weak magnetic fields which can be influenced by an external magnetic field
• Hydrogens nucleus can align w/ (stable) or against (unstable) an applied field

Question 2

How is it possible to make H flip from more to less stable alignment?
2

Answer 2

• Supplying an exact amount of energy

- the amount of energy depends on the strength of the magnetic field

Question 3

What is the resonance condition?

Answer 3

Energy peak when (H) flipped form more to less stable alignment on a graph

Question 4

What solvents are used for NMR?

2

Answer 4

CCl4

CDCl3 (sometimes gives a clearer split of peaks so used for complex spectra

Question 5

What is TMS and why is it the standard?

(1H nuclei of TMS given chemical shift value of 0)

7
Environment?
Resonance?
Cost?
Reactivity?

Answer 5

TMS: tetramthylsilane SiC4H12

• all H and C are in the same environment so one intense peak
• H nuclei are highly shielded due to Si’s low electronegativity so it has high resonance and strong sharp easily detected absoptions
• non-toxic
• inert
• cheap
• easy to remove from sample (low b.p)

Question 6

What are spectra?

1

Answer 6

Resonance conditions

Question 7

What are spectra affected by?

3

Answer 7

• Environment of H: causes different peak shapes and chemical shifts
• T4 affected by no. H in molecule
• T4 affected by electronegativity of surrounding atoms

Question 8

What is the position of the peak influenced by?

1

Answer 8

Electronegativity of surrounding atoms

Question 9

What is the size of the peak influenced by?

1

Answer 9

Relative to number of H’s in that environment

Question 10

What does the amount of absorptions in NMR spectra represent?
1

Answer 10

Amount of non-equivalent protons

Question 11

What does the intensity of absorptions in NMR spectra represent?
1

Answer 11

Quantity of protons with each peak

Question 12

What does the position of absorptions in NMR spectra represent?
1

Answer 12

Environment of each proton (electronegativity of surrounding atoms)

Question 13

What does the splitting of absorptions in NMR spectra represent?
1

Answer 13

Amount of neighbouring inequivalent protons on C (or other atom) directly adjacent

Question 14

What is the split with 0 inequivalent H’s?

Answer 14

Singlet

Question 15

What is the split with 1 inequivalent H’s?

Answer 15

Doublet

Question 16

What is the split with 2 inequivalent H’s?

Answer 16

Triplet

Question 17

What is the split with 3 inequivalent H’s?

Answer 17

Quartet

Question 18

What is the split with 4+ inequivalent H’s?

Answer 18

Multiplet

Question 19

What stationary phase and mobile phase is used in column chromatography?

Answer 19

Stationary: powder
Mobile: solvent

Question 20

What stationary phase and mobile phase is used in paper chromatography?

Answer 20

Stationary: absorbent paper
Mobile: solvent

Question 21

What stationary phase and mobile phase is used in TLC (thin layer) chromatography?

Answer 21

Stationary: powder on glass/plastic plate
Mobile: solvent

Question 22

What stationary phase and mobile phase is used in gas chromatography?

Answer 22

Stationary: powder (sometimes) coated with liquid
Mobile: inert gas

Question 23

What is chromatography?

1

Answer 23

Process of separating substances by their relative affinity to a stationary and mobile phase

Question 24

How fast do substances with a stronger affinity for the mobile phase move?

Answer 24

More quickly

Question 25

How fast do substances with a stronger affinity for the stationary phase move?

Answer 25

More slowly

Question 26

What (type of) powders are used for stationary phase?

Answer 26

SiO2 (silica) (or Al2O3 (alumina)) | -Polar powders

Question 27

What are some examples of solvents used as mobile phases?

Answer 27

Non-polar: alkanes | Polar: water, alcohols

Question 28

What type of solvent is used with a polar stationary phase and why? 3/4

Answer 28

Non-polar For separation -Polar molecules will move slowly as higher affinity for the polar stationary phase -Non-polar molecules will move quickly as higher affinity for the non-polar mobile phase

Question 29

What is the Rf value? | Calculation?

Answer 29

-always less than 1 Distance moved by sample / distance moved by solvent

Question 30

How do you view results of colourless samples?

Answer 30

UV light

Question 31

What is column chromatography?

Answer 31

Large scale TLC

Question 32

How does column chromatography work? 3

Answer 32

- Apply solvent and pass it through column - Sample separates into coloured bands - If keep passing through solvent the bottom band will reach the bottom of the column and can be collected for further testing

Question 33

How does gas chromatography work? 4

Answer 33

- Long coiled tube packed with powder - Inert gas passes through tube (H2, N2, He) - Retention time: time to travel through tube - Samples are often analysed by mass spec as they leave

Question 34

What is the likely group with triplet:quartet peaks at a 3:2 ratio?

Answer 34

Ethyl group

Question 35

What is the group with singlet with intensity of 3?

Answer 35

Methyl group

Question 36

What is the group with a singlet at intensity of 6?

Answer 36

Two methyl groups on same carbon with two other non-H molecules bonded

Question 37

What is the group with triplet:triplet at 2:2 ratio of relative intensity?

Answer 37

Middle of carbon chain CH2-CH2

Question 38

What is group with doublet:quartet at 3:1 ratio?

Answer 38

CH3-CHR2

Question 39

What is group with singlet at 9 intensity?

Answer 39

3 methyl groups on one carbon

Question 40

What is group with singlet with intention of 1?

Answer 40

Likely alcohol group Other wise CHR3