Topic 19: Modern Analysis Techniques 2 2️⃣ Flashcards

1
Q

What is high resolution mass spectrometry (HMRS) used for ?

A

Provide an Mr value to four or more decimal places
Values with this degree of precision can sometimes enable a compound to be identified from the Mr alone
Useful if compounds have the same Mr when rounded

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What is the theory behind NMR

A

Makes use of a strong magnetic field and radio frequency radiation
Nucleus absorbs energy if both are present
If odd number of nucleons present, the spin state of nucleons is flipped = resonance

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Describe an NMR spectrum

A

Vertical axis sometimes labelled absorption (no units), often label is omitted however
Horizontal axis is labelled chemical shift / δ symbol (units ppm)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

What are the 2 types of NMR we need to be aware of ?

A

1H (proton) NMR
Carbon 13 NMR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Name and explain the most common solvent for 13C and 1H NMR

A

CDCl3
- symbol D stands for deuterium, 2H isotope of hydrogen, containing a proton and a neutron whose spins cancel eachother out
- produces no signals which can interfere with the signals from hydrogen atoms
- produces only one signal that can interfere with signals from carbon atoms but can be easily removed from spectrum

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What is the reference standard for a chemical shift (NMR)

A

(CH3)4Si

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

Why is (CH3)4Si used as the reference standard for a chemical shift

A
  • highly shielded 1H nuclei
  • unreactive & non-toxic
  • low boiling point
  • produces a single strong signal which is easy to identify
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is a chemical shift of a proton (or group of protons)

A

A number (in units ppm) that indicates its behaviour in a magnetic field relative to (CH3)4Si.
It can be used to identify the environment of the carbon atoms or hydrogen atoms attached to it

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What does the number of peaks on an NMR spec tell us

A

The number of different chemical environments

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What do the positions of each of the peaks on the NMR spectrum tell us?

A

Tell you the type of chemical shift

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What do differences in height of peaks in 13C NMR spectrum tell us ?

A

Indicates greater number if carbon atoms causing that peak

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What are the 2 types of 1H NMR spectrometry

A

Low resolution and high resolution.

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

What do the areas under the peaks in 1H NMR spectrometry correspond to

A

The number of hydrogen atoms in each environment

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Give the framework of how to interpret a chemical shift

A

δ ~ 45-62ppm C -OH

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

If a peak is seen at 0ppm what does it correspond to (NMR)

A

(CH3)4Si - should be ignored

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

What is the difference between low and high resolution 1H NMR spectrometry?

A

Share same features bar one :
- high res proton NMR has splitting patterns

17
Q

What causes the splitting of peaks in high resolution 1H NMR spectrometry

A

The influence of any other protons on adjacent carbon atoms causes splitting (sometimes referred to as spin-spin coupling)

18
Q

What are the types of splitting that can occur in 1H NMR spectrometry

A

If peak is not split = singlet
If peak is split into 2 sub peaks = doublet
If peak is split into 3 sub peaks = triplet
If peak is split into 4 sub peaks = quartet

19
Q

How is the n+1 rule used in splitting patterns of high res 1H NMR spectrometry

A

Splitting tells us number of protons on adjacent carbon(s) - count subpeaks minus 1
OH doesn’t get split

20
Q

If unsure which peak is due to OH in high res 1H NMR what should be done ?

A

Add heavy water (D2O) and re-run, peak will disappear