Topic 19

Question 1

What does NMR do?

Answer 1

• Nuclear Magnetic Resonance
• Helps determine the structure of a molecule
• How much energy is required to flip the spin of nuclei

Question 2

What are the different types of NMR?

Answer 2

• 13C NMR - info on how carbon atoms are arranged
• High resolution 1H NMR - how hydrogen atoms are arranged

Question 3

What is low resolution NMR?

Answer 3

Question 4

What is high resolution NMR?

Answer 4

Question 5

What is nuclear spin?

Answer 5

• Atomic nucleus has an odd number of protons and neutrons
• It has nuclear spin which creates a magnetic field
• NMR detects how the magnetic fields are affected by a larger external magnetic field

Question 6

How does external magnetic fields effect nuclei spin?

Answer 6

• Nuclei spin in random directions
• External magnetic field will align in two directions
• Where the nuclei will flip in the same direction

Question 7

How does radio waves effect nuclei?

Answer 7

• Radio waves at a specific frequency
• Forces aligned with the magnetic field will absorb the energy and move to a higher energy level

Question 8

How does the nuclei absorb energy in NMR?

Answer 8

• Energy absorbed by the nuclei is dependent on the environment it is in
• Electron shielding effects the extent an external magnetic field has on the nucleus
• Which creates variation in the energy absorbed and various frequency which NMR detects
• Absorbance of energy to turn one of the nucleons into the higher energy level

Question 9

What determines in environment in NMR?

Answer 9

• Group of atoms that exist neat the examined nuclei
• Looking along the full chain not just the atoms immediately bonded to the examined atom

Question 10

What is required for an atom to be in the same environment?

Answer 10

• Must be bonded to an atom or atoms are identical
• There is only one hydrogen environment in example

Question 11

What is TMS and what is it used for?

Answer 11

• Tetramethylsilane (TMS)
• Chemical used as a standard
• For looking at chemical shift in NMR
• As its difficult to measure the magnitude of energy absorbed

Question 12

What makes TMS good as a standard?

Answer 12

• 12 Hydrogens all in identical environments
• Producing a single large peak in the spectra
• Inert, Non-toxic and Volatile

Question 13

What is a chemical shift?

Answer 13

• the difference between the TMS peak and the peaks produced by the tested substance
• Uses the δ, measured in parts per million
• Where standard is δ=0 (TMS)
  
  • Used to calibrate NMR machines

Question 14

What does 13C NMR tell you?

Answer 14

• Tells us the number of different carbon environments in a sample
• Where the number of peaks reflect this
• Carbon closest to the electronegative chlorine has less electron shielding
  
  • Meaning the chemical shift is higher

Question 15

Why are cyclic compounds difficult to use with 13C NMR?

Answer 15

• Not an easily identifiable chain
• Symmetry is used instead to identify different carbon environments

Question 16

What can you do which 13C NMR Chemical Shifts?

Answer 16

• You can reference the data table and identify what carbons exist
• But a peak at 190 suggest a carbonyl group (aldehyde or ketone)

Question 17

What are the steps in using 13C NMR?

Answer 17

Question 18

What are the steps in using 13C NMR?

Answer 18

Question 19

What does proton NMR tell you?

Answer 19

• Tells you the number of different hydrogen environments
• As well as how many hydrogens in each environment
• Numbers above peak tells the ration of the area under the peak
  
  • Allows for calculating the relative number of Hydrogens
  • 3 : 1 environment ratio

Question 20

What is a splitting pattern?

Answer 20

• 1H NMR spectrum have peaks that split allowing for structure to be determined
• Peaks splitting into smaller peaks corresponds to the number of hydrogen atoms on the adjacent carbon + 1
  
  • Spin-spin coupling
  • n+1 rule

Question 21

What are integration traces?

Answer 21

• Shows area under peak more clearly helping calculate hydrogen ratio
• Split peaks means its difficult to work out the area underneath
• Allows for the height and area ratio
• Use a ruler to measure the vertical parts and find a ratio

Question 22

What is HPLC?

Answer 22

Question 23

What are the differences of column chromatography?

Answer 23

• Solvent forced through metal tube under high pressure
  
  • Rather than passing through gravity
• Particle size of stationary phase
  
  • Leading to better separation of components
• Sample injected into column
• Components detected after passing through column
  
  • By absorption of UV radiation
• Process is automated and results are quickly available on computer

Question 24

What is retention time?

Answer 24

• Similar to Rf on paper chromatography
• Time taken from injection to detection
• Important for identifying components
  
  • Depends on
    
    • Nature of solvent
    • Pressure used
    • Temp inside column

Question 25

What are the main differences of GC?

Answer 25

• Metal tube several meters long
  
  • Coiled to save space
• Stationary phase is solid or liquid coated on the inside of the tube
• Mobile phase in inert carrier gas (nitrogen or helium)
• Sample injected into column (as in HPLC)
• Components passing through column as in HPLC
• Components passed in column are detected
• Whole process is automated and results are quickly available on computer

Question 26

What is the setup of GC?

Answer 26

Question 27

How does GC work?

Answer 27

1. Sample is injection and components vaporised
2. Moves through coiled tube with the carrier has
3. Moving at different speeds depending on strength of attraction to stationary phase
   
   1. Weaker attractions move more quickly
   2. Resulting in shorter retention time

Question 28

What is the main limitation of HPLC and CG?

Answer 28

• Both cant positively identify components
• Due to difficulty in controlling all variables
  
  • Such as solvent, pressure and temp
  • With substances having different retention times
• With a record of retention times of known substances needed for data to be useful

Question 29

What are the two areas in which chromatography methods are correct?

Answer 29

1. Providing forensic evidence (In court)
2. In detecting banned drugs in athletes and racehorses

Question 30

What is the setup for GC-MS?

Answer 30

Question 31

What are the steps for CG-MS?

Answer 31

1. Mixture injected into gas chromatography
2. Each component has a different retention time so emerges at the same time
3. One at a time, component enters the mass spectrometer
4. Component C has mass spectrum displayed
5. m/z values and relative abundances of components are compared with a database of know substances
6. When match is found component C has been positively identified

Question 32

How is seperation achived in GC?

Answer 32

• Seperation is achived due to components in the mixture
• Having different interactions in the stationary phase

Question 33

What are the differences between GC and HPLC

Answer 33

• In GC the mobile phase uses an intert gas(carrier gas)
• In GC the stationary phase uses a liquid on a solid
• In HPLC the mobile phase uses a liquid