Topic 19 Flashcards
1
Q
What does NMR do?
A
- Nuclear Magnetic Resonance
- Helps determine the structure of a molecule
- How much energy is required to flip the spin of nuclei
2
Q
What are the different types of NMR?
A
- 13C NMR - info on how carbon atoms are arranged
- High resolution 1H NMR - how hydrogen atoms are arranged
3
Q
What is low resolution NMR?
A
4
Q
What is high resolution NMR?
A
5
Q
What is nuclear spin?
A
- 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
6
Q
How does external magnetic fields effect nuclei spin?
A
- Nuclei spin in random directions
- External magnetic field will align in two directions
- Where the nuclei will flip in the same direction
7
Q
How does radio waves effect nuclei?
A
- Radio waves at a specific frequency
- Forces aligned with the magnetic field will absorb the energy and move to a higher energy level
8
Q
How does the nuclei absorb energy in NMR?
A
- 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
9
Q
What determines in environment in NMR?
A
- Group of atoms that exist neat the examined nuclei
- Looking along the full chain not just the atoms immediately bonded to the examined atom
10
Q
What is required for an atom to be in the same environment?
A
- Must be bonded to an atom or atoms are identical
- There is only one hydrogen environment in example
11
Q
What is TMS and what is it used for?
A
- Tetramethylsilane (TMS)
- Chemical used as a standard
- For looking at chemical shift in NMR
- As its difficult to measure the magnitude of energy absorbed
12
Q
What makes TMS good as a standard?
A
- 12 Hydrogens all in identical environments
- Producing a single large peak in the spectra
- Inert, Non-toxic and Volatile
13
Q
What is a chemical shift?
A
- 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
14
Q
What does 13C NMR tell you?
A
- 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
15
Q
Why are cyclic compounds difficult to use with 13C NMR?
A
- Not an easily identifiable chain
- Symmetry is used instead to identify different carbon environments
16
Q
What can you do which 13C NMR Chemical Shifts?
A
- You can reference the data table and identify what carbons exist
- But a peak at 190 suggest a carbonyl group (aldehyde or ketone)
17
Q
What are the steps in using 13C NMR?
A
18
Q
What are the steps in using 13C NMR?
A
19
Q
What does proton NMR tell you?
A
- 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
20
Q
What is a splitting pattern?
A
- 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
21
Q
What are integration traces?
A
- 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
22
Q
What is HPLC?
A
23
Q
What are the differences of column chromatography?
A
- 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
24
Q
What is retention time?
A
- 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
- Depends on
25
What are the main differences of GC?
- 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
26
What is the setup of GC?
27
How does GC work?
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
28
What is the main limitation of HPLC and CG?
- 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
29
What are the two areas in which chromatography methods are correct?
1. Providing forensic evidence (In court)
2. In detecting banned drugs in athletes and racehorses
30
What is the setup for GC-MS?
31
What are the steps for CG-MS?
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
32
How is seperation achived in GC?
- Seperation is achived due to components in the mixture
- Having different interactions in the stationary phase
33
What are the differences between GC and HPLC
- 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
