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
Q
What are the main differences of GC?
A
- 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
Q
What is the setup of GC?
A
27
Q
How does GC work?
A
- Sample is injection and components vaporised
- Moves through coiled tube with the carrier has
- Moving at different speeds depending on strength of attraction to stationary phase
- Weaker attractions move more quickly
- Resulting in shorter retention time
28
Q
What is the main limitation of HPLC and CG?
A
- 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
Q
What are the two areas in which chromatography methods are correct?
A
- Providing forensic evidence (In court)
- In detecting banned drugs in athletes and racehorses
30
Q
What is the setup for GC-MS?
A
31
Q
What are the steps for CG-MS?
A
- Mixture injected into gas chromatography
- Each component has a different retention time so emerges at the same time
- One at a time, component enters the mass spectrometer
- Component C has mass spectrum displayed
- m/z values and relative abundances of components are compared with a database of know substances
- When match is found component C has been positively identified
32
Q
How is seperation achived in GC?
A
- Seperation is achived due to components in the mixture
- Having different interactions in the stationary phase
33
Q
What are the differences between GC and HPLC
A
- 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
