Topic 19: Modern Analytical Techniques II Flashcards
1
Q
What are high resolution mass spectrometers?
A
- they can measure atomic and molecular masses extremely accurately
- useful for identifying compounds that appear to have the same Mr when they’re rounded to the nearest whole number
2
Q
How does nuclear magnetic resonance spectroscopy work?
A
- a sample of a compound is placed in a strong magnetic field and exposed to a range of different frequencies of radio waves
- the nuclei of certain atoms within the molecule absorb energy from the radio waves
- the amount of energy that a nucleus absorbs at each frequency will depend on the environment that it’s in
- the pattern od these absorptions gives you information about the positions of certain atoms within the molecule and about how many atoms of that type the molecule contains
- you can use this to work out the structure of the molecule
- the two types are carbon-13 NMR and high resolution proton NMR
3
Q
What is carbon-13 NMR?
A
- give you the information about the number of carbon atoms that are in a molecule and the environments that they are in
4
Q
What is high resolution proton NMR?
A
- gives you informations about the number of hydrogen atoms that are in a molecule, and the environments that they are in
5
Q
How do nuclei in different environments absorb different amounts of energy?
A
- a nucleus is partly shielded from the effects of external magnetic fields by its surrounding electrons
- any other atoms and groups of atoms that are around a nucleus will also affect its amount of electron shielding
- this means that the nuclei in a molecule feel different magnetic fields depending on their environments
- nuclei in different environments will absorb different amounts of energy at different frequencies
- an atom’s environment depends on all the groups that it is connected to, going right along the molecule
6
Q
Explain why tetramethylsilane is used as a standard in NMR spectroscopy?
A
- the differences in absorption are measured relative to a standard substance, which is tetramethylsilane
- TMS produces a single absorption peak in both types of NMR because all its carbon and hydrogen nuclei are in the same environment
- it is a standard because the absorption peak is at a lower frequency than just about everything else
- the peak is given a value of 0 and all the peaks in other substances are measured as chemical shifts relative to this
7
Q
What is a chemical shift?
A
- it is the difference in radio frequency absorbed by the nuclei in the molecule being analysed and that absorbed by the same nuclei in TMS
- it is given the symbol delta and is measured in parts per million, ppm
- a small amount of TMS is often added to samples to give a reference peak on the spectrum
8
Q
How do you interpret carbon-13 NMR spectra?
A
9
Q
What is the main difference between proton NMR and carbon-13 NMR?
A
- the peaks in a proton NMR spectrum split according to how the hydrogen environments are arranged
- only the peaks of hydrogen bonded to carbon atoms split
- the splitting is caused by the influence of hydrogen atoms that are bonded to neighbouring carbons
- these are carbons one along in the carbon chain from the carbon the hydrogen is attached to
- this is called spin-spin coupling
- only hydrogen nuclei on adjacent carbon atoms affect each other
- these split peaks are called multiplets
- they are always split into one more than the number of hydrogens
- the n+1 rule
10
Q
What do integration traces tell you about the environments?
A
- in proton NMR, the relative area under each peak tells you the relative number of H atoms in each environment
- if the area under two peaks is in the ratio 2:1, there will be two H atoms in the first environment for every one in the second environment
- area can be shown using numbers above the peaks or with an integration trace
11
Q
How do you use proton NMR to work out structures?
A
- use clues from integration traces or numbers above the peaks, the chemical shift values and splitting patterns to work it out
12
Q
What is the mobile phase in chromatography?
A
- where the molecules can move
- this is always a liquid or a gas
13
Q
What is the stationary phase?
A
- where the molecules can’t move
- this must be a solid, or a liquid on solid support
14
Q
How do you calculate Rf values?
A
- distance travelled by spot / distance travelled by solvent
15
Q
Describe high-performance liquid chromatography
A
- in HLPC the stationary phase is small particles of a solid packed into a column
- often silica bonded to various hydrocarbons
- the liquid mobile phase is often a polar mixture such as methanol and water
- it is forced through the column under high pressure
- the mixture to be separated is injected into the stream of solvent and is carried through the column as a solution
- the mixture is separated because the different parts are attracted by different amount to the solid, so they take different length of time to travel through tthe column
- as the liquid leaves the column, UV light is passed through it
- the UV light is absorbed by the parts of the mixture as they come through and a UV detector measures the UV light absorbed by the mixture
- a chromatogram is produced
- this shows the retention times of the components of the mixture
- this is the time taken for a substance to pass through the column and reach the detector
- you can compare these to reference books etc
16
Q
Describe gas chromatography
A
- the sample to be analysed is injected into a stream of gas, which carries it through a coiled column coated with a viscous liquid or a solid
- the components of the mixture constantly dissolve in the oil or absorb onto the solid, evaporate back into gas and then redissolve as they travel through the column
- the different components in the mixture can be identified by their time taken to travel through the column
18
Q
How do you use data from several spectra to work out a structure?
A
