Topic 19 - Modern Analytical Techniques II Flashcards
On a mass spectrum, what is the name for the peak formed by the compound having lost an electron?
Molecular ion peak
How can the molecular mass of a compound be found from a mass spectrum?
It is the m/z ratio of the molecular ion peak.
Assuming the ion has a +1 charge
What are high resolution mass spectrometers?
Mass spectrometers that measure the atomic and molecular masses extremely accurately (to several decimal places).
What are high resolution spectrometers useful for?
Identifying compounds that appear to have the same Mr when they’re rounded to the nearest whole number.
Remember to look at the example for high resolution mass spectrometry on pg 230 of revision guide.
Do it.
What is does NMR spectroscopy stand for?
Nuclear magnetic resonance spectroscopy
What is NMR spectroscopy used for?
Working out the structure of an organic molecule.
Describe the environment used in NMR spectroscopy.
- Strong magnetic field
* Radio waves
Describe the basic principle of NMR spectroscopy.
1) A sample of a compound is placed in a strong magnetic field and exposed to a range of different frequencies of radio wave.
2) The nuclei of certain atoms within the molecule absorb energy from the radio waves.
3) The amount of energy that a nucleus absorbs at each frequency will depend on the environment that it’s in.
4) The pattern of these absorptions gives you information about the positions of certain atoms within the molecule, and about how many atoms of that type the atom contains.
5) You can piece these bits of information together to work out the structure of the molecule.
What are the two types of NMR spectroscopy?
- Carbon-13 NMR
* High resolution proton NMR
What does carbon-13 NMR give you information about?
The number of carbon atoms that are in a molecule, and the environments that they are in.
What does high resolution proton NMR give you information about?
The number of hydrogen atoms that are in a molecule, and the environments that they are in.
In NMR spectroscopy, what shields a nucleus from the effect of the magnetic field?
- The electrons around it
- These are affected by the other atom and groups of atoms that are around the nucleus (due to electron repulsion/attraction)
Describe how the environment an atom is in changes how it appears on an NMR spectrum.
- The atoms or groups of atoms around an atom affect the electrons around it -> e.g. Oxygen will drag electrons away from a carbon atom
- The nucleus is partly shielded from the external magnetic field by these electrons
- So nuclei in different environments will absorb different amounts of energy at different frequencies of radio wave
- These will cause a peak at a different point in the NMR spectrum
What does an atom’s environment depend on in NMR spectroscopy?
All of the connected atoms, all along the molecule (not just the ones it is directly bonded to).
Remember to practise working out the number of environments in a molecule in terms of NMR spectroscopy.
Pg 231 of revision guide
What is on the x-axis and y-axis of an NMR spectrum?
- x-axis -> Chemical shift
* y-axis -> Absorption
On a carbon-13 NMR spectrum, what does each peak show?
The frequency at which a different carbon environment absorbs energy.
What is the absorption in NMR measured relative to and how is this shown on an NMR spectrum?
- Tetramethylsilane (TMS)
* There is a small TMS peak at the 0 chemical shift point
Does TMS produce a single absorption peak in both types of NMR? Why?
Yes, because all of its carbon and hydrogen nuclei are in the same environment.
Why is TMS chosen as a standard for NMR and how does it work?
- TMS gives a single absorption peak at a lower frequency than almost anything else
- This means it can be assigned a value of 0 and all other peaks are measured as chemical shift relative to this
What is a chemical shift in NMR?
The difference in radio frequency absorbed by the nuclei (hydrogen or carbon) being analysed, relative to a reference peak (TMS).
Wha is the symbol for chemical shift?
δ
What are the units for chemical shift in NMR?
- Parts per million
* ppm
Before NMR is performed on a sample, what is usually done?
A small amount of TMS is added to give a reference peak on the spectrum.
What is the formula for TMS?
Si(CH₃)₄
What is the small peak at 0 chemical shift on an NMR spectrum?
The TMS peak. It can be ignored.
Describe how to interpret carbon-13 NMR spectra to determine the structure of the molecule.
1) Count the number of carbon environments
• Ignore the peak at δ=0 (the is the reference peak from TMS)
• This gives the number of different carbon environments in the spectrum
2) Look up the chemical shifts in a shift diagram.
• This tells you which carbon environments are present (e.g. C=C)
3) Try our possible structures of the molecule, bearing in mind the number of different environments and the types of environment involved
Do the heights of peaks in a carbon-13 NMR spectrum have any meaning?
No
How can you predict the carbon-13 NMR spectrum for a molecule?
- Identify the number of unique carbon environments
* Use the shift diagram in your data booklet to work out where the peaks of each carbon environment would appear
Remember to practise interpreting carbon-13 NMR spectra.
Pgs 232 and 233 of revision guide
What does each peak on a proton NMR spectrum represent?
A different hydrogen environment (bearing in mind that a single peak can be split into many).
How can you work out the environment that a peak on a proton NMR spectrum represents?
Look up the chemical shift in on a data diagram to see what it corresponds to.
What is the big difference between carbon-13 NMR and proton NMR spectra?
In a proton NMR spectrum, the peaks are split according to the arrangement of the hydrogen environments.
Are all hydrogen peaks on a proton NMR spectrum split?
- No, only the peaks of hydrogens bonded to carbon atoms.
* Peaks of, for example, -OH and -NH hydrogens are not split
What causes the splitting of peaks in a proton NMR spectrum?
Hydrogen atoms that are bonded to adjacent carbons (which are inequivalent!)
Do hydrogens on non-carbons cause splitting of hydrogen peaks on an NMR spectrum?
No, only hydrogen attached to adjacent carbons.
Will all hydrogens on adjacent carbon atoms cause splitting of hydrogen peaks on an NMR spectrum?
- No, only inequivalent carbons
* If the carbons are in exactly the same environment, no splitting occurs
What is the name for the effect that causes splitting of peaks in a proton NMR spectrum?
Spin-spin coupling
What is the name for split peaks on a proton NMR spectrum?
Multiplets
What rule is used to calculate the split peaks in a peak on a proton spectrum NMR spectrum?
n+1 rule
Describe how you can calculate the number of split peaks in a peak on a proton NMR spectrum.
- Look at the number of hydrogens attached to the adjacent carbons (except hydrogens in the exact same environment)
- Add one to this number
- This is how many split peaks there are
If there are two hydrogens on an adjacent carbon to a hydrogen, how many split peaks will be in that peak in the proton NMR spectrum?
3
In a proton NMR spectrum, how may the area under each peak be symbolised?
An integral symbol next to each one of a relative height proportional to the area under the graph.
In a proton NMR spectrum, what does the area under each peak tell you?
The relative number of protons (compared to the other environments).
Describe how to interpret proton NMR spectra to determine the structure of the molecule.
1) Count the number of clusters of peaks
• Ignore the peak at δ=0 (the is the reference peak from TMS)
• This gives the number of different hydrogen environments in the spectrum
2) For each cluster of peaks, look at the chemical shift and look it up in a data diagram
• This tells you which hydrogen environments are present (e.g. C=C)
3) For each cluster of peaks, count the number of split peaks
• This tells you the total number of hydrogens on adjacent carbons
4) For each cluster of peaks, look at the relative area under it.
• This tells you the relative number of hydrogens in that environment
5) Try our possible structures of the molecule, bearing in mind the things found.
What is the name for 1 peak in a cluster in proton NMR?
Singlet
What is the name for 2 peaks in a cluster in proton NMR?
Doublet
What is the name for 3 peaks in a cluster in proton NMR?
Triplet
What is the name for 4 peaks in a cluster in proton NMR?
Quartet
Remember to practise drawing out the proton NMR spectrum for 1,1,2-trichloroethane.
See diagram pg 235 of revision guide.
What solvent is often used in both types of NMR and why?
- CDCl₃
- This is trichloroethane where the hydrogen has been replaced by a deuterium
- The deuterium means it gives no line in the proton NMR spectrum, while the carbon line seen in the carbon-13 NMR spectrum is easily recognised and removed
What happens to -OH or -NH peaks in a proton NMR spectrum?
They are not split and they do not cause splitting.
How can an -OH peak on a proton NMR spectrum easily be detected and why?
- D₂O (heavy water) is added to the alcohol
- The H in the OH is replaced by the D, which does not produce a peak
- Therefore, the OH peak disappears
Remember to revise the principle on which carbon-13 and proton NMR work.
Chemguide is very good
NOTE: Check with Sir whether this is necessary
What are the two parts of a chromatography setup?
- Mobile phase -> Where the molecules can move
* Stationary phase -> Where the molecules can’t move
What state is the mobile phase always in?
Liquid or gas
What state is the stationary phase always in?
Solid or liquid on solid support
What is the principle on which chromatography works?
1) The mobile phase moves through or over the stationary phase
2) The distance each substance moves up the plate depends on its solubility in the mobile phase and its retention by or adsorption to the stationary phase
3) Components that are more soluble in the mobile phase will travel further up the plate or faster through the column. This allows the components to be separated out.
What factors determine the distance a substance will move up a plate in chromatography?
- Solubility in the mobile phase
* Retention by or adsorption to the stationary phase
What is the formula for the Rf value in chromatography?
Rf = Distance travelled by spot / Distance travelled by solvent
What factors does the Rf value depend on?
- Paper
- Solvent
- Temperature
What are the two additional types of chromatography you need to know about (aside from paper chromatography and thin-layer chromatography)?
- High-performance liquid chromatography (HPLC)
* Gas chromatography
What does HPLC stand for?
High-performance liquid chromatography
HPLC and gas chromatography are types of what?
Column chromatography
In HPLC, what is the mobile phase and stationary phase?
- Mobile phase -> Polar liquid mixture (e.g. ethanol and water)
- Stationary phase -> Solid (e.g. silica bonded to various hydrocarbons)
Describe how high performance liquid chromatography (HPLC) works.
1) Liquid mobile phase (often a polar mixture such as methanol and water) is forced through a column at high pressure.
2) Mixture to be separated is injected into the stream of solvent and is carried as a solution into the column.
3) Small particles of solid (such as silica bonded to various hydrocarbons) are in the tube.
4) The mixture is separated because different parts are attracted by different amounts to the solid.
5) As liquid leaves the column, UV light is passed through it. The UV is absorbed by parts of the mixture as they come through, and a UV detector measures the light absorbed by the mixture.
6) This produces a chromatogram, showing the retention times of the components. These can be compared to a reference.
Remember to practise drawing the diagram for HPLC.
Pg 236 of revision guide
In gas chromatography, what is the mobile phase and the stationary phase?
- Mobile phase -> Gas
* Stationary phase -> Viscous liquid (e.g. oil) or solid
Describe how gas chromatography works.
1) The sample is injected into a stream of gas, which carries it through a coiled column coated with viscous liquid (e.g. oil) or a solid.
2) The components of the mixture constantly dissolve in the oil or adsorb onto the solid, evaporate back into the gas and then redissolve as they travel through the column.
3) There is a defector at the end of the coil and the components can be identified by their retention times (time taken to travel through the coil).
Remember to practise drawing the diagram for gas chromatography.
Pg 237 of revision guide
When can gas chromatography not be used?
- When the sample is heat-sensitive
* When the sample has a high boiling point
How can mass spectrometry be used together in general?
Gas chromatography-mass spectrometry:
• Sample is separated using gas chromatography, but instead of going to a detector, the components are fed into a mass spectrometer
• This produces a mass spectrum for each component, which can be used to identify each one and show what the original sample consistent of
In what real-world applications are HPLC, gas chromatography and mass spectrometry combined?
- Forensics
- Drug testing
Together, these techniques can separate and detect trace amounts of substances.
Remember to to practise using multiple spectroscopy techniques together to work out a structure.
Pg 238 of revision guide
What is elemental analysis?
When experiments determine the masses or percentage composition of different elements in a compound.
How can elemental analysis be used to work out a structure alongside combined spectroscopy techniques?
- In elemental analysis, experiments determine the masses or percentage composition of different elements in a compound.
- This helps work out the empirical and molecular formulae.
- This is useful in working out the structure when combined with spectroscopy techniques.
