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