19:Modern Analytical Techniques II
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What is mass spectrometry used to find?
The relative molecular mass (Mr) of a compound
What do the peaks on a mass spectrometer show?
They show fragments of the original molecule
What is the last peak on a mass spectrometer?
The M+ peak (or molecular ion peak)
What does the M+ peak tell us?
The relative molecular mass of the substance under test
What is high resolution mass spectrometry used for?
To identify different molecules with the same molecular mass, rounded to the nearest whole number) - they measure the Mr to several decimal places (whereas standard mass spec only measures to the nearest whole number)
What is NMR Spectroscopy used for?
To help determine the strucutre of a molecule
What are the two types of NMR Spectroscopy?
1) ¹³C NMR 2) High-resolution ¹H NMR
What does ¹³C NMR tell us?
Gives us information on how carbon atoms are arranged
What does High-resolution ¹H NMR tell us?
Gives us information on how hydrogen atoms are arranged
How is nuclear spin formed?
If an atomic nucleus had an odd number of nucleons then it has a nuclear spin
What are nucleons?
The total number of protons and neutrons
What happens when there is a nuclear spin?
A weak magnetic field is created - NMR detects how these magnetic fields are affected by a larger external magnetic field
Why does ¹H have a nuclear spin?
It only has one proton in total and no neutrons so it has a nuclear spin
Why is ¹³C used rather than ¹²C for NMR?
Carbon-12 has an even number of nucleons so it doesn’t have a nuclear spin
What happens when nuclear spin occurs?
Nuclei spin in random directions, however when an external magnetic field is applied they align in two directions
How does an NMR spectrometer work?
1) Nuclei spin either the direction of the external magnetic field or against it 2) Those that spin in the direction of the magnetic field have lower energy 3) NMR fires out radio waves, at a specific frequency the nuclei that are aligned with the magnetic field absorb the energy and flip it to a higher level 4) Those with higher energy can also drop to a lower level, so they admit radio waves 5) Initially there are more nucleus aligned with the magnetic field so overall more energy is absorbed than emitted NMR measures the amount of energy absorbed
What is energy absorbed by the nucleus dependent on?
The environment it is in
Why does the energy absorbed by the nucleus depend on the environment?
1) A nucleus can be shielded from an external magnetic field from electrons surrounding the nucleus -> atoms and groups of atoms adjacent to the nucleus affect the level of electron shielding (electronegative elements like Cl near a carbon atom will reduce electron shielding of carbon atom 2) This magnetic field will be felt by the nuclei different depending on the enviroment it is in as they absorb different amounts of energy and various frequencies 3) This difference in energy is what NMR picks up
What is tetramethysilane (TMS)?
A chemical used as a standard when looking at chemical shift in NMR spectrum
Why is it important that we use tetramethysilane (TMS) as a baseline?
As nuclei absorb different amounts of energy at different frequencies to measure the magnitude without a reference - TMS is the reference, it has a chemical shift of 0 or In other words it is used to calibrate the NMR spectroscope
What is the chemical shift?
The difference between TMS peak and the peaks produced by the substance under test
What is the chemical shift of TMS?
0
What is chemical shift measured in?
Parts per million (ppm)
Why is TMS good as a baseline?
It has 12 hydrogen atoms all in identical environments, this will produce a large single peak well always from the sample