Module 4 Chapter 17
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What is a mass spectrometer used from
To identify the molecular mass (Mr) of an unkown (or known) compound
What is the molecular ion (M+)
The organic compound that is put into the mass spectrometer, loses an electron and forms a positive ion.
What does a mass spectrometer detect and what does this tell is
it detects the mass:charge ratio —> this gives the molecular mass of the compounds.
Where is the molecular ion peak located
It is the furthest peak found to the right of your mass spectrum (has the highest m/z value)
What is the small peak after M+ and why does it exist
The M+1 peak And it exists due to 1.1% of carbon being the carbon-13 isotope
What is a fragment
It is caused when a molecular ion breaks down into smaller pieces called fragments
What are the other parts of a mass spectrum caused by
By fragment ions which are formed from the breakdown of the molecular ion.
What does the simplest fragmentation do
It breaks down the molecular ion into two species: The positively charged fragment ion and a radicals
What is detected by the mass spectrometer
The positively charged ion fragments are detected however uncharged radicals are not
What is a feature of a mass spectrum and why
It is unique for each compound As they all fragment in different ways depending on their structure
What can you use fragments to do
As clues to build and identify molecules
What are the stages of a mass spectrometer working
Ionisation, acceleration, deflection and detecting
What is the most abundant peak called
The base peak
What are features of a mass spectrum
the molecule is ionised and fragmented The fragment with the greatest mass/charge ratio is called the molecular ion The parent ion gives the molecular mass The base peak is the most intense peak Isotope peaks may be observed
What is a disadvantage of a mass spectrum
You are destroying the sample when analysing it
What does IR (infrared radiation) do (vibrate)
It causes covalent bonds to vibrate more and absorb energy
What is a feature of the IR energy that molecules absorb
They absorb IR energy at a frequency corresponding to the frequency of the bonds natural vibration
What happens when the bonds absorb IR
They bend and stretch more
What does the IR detect (absorption and wave number)
How the absorption of a sample varies with wave number cm ^-1, which is proportional to the frequency of the vibration of bonds in the molecule.
What is IR used to do
Identify the functional groups that are present in a molecule
What can IR be absorbed by (atmosphere) and what can it cause
It can be absorbed by atmospheric gases containing C=O, O-H and C-H bonds which is the suspect link to global warming and resulting changes to energy usage.
What do the bonds that absorb the energy in these gases do
The vibrating bonds re-emit the absorbed energy from the sun as radiation that increases the temperature of the atmosphere close to the earths surface leading to global warming.
What are the three factors affecting vibration and why
Type of vibration: absorptions due to bending tend to occur at lower wave numbers than stretching Strength of bonds: stronger bonds absorb higher wave numbers Mass of atoms: heavier atoms vibrate at lower frequencies than lighter ones
How can IR be used to identify functional groups (method)
• Sample placed in Ir spec • A beam of Ir radiation (200-4000 cm^-1) is passed through the sample • Molecule absorbs some IR frequencies, the emerging beam is analysed • connected computer plots graph of transmittance against wave number