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Summary Mass Spec, Infrared Spec, NMR
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Mass Spec, Infrared Spec, NMR
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Mass Spec, Infrared spec, NMR – Consolidation notesMass Spectroscopy
• A mass spectrum is produced by a mass spectrometer.
• The molecules are bombarded with electrons, which remove an electron from a molecule to form a molecular
ion (M+(g))
• To find the relative molecular mass of a compound you look at the molecular ion peak (M peak) the mass/charge
value of the molecular ion peak is the molecular mass.
• The molecular peak is found near the higher values on the mass spectra.
• You also get an M+1 peak which is caused by the presence of the carbon-13 isotope.
Fragmentation à the breaking up of the molecule into fragments
à FREE RADICALS ARE LOST AND DON’T APPEAR ON THE SPECTRA.
1 à Identify the fragments
2 à piece them together to
Form a molecule with the 1 à sample is vapourised
Correct molecular mass. 2 à sample is ionised.
3 à accelerated through MS.
4 à different fragments detected.
https://www.youtube.com/watch?v=VmgR1rosQ4g
Infrared Spectroscopy
1) Beam of IR radiation is passed through a sample of a chemical.
2) IR radiation is absorbed by the covalent bonds in these molecules increasing their vibrational energy.
3) Bonds between different atoms absorb different frequencies of IR radiation.
4) Infrared spectrum is produced that shows you what frequencies of radiation the molecules are absorbing.
• Infrared Spectroscopy is useful for distinguishing the functional group of a compound as they have very distinct
peaks.
• You can also use IR spec to work out how something has changed during a reaction (oxidation of alcohols.)
Functional group Where it is found Frequency/wavenumber
C—H Alkyl groups, alkenes, arenes 2850-3100
O—H Alcohols 3200-3600
O—H Carboxylic acids 2500-3300 (BROAD)
C=O Aldehydes, ketones, COOH, esters 1630-1820
Uses of IR Spectroscopy
1) Breathalysers à works out the amount of
ethanol vapour in the breath by measuring
the intensity of the peak corresponding to
the C—H bond in the IR spectrum
2) Monitor polluting gases à including carbon
monoxide, nitrogen monoxide – intensity of
these peaks corresponding to the CO or NO
bonds can be studied to monitor their levels.
https://www.youtube.com/watch?v=IXTsIFhOMr8
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