Summary
Summary Module 4.2.4 Chapter 17 Spectroscopy
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- OCR
notes on: 1 - mass spectrometry 2 - infrared spectroscopy
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17.1 mass spectrometryMolecular and - In spectrometer – compound loses e- -> +ve ion = molecular ion (M+)
fragment ions - Mass spectrometer – detects mass: charge of ion – gives molecular mass of compound.
Molecular mass - To find mass – M+ peak (furthest on x axis) = molecular mass.
- M+1 peak – small peak after M+ - 1.1% of carbon is present as C13 isotope.
fragmentation - Some molecular ions break down into fragments – other peaks are caused by fragment ions.
- Fragment ions – formed from breakdown of the molecular ion.
- Fragmentation – breaks ion into 2 species – radical and +ve fragment.
- +ve is detected by spectrometer but radicals aren't.
Fragmentation - Find the M+ and the M+1 peaks.
peaks – organic - Break the molecule you're looking for up.
molecule - Using peaks on spectrum + molecular mass of fragments – find fragments.
M+ peak - Used to identify number of C atoms present in the compound.
- C = (height of M+1/height of M+) x 100
- Using ruler to measure in mm.
17.2 infrared spectroscopy
Infrared radiation - Bonds have energy and vibrate – as temperature increases, vibration does too.
and covalent bonds - Bonds absorb IR radiation – bend/stretch more (usually 200cm-1 to 4000cm-1)
- Stretch – along the line between the atoms so the distance between 2 centres changes.
- Bend – change in bond angle = mass and strength
- Mass – heavier = slower, lighter = faster
- Stronger = faster, weak = slower.
- Any bond can absorb radiation that has same frequency as natural frequency of bond.
- Frequency too large – wavenumber is used = proportional to frequency.
Infrared and - Sun and IR are unaffected by atmospheric gases – passes through atmosphere to Earth.
atmospheric gases - Some is re-emitted from Earth’s surface in longer-wavelength IR radiation.
- Greenhouse gases absorb the longer wavelength as it has same natural frequency – vibrating
bonds in molecules re-emit energy as radiation – increases temperature of atmosphere -
global warming. - water vapour, CO2 and methane.
Infrared and organic - Infrared spectroscopy – identifying functional groups.
molecules 1) sample placed inside IR spectrometer.
2) beam of IR radiation is passed through sample.
3) molecule absorbs IR frequency and emerging beam of radiation is analysed to find frequencies.
4) IR spectrometer connected to computer – plots graph of transmittance x wavenumber.
- Peaks – dips in graph = observes at wavenumber related to bond in molecule.
- Fingerprint region – area of smaller peaks after a certain point = find molecule using spectra.
Infrared spectra of - From outside region – peaks are clear – using wavenumber from data book.
functional groups - C-H bonds = 2850-3100cm-1
Alcohol - Has absorbance peak of 3200-3600cm-1 = OH bond + Also has 1000-1300cm-1 by CO bond.
Aldehyde or ketone - Has 1630-1820cm-1 = C=O but typically 1700cm-1
Carboxylic acid – 1630 – 1820cm-1 (C=0), 2500-3330cm-1 (OH)
applications - Pollutants can be identified by IR fingerprints – remote sensors - analyse vehicle emissions to
measure + detect carbon monoxide + carbon dioxide and hydrocarbons.
- Breathalysers – pass beam of IR through breath and detect IR absorbance – find ethanol =
more IR = higher reading = more ethanol in breath.
identification Elemental analysis – percentage composition data to determine empirical formula.
Mass spectrometry – molecular ion peak from spectrum to find MR – fragment ions for sections.
Infrared – absorption peaks from spectrum to find bonds and functional groups.
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