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What does the peak at the end of a mass spectrometry graph tell you? What is it called?
- Molecular ion peak (the M peak) - shows the molecular mass of the compound.
- (be careful, as there is small peak to the right of this, which is dues to presence of carbon-13 isotope)
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What are the 5 processes involved in mass spectroscopy?
- Vaporisation - heated
- Ionisation - bombard electrons
- Acceleration - electric field
- Deflection - magnetic field
- Detection - electric plate detecting charge (photoelectric methods)
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In a mass spectra, why are there many peaks for one compound? What is this called?
- Because the bombarding electrons break some of the molecular ions up into fragments.
- Fragments shown up in the spectrum are called fragmentation pattern.
- You can use these to identify the molecule and their structure.
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When a molecule is fragmented in mass spectroscopy by bombarding electrons, what 2 things are created, and what does the spectrometer detect?
- into one free radical and one ion.
- The ions are shown up on spectrum but free radicals don't.
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What are some of the common fragments from an organic compound and their mass?
- CH3 - 15
- C2H5 - 29
- C3H7 - 43
- OH - 17
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Mass spectroscopy can be used to differentiate between similar molecules. eg. How can it identify whether its propanal or propanone?
- They both have same Mr but different structures so - different fragments!
- eg. Propanal has C2H5 fragment but propanone won't.
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Only what molecules can absorb infrared radiation?
- Those that have bonds in molecule that when it absorbs IR, the bending or stretching vibrations causes a change in net dipole moment.
- (eg. O2 or N2 cannot because polarity will not change as it vibrates)
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Why is the spectrum from infrared spectroscopy produced?
Different bonds absorb different wavelengths of IR.
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What are the factors that influence what wavelength of IR is absorbed by a bond?
- Bond strength
- Bond length
- Mass of each atom involved in bond
- Rest of molecule also has an effect.
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What shows up in a infrared spectrum for a carbonyl (C=O) bond?
Strong, sharp absorption around 1700cm-1
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Infrared spectrum of alcohol (OH bond)?
Strong, broad absorption around 3200 - 3600cm-1
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Infrared spectrum of carboxylic acids?
- Medium, broad absorption between 3200 - 3600 (OH bond)
- Strong, sharp absorption around 1700 (due to C=O bond)
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