Chapter 17 - Spectroscopy Flashcards
What does the mass spectrometer detect ?
The mass to charge ratio of the molecular ion, which gives the molecular mass of the compound
How are molecular ions formed in mass spectrometry?
- An electron is lost to form a positive ion
- Electrons from an electron gun knock off electrons in the outer shell
How is molecular mass found from a mass spectrum?
The molecular ion peak (M+ peak) has to be located
What is the cause of the M + 1 peak?
Some carbon is present as the carbon-13 isotope
What is fragmentation?
The molecular ions break down into smaller pieces called fragments
What causes the other peaks in a mass spectrum ?
Fragment ions
How can a mass spectrum differentiate between structural isomers?
They will break apart differently and thus will have
different mass spectra
What does the mass to charge ratio give you ?
It gives you the molecular mass of the compound
How is an electron lost ?
Electrons from an electron gun knock off electrons in the outer shell
How do you locate the M+ peak ?
It is the big peak furthest to the right of the graph
What is the effect on a bond when it absorbs IR ?
It bends or stretches more as its gains energy
What does the amount a bond stretches or bends depend on ?
- Mass of the atoms in the bond - heavier atoms vibrate more slowly than lighter atoms
- Bond strength - stronger bonds vibrate faster than weaker bonds
What is unique about each bond with regards to IR absorption?
Can only absorb radiation that has the same frequency as the natural frequency of the bond.
What scale is used for the frequency values?
Wavenumber (200/cm to 4000/cm)
How does infrared spectroscopy work?
- Sample placed inside an IR spectrometer
- A beam of IR radiation is passed through the sample
- Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.
