ch17 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?
Electron is lost to form a positive ion
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 is the effect on a bond when it absorbs IR?
Bend or stretch more as they gain energy.
What does the amount a bond streches 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 in range 200/cm to 4000/cm passed through the sample
Molecule absorbs some of the IR frequencies and the emerging beam is analysed to find the absorbed frequencies.
What is the term given to the dips in the graph?
peaks
What do peaks help us predict?
bond types in the molecules
What is below 1500/cm known as on the IR spectrum and why?
Fingerprint region
The peaks in this region are unique to each molecule.
What is the typical sequence for identification of an unknown organic molecule?
Elemental analysis- use of percentage composition data to determine the empirical formula
Mass spectrometry- Use of molecular ion peak from a mass spectrum to determine the molecular mass; use of fragment ions to identify sections of a molecule
Infrared spectroscopy- use of absorption peaks from an infrared spectrum to identify bonds and functional groups present in the molecule.
