Chapter 17 Spectroscopy Flashcards
molecular ion
a positive ion formed when an organic compound is placed in a mass spectrometer and loses an electron
What does a mass spectrometer detect?
mass-to-charge ratio (m/z) of molecular ion
–> gives molecular mass of compound
e.g. CH3CH2CH2OH –> CH3CH3CH3OH+ + e-
Finding molecular mass from a mass spectrum
molecular ion peak (M+ peak) is the clear peak at the highest m/z value on the right hand side e.g propan-1-ol is 60
Finding molecular mass from a mass spectrum
molecular ion peak (M+ peak) is the clear peak at the highest m/z value on the right hand side e.g propan-1-ol is 60
What is the small peak after M+ peak?
very small peak called M+1 peak
–> exists because 1.1% of carbon is present as C-13 isotope
e.g for propan-1-ol the molecular mass of the M+1 peak is 61
Fragmentation and what causes other peaks
the breaking down of some molecular ions into smaller pieces known as fragments (unstable ions)
–> other peaks are caused by fragment ions
simple fragmentation and its equation
breaks a molecular ion into two species (positively charged fragment ion and radical)
=> positive ions are detected but uncharged radical is not
In propan-1-ol largest peak has m/z value of 31
e.g. CH3CH2CH2OH+ –> CH2OH+ + CH3CH2*
How can fragmentation peaks be used to identify organic molecules?
Molecules will all fragment slightly differently depending on strcture (even if molecular mass and molecular ion peak are same, fragment ions are different
number of carbon atoms formula
(height of M+1 peak/ height of M peak) x 100
What does a mass spectrometer find?
percentage abundances of isotopes in a sample of an element
–> isotopic signature
How does a mass spectrometer function
- sample is placed on mass spectrometer
- Sample is vaporised and ionised with an electron gun causing electrons to be displaced
- Ions accelerated => heavier ions move more slowly and are more difficult to deflect than lighter ions, so ions of each isotope are separated
- Ions are detected on a mass spectrum as mass-to-charge ratio. Each ion reaching detector adds to signal so greater abundance= larger signal
mass-to-charge ratio formula m/z
relative mass of ion/ relative charge on ion
For an ion with +1 charge, ratio is equivalent to relative isotopic mass
What makes the ions more stable?
alkyl groups are good at stabilising positive charges because they are electron rich
elements like N,O because they have lone pairs
more stable= more abundance
2 types of bond vibrations
stretch: a rhythmic movement along the line between atoms so distance between two atomic centres increases and decreases
bend: a change in bond angle
amount that a bond stretches or bends depends on
mass of atoms in bond (heavier atoms vibrate more slowly than lighter ones)
strength of bond: stronger bonds vibrate faster
