17 - Spectroscopy Flashcards
Molecular ion
When compound loses electrons and forms positive ion
Mol ion peak
clear peak on right side
Why small other peak
This is the M+1 peak, as 1.1% of carbon is present in c-13 isotope
Fragmentation
Some mol ions break down into smaller pieces known as fragments and called fragment ions
How does fragmentation occur
Breaks mol ion into +fragment ion and a radical where only positive charge ion is detected
Common fragment ions
- Ch3 + = 15
- C2H5 + = 29
- C3H7 + = 43
- C4H9 + = 57
Infrared spectroscopy
Atoms in mols are in constant motion due to vibrations and can absorb IR which makes them bend and stretch more
Different vibrations
Stretch - so that the distance between atomic Centre changes - like a spring
Bend - changing the bond angle = depends on mass of atom and strength of bonds (stronger- the faster)
How does it work
Any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond
How does IR spec work
- sample is placed in IR spec
- beam of IR from 2000-4000 is passed through sample
- mols absorb some IR and emit to analyse
- IR spec is connected to computer which shows which frequency of rad is emitted
IR of alcohol
Range of 3200-3600 and is usually wide and curved
C=O IR FOR ALDÉHYDE OR KETONE
Range from 1630-1820 and is very steep and long and thin
C=O for CARBOXLYIC
Will have normal C=O from 1630… and is steep
However will have different OH group than alc: here has much wider from 2500-3330
Uses of IR
- remote sensors analyse IR of vehicle emissions to measure CO, CO2 on busy town centres or motorways to measure pollution
- breath analysis to pass IR through breath sample to detect IR absorption of compounds. More IR absorbed, higher the reading and more ethanol present
Order to analyse
- elemental analysis = percentage composition to determine empirical formula
- mass spectrometry = use of mol ion peak to determine mass and use frag ions to see sections of mol
- IR spec = identity bonds and functional groups present
