Lecture 5 Flashcards
(13 cards)
Why does fragmentation occur?
If the technique used to create gas-phase ions impacts sufficient vibrational energy into the ion, then it might fragment to produce different ions
Why is fragmentation useful?
- data from fragmentation was used to elucidate the structure of molecule
- data used as rapid way to identify molecules
Describe the conditions of an ionisation chambre
- kept at low pressure so the actual amounts of ions created is low
- means the reactions where ion collide are rare
Give the factors which influence which ions appear in fragmentation
- how quickly ion is formed- kinetics
- how stable the ion is- thermodynamics
What is the favoured form of ions and radicals?
- tertiary>secondary>primary ions
- formation of small ions disfavoured
- formation of small radicals favoured
Which bonds are broken during fragmentation?
C-C
O-H
C-O
Give the relationship between molecular ion m/z and fragmentation m/z
usually an even m/z for the molecular ion gives an odd m/z ion and odd mass for radical (invisible)
State the common fragmentation pathways for organic compounds
- Alpha fragmentation
- Inductive fragmentation
- Sigma fragmentation
Describe alpha fragmentation
- In this fragmentation pathway, a bond breaks alpha to the ionisation site
- fishook arrows are used here
Describe Sigma fragmentation
- molecules with no heteroatoms can undergo fragmentation, forming stabilised carbocations where possible
- tertiary>secondary>primary
Give examples of further fragmentation
- more than 1 bond can break
- fragments form cations and neutral species
Describe inductive fragmentation
- In this fragmentation pathway, a bond breaks adjacent to the ionisation site
- normal arrows used here
- bond breaks between C and heteroatom?
What do fragments form if they undergo further fragmentation?
its possible for the cation fragment to undergo further fragmentation- this would generate 2 new species- cationic species and neutral species (invisible)
