Mass Spectrometry: Fragmentation
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Transcript of Mass Spectrometry: Fragmentation
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Mass Spectrometry:Fragmentation
Chem 4010/5326:Organic Spectroscopic Analysis
© 2015 Andrew Harned
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Initial ChargeWhere is it localized?
– In general HOMO-controlled → lost first– Lost from heteroatoms in preference
– Usually several fragmentation pathways occur
Where charge begins determines how all otherfragments are formed
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FragmentationHow to Represent Initial Radical Cation
Three ways to represent molecular ion of 1,3-cyclohexadiene
A. Delocalized structure with one less electronboth electron and charge delocalized over π-system
B. & C. electron and charge localized on π-systemboth are possible and each could give different fragments
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FragmentationGeneral Considerations
1. Even-electron ions cannot cleave to a pair of odd-electron fragments
• from thermodynamics2. Mass loss of 14 from M+ is extremely rare
• will see separation of 14 in hydrocarbons• arise from 2 different fragmentations
3. Order of radical/carbocation loss• tertiary > secondary > primary > methyl > hydrogen• loss of longest is preferred
4. Fragment ion stability takes precedence over radical5. Vinyl radical loss is not favorable, but possible
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FragmentationGeneral Considerations
6. Several fragmentations can occur, especially if it leads to a highly stabilized cation/radical
7. If loss of stable neutral molecule is possible, will do so (simple alkenes, H2O, CO, etc.)
8. Probability of cleavage is related to bond strength and stability of fragments.
Lastly...It is not possible to identify every peak in a massspectrum. The key is to remember a few diagnostic
fragment ions (e.g. m/z = 77, 91) and a few key masslosses in order to gain some insight into subunits.
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FragmentationFragmentation of Molecular Ion
The initial radical cation can fragment in a number of ways
I. Homolytic (one electron at a time) cleavage• products are even-electron cation and a free radical
II. A less cluttered representation of reaction IIII. Heterolytic (two electrons at a time) cleavage
• electrons move toward charged site• products are even-electron cation and a free radical
IV. Secondary fragmentation of even-electron cation by two electron process
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FragmentationA Few General Mechanisms
A – one-bond σ-cleavage
(a) cleavage at C–C producing cation & radical
(b) cleavage at C–heteroatom producing cation & radical- 2 electron process- works well for Z = O, halogen
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FragmentationA Few General Mechanisms
(c) α-cleavage
Charged species has heteroatom-isotope patterns in fragment ions
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FragmentationA Few General Mechanisms
(c) α-cleavage
If loss of stable neutral molecule is possible, will do so(simple alkenes, H2O, CO, etc.)
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FragmentationA Few General Mechanisms
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FragmentationA Few General Mechanisms
B – two-bond σ-cleavage or rearrangement(d) elimination of vicinal H and heteroatom
- common with alcohols- difficult to see M+
(e) retro Diels-Alder cleavage- mechanism involving single electrons also possible
Similar fragmentations with alcohols
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FragmentationA Few General Mechanisms
(f) McLafferty rearrangement- common with ketones, esters, carboxylic acids
Multiple McLafferty is possible
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FragmentationOther “Unusual” Mechanisms
Ortho effect
also with esters, amides, OH, etc.
“McLafferty + 1”- esters only
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FragmentationOdd-electron vs. Even-electron
(Nitrogen rule revisited)
For non-nitrogen-containing molecular ions, odd-electron ions occur as even mass.
molecular ions (OE) => even mass
most fragment ions (EE) => odd mass
– fragment
If ion has even mass then “unusual” fragmentation hasoccurred (e.g. retro Diels-Alder, McLafferty)
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FragmentationOdd-electron vs. Even-electron
(Nitrogen rule revisited)Example – 3-methyl-1-cyclohexene
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FragmentationOdd-electron vs. Even-electron
(Nitrogen rule revisited)Example – 3-methyl-1-cyclohexene
– e
m /z = 96 (OE)
m /z = 68 (OE)
– CH2=CH2(retro DA)
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FragmentationOdd-electron vs. Even-electron
(Nitrogen rule revisited)Example – 3-methyl-1-cyclohexene
m /z = 96 (OE)
m /z = 54 (OE)
– CH2=CHCH3(retro DA)
double bondisomerization
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Fragmentation
See Pretsch Chapter 8 for common fragmentation pathways of various functional
groups/compound classes