Ionization OE+•
Electron Ionization (EI): Even-electron neutrals yield odd-electron radical cations.
M(EE) M (OE)- 1e-
+•
Electron can come from anywhere.
- 1e- (n) - 1e- (π)
- 1e- (σ)
EI
EI EI
EI
evenelectron
evenelectron
evenelectron
oddelectron
oddelectron
oddelectron
Ionization OE+•
Electron can (and does) come from anywhere.
EI- 1e- (n)- 1e- (π)
- 1e- (σ)EIEI
Likelihood of each of these depends on energy levels in molecular orbitals: π
σ
nπ*σ*
vacuumlevel
IEn IE
π
IEσ
mostlikely
leastlikely
Ionization OE+•
Electron can (and does) come from anywhere.
EI- 1e- (n)- 1e- (π)
- 1e- (σ)EIEI
Naturally, cannot distinguish these in mass spectrometer (all have m/z = 100).
But fragmentation patterns will be different.
Ionization EE+
CI, MALDI and ESI: Even-electron neutrals yield even-electron cations.
M(EE) + H+ MH+(EE)
Like EI, ionization may occur at multiple places.
+ +
+ H+
+ H+
Again, instrument cannot distinguish.
Fragmentation Mechanisms: EE+
• EE+ ions don't fragment to form OE+•
• Fragmentations more familiar and spectra generally less complex than OE+•
• More frequent and varied rearrangements can make interpretation more difficult
• Spectra can be more sensitive than EI to small structural changes
• Sensitivity to experimental conditions incompatible with reference libraries
• Bond cleavage with charge migration– R-OH2+ → R+ + H2O
• Cleavage with cyclization and migration
Fragmentation Mechanisms: EE+
• Bond cleavage with charge migration– R-OH2+ → R+ + H2O
• Cleavage with cyclization and migration
–
Fragmentation Mechanisms: EE+
O
HN
O
HN
HN CR2
HN CR2+
• Bond cleavage with charge migration– R-OH2+ → R+ + H2O
• Cleavage with cyclization and migration
–
• Two-bond cleavage with charge retention
Fragmentation Mechanisms: EE+
O
HN
O
HN
HN CR2
HN CR2+
• Bond cleavage with charge migration– R-OH2+ → R+ + H2O
• Cleavage with cyclization and migration
–
• Two-bond cleavage with charge retention
–
Fragmentation Mechanisms: EE+
O
HN
O
HN
HN CR2
HN CR2+
H H-H2
• Bond cleavage with charge migration– R-OH2+ → R+ + H2O
• Cleavage with cyclization and migration
–
• Two-bond cleavage with charge retention
–
• Other fragmentations require MS/MS
Fragmentation Mechanisms: EE+
O
HN
O
HN
HN CR2
HN CR2+
H H-H2
Fragmentation Mechanisms in MS: OE+•
Electron Ionization: Fragmentation is always unimolecular. Two possible categories of fragmentation:
M (OE)+• A
(OE)
+•B(EE)
M (OE)+•
B • (OE)A+(EE)
+
+
parents daughters
charge migration(radical and charge part ways)
charge retention(neutral molecule is ejected)
Fragmentation Mechanisms in MS: OE+•
Electron Ionization: Fragmentation is always unimolecular. Two possible categories of fragmentation:
M (OE)+• A
(OE)
+•B(EE)
M (OE)+•
B • (OE)A+(EE)
+
+
parents daughters
charge migration(radical and charge part ways)
charge retention(neutral molecule is ejected)
Important: Only daughter ions are detected by MS instrument. Released neutrals are only inferred.
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage– R-CH2-Y-R' → R• + CH2=Y-R'+
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage– R-CH2-Y-R' → R• + CH2=Y-R'+
– R-CH2-Y-R' → R+ + •CH2-Y-R'
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage– R-CH2-Y-R' → R• + CH2=Y-R'+
– R-CH2-Y-R' → R+ + •CH2-Y-R'• Two-bond cleavage
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage– R-CH2-Y-R' → R• + CH2=Y-R'+
– R-CH2-Y-R' → R+ + •CH2-Y-R'• Two-bond cleavage
– Retro Diels-Alder
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage– R-CH2-Y-R' → R• + CH2=Y-R'+
– R-CH2-Y-R' → R+ + •CH2-Y-R'• Two-bond cleavage
– Retro Diels-Alder• Rearrangement
Fragmentation Mechanisms: OE•+
• Direct dissociation– R-R' → R+ + R'•
• Cleavage adjacent to a heteroatom– R-CH2-Y-R' → R-CH2+ + •Y-R'– R-CH2-Y-R' → R-CH2• + +Y-R'
• α-Cleavage– R-CH2-Y-R' → R• + CH2=Y-R'+
– R-CH2-Y-R' → R+ + •CH2-Y-R'• Two-bond cleavage
– Retro Diels-Alder• Rearrangement
– McLafferty Rearrangement
Fragmentation Mechanisms in MS:Direct Cleavage
(electron can come from any bond)
Fragmentation
+ ++ +
EI
(one-electron bond can break either way)
+
+
+ +
+
+ +
Fragmentation Mechanisms in MS:Direct Cleavage
(electron can come from any bond)
Fragmentation
+ ++ +
EI
(one-electron bond can break either way)
+
+
++
+
+
+ +
15
1571
7143
43
57
29
Fragmentation Mechanisms in MS
+
+
++
+ +
15
1571
7143
43
57
29
What governs which ions are predominant?
+
+
Fragmentation Mechanisms in MS
+
+
++
+ +
15
1571
7143
43
57
29
What governs which ions are predominant?
1. Most ionizeable type of electrons. Here, all electron sources are σ bonds.
+
+
Fragmentation Mechanisms in MS
+
+
++
+ +
15
1571
7143
43
57
29
What governs which ions are predominant?
1. Most ionizeable type of electrons. Here, all electron sources are σ bonds.
2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.)
Here, secondary cation/radical combination favored over primary.
+
+
Fragmentation Mechanisms in MS
+
+
++
+ +
15
1571
7143
43
57
29
What governs which ions are predominant?
1. Most ionizeable type of electrons. Here, all electron sources are σ bonds.
2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.)
Here, secondary cation/radical combination favored over primary.
+
+
Fragmentation Mechanisms in MS
+
+
++
+ +
15
1571
7143
43
57
29
What governs which ions are predominant?
1. Most ionizeable type of electrons. Here, all electron sources are σ bonds.
2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.)
Here, secondary cation/radical combination favored over primary.
3. In charge separation, cation stability is more important than radical stability. Here, masses 71, 43 favored over 15, 57.
+
+
Fragmentation Mechanisms in MS
+
+
++
+ +
15
1571
7143
43
57
29
What governs which ions are predominant?
1. Most ionizeable type of electrons. Here, all electron sources are σ bonds.
2. Combination of most stable cation and radical. (Actually, this addresses most ionizeable bond within type.)
Here, secondary cation/radical combination favored over primary.
3. In charge separation, cation stability is more important than radical stability. Here, masses 71, 43 favored over 15, 57.
+
+
t-butyl ethyl ether
O
102
Heteroatoms: Induced vs. Alpha Cleavage
CH3•O
HO
Hα
O
87
5987
59
O Oi
57
57
Competition: i vs α Cleavage
• Induced cleavage is favored for larger heteroatoms
• α-cleavage is favored for electron donating substituents
Competition: i vs α Cleavage
• Induced cleavage is favored for larger heteroatoms
• α-cleavage is favored for electron donating substituents
• Br,Cl < R•,π bond,S,O < N
OE+• Fragments
• Odd electron fragments are less common, especially at low mass
– Even mass ions at low mass likely contain N
OE+• Fragments
• Odd electron fragments are less common, especially at low mass
– Even mass ions at low mass likely contain N
• These fragments arise from two-bond cleavages, or rearrangements
OE+• Fragments
• Odd electron fragments are less common, especially at low mass
– Even mass ions at low mass likely contain N
• These fragments arise from two-bond cleavages, or rearrangements
• Even mass OE+• fragments are important ions, as they can be highly sensitive to structural changes
100
43
29
85
O
2-methyl 3-pentanone
McLafferty rearrangement is precluded here
No major OE+•
5757
71
OO+
OO
+
71
177
192159149
135
OO
O
- •CH3
XRetro Diels-Alder reaction is not observed
Aromatization via substitution dominates the spectrum
177
164
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