MS Interpretation II Fragmentation Mechanisms in MS + + + + + + 15 71 15 43 71 43 57 29 What governs

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Transcript of MS Interpretation II Fragmentation Mechanisms in MS + + + + + + 15 71 15 43 71 43 57 29 What governs

  • MS Interpretation II

    Fragmentation

  • 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

    even electron

    even electron

    even electron

    odd electron

    odd electron

    odd electron

  • Ionization OE+•

    Electron can (and does) come from anywhere.

    EI- 1e- (n) - 1e- (π)

    - 1e- (σ)EI EI

  • Ionization OE+•

    Electron can (and does) come from anywhere.

    EI- 1e- (n) - 1e- (π)

    - 1e- (σ)EI EI

    Likelihood of each of these depends on energy levels in molecular orbitals: π

    σ

    n π* σ*

    vacuum level

    IEn IE

    π

    IEσ

    most likely

    least likely

  • Ionization OE+•

    Electron can (and does) come from anywhere.

    EI- 1e- (n) - 1e- (π)

    - 1e- (σ)EI EI

    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

  • Fragmentation Mechanisms: EE+

  • • Bond cleavage with charge migration

    Fragmentation Mechanisms: EE+

  • • Bond cleavage with charge migration – R-OH2+ → R+ + H2O

    Fragmentation Mechanisms: EE+

  • • 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

    H N 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

    H N 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

    H N 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

    H N 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•+

  • Fragmentation Mechanisms: OE•+ • Direct dissociation

  • Fragmentation Mechanisms: OE•+ • Direct dissociation

    – R-R' → R+ + R'•

  • 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

    m/z

    R el

    at iv

    e A

    bu nd

    an ce

    +

    +

  • 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 radic