Odd-Electron Ionization

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Odd-Electron Ionization Electron Ionization (EI): Even-electron neutrals yield odd-electron radical cations. EI + M(EE) M (OE) -1e- Electron can come from anywhere. EI EI O O -1e- (n) -1e- () even odd even odd electron electron electron electron EI -1e- () even odd electron electron Odd-Electron Ionization Electron can (and does) come from anywhere. OH - - -1e (n) EI EI -1e () EI - 1e- () OH OH OH vacuum level * Likelihood of each of these * IEn IE IE depedependsnds oonn eenergynergy levelslevels in n molecular orbitals: most least likely likely Odd-Electron Ionization Electron can (and does) come from anywhere. OH - - -1e (n) EI EI -1e () EI - 1e- () OH OH OH Naturally, cannot distinguish these in mass spectrometer (all have m/z = 100). But fraggpmentation patterns will be different. Even-Electron Ionization CI, MALDI and ESI: Even-electron neutrals yield even-electron cations. + + M(EE) + H MH (EE) Like EI, ion iza tion may occur a t mu ltip le p laces. H OO + H+ + H+ H + H + H H OO OO Again, instrument cannot distinguish. Fragmentation Mechanisms in EI-MS Electron Ionization: Fragmentation is always unimolecular. Two possible categories of fragmentation: parents daughters + + M (OE) A (EE) + B (OE) charge migration (radical and charge part ways) + + M (OE) A (OE) + B(EE) charge retention (neutral molecule is ejected) Important: Only daughter ions are detected by MS instrument. Released neutrals are only inferred. Alkane Fragmentation in EI-MS: EI (electron can come from any bond) or + + + + Fragmentation (one-electron bond can break either way) + + + CH 3 + CH3 15 43 57 71 + + + CH CH 3 + 3 71 43 29 15 Alkane Fragmentation in EI-MS dance n lative Abu e R m/z + + + CH3 + CH3 15 43 57 71 + + + CH CH 3 + 3 71 43 29 15 Alkane Fragmentation in EI-MS What governs which ions are predominant? 1. Most ionizeable type of electrons. Here, all electron sources are 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. + + + CH3 + CH3 15 43 57 71 + + + CH CH 3 + 3 71 43 29 15 Alkane Fragmentation in EI-MS dance n lative Abu e R m/z + + + CH3 + CH3 15 43 57 71 + + + CH CH 3 + 3 71 43 29 15 Sequential Fragmentations + + + 57 Further fragmentation is + + common; driven primarily by entropy. 71 + + 43 Allylic Fragmentation EI 56 + + CH3 + 41 -Cleavage from Lone-Pair Ions Alcohols + + OH OH OH EI or m = 102 59 73 73 59 -Cleavage from Lone-Pair Ions Amines EI + 58 73.

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