MS Lecture Fragmentation

MS Lecture Fragmentation - MS Fragmentation Molecular ion...

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Unformatted text preview: MS Fragmentation Molecular ion must be capable of producing the important ions in the high-mass region of the spectrum by loss of logical neutral species C 5 H 10 O MW = 86 M+ M+ M+ 71 43 71 59 43 85 58 O OH O Structural Analysis and Fragmentation Data M MW and MF Other data NMR, IR, UV etc Possible structures Predict fragments computer Correct structure MS Fragmentation Simple bond cleavages and rearrangements Primary and secondary fragmentations Both charged and neutral fragments are formed A B [A B] + M e A + B observable Neutral fragments are not directly observed. M A loss of neutral fragment (B) What Influences Fragmentation? Bond strength (C-H > C-C > C-X) G # (free energy of activation) Stability of fragments C CH 3 H 3 C CH 3 CH 3 C CH 3 H 3 C CH 3 CH 3 + C CH 3 H 3 C CH 3 CH 3 + A B ? Stability of Cations and Radicals H H H H H H 18 28 37 E (kcal/mol) 3.5 3.5 H H H In gas phase, the difference in energy between ions are more important than neutral species. Dominant fragmentation pathways often produce stable cations. MS of Isooctane 99 (M-CH3) 57 ( t-butyl cation) H 3 C C CH 2 H C CH 3 CH 3 CH 3 CH 3 Stevensons Rule [AB] + A + B + Consider ionization potential (IE) for A and B. A A + + electron IP(A) B B + + electron IP(B) A A + + free electron IP (ionization energy) If IE(A) > IE(B), B will take the positive charge. MS of 2-Methylhexane Stability of Cations N F > N F Odd- and Even-electron Ions CH 3 O O CH 3 + OH + H 2 O O + CO odd odd odd odd (with some exceptions) even even even Br Br Br- Br- Br Common Fragment Ions Common Neutral Fragments...
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MS Lecture Fragmentation - MS Fragmentation Molecular ion...

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