1 / 22

Mass Spectrometry

Mass Spectrometry. Reading EI Spectra: Recognition of M + Deduction of Formula (3) Fragmentation of M +. Recognition of Molecular Ion. Is h ighest m/z value even? If odd then what? Do highest m/z fragments make sense? M-15 present? M-18? M-31? M-29? M-3  M-14 are impossible

shelby-fry
Télécharger la présentation

Mass Spectrometry

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Mass Spectrometry Reading EI Spectra: Recognition of M+ Deduction of Formula (3) Fragmentation of M+

  2. Recognition of Molecular Ion • Is highest m/z value even? • If odd then what? • Do highest m/z fragments make sense? • M-15 present? M-18? M-31? M-29? • M-3  M-14 are impossible • Even if 1 and 2 are true the highest m/z might not be M+

  3. Nitrogen Rule

  4. Spectrum of an Amine

  5. Spectrum of an Alcohol

  6. M+ present?

  7. Deducing molecular formula • Use %m+1 to get # of Cs • Use Nitrogen rule • M-18 peak indicates oxygen • Doublet M+ peaks indicate Cl (3:1) and Br (1:1) • Peak at 127 = I • Peak at 77 is phenyl group (C6H5) • M+2 peak = 4-5% indicates S • Deduce molecular formula!

  8. Molecular Formula?

  9. Molecular Formula?

  10. Molecular Formula? m/z %abund • 20.2 • 1.2

  11. Index of Hydrogen Deficiency • Units of Unsaturation = r + db • CnHmXxNyOz r + db = n – m/2 – x/2+ y/2 + 1

  12. Fragmentation of M+ • Draw structure of M+ if possible and then draw possible heterolytic cleavages (curved arrows) or homolytic cleavages (fish hooks) • Alternatively, show homolytic or heterolytic cleavage cleavage of the molecule and then remove an electron from one of the fragments. • Fragmentation to give neutral molecule and a radical cation must involve some kind of rearrangement.

  13. General Rules Probablity of bond cleavage depends on stability of fragments produced. (3º > 2º > 1º > CH3) (oxonium > carbocation) (allylic, benzylic > no resonance) • Increase branching  decrease M+ • Add carbons  decrease M+ • Cleave more likely at higher º C • Unsubst cyclic have strong M+ (especially aromatic) • Alkenes cleave to allylic species • R on cycloalkanes α cleave. Cyclohexenes do retro Diels-Alder. • R on aromatic rings β-cleave to benzylic • Bonds β to heteroatoms cleave • Loss of small stable molecules, H2O, NH3, C2H4, HCN, CO, H2S,etc often occur

  14. Alkanes • Straight chain alkanes – homologous series of ions – smooth curve max at 57 or 71. M-15 absent. “picket fence” • Discontinuity = branching. Absence of homolog  no single cleavage possible • Cycloalkanes. Larger M+. More even mass fragments (M-C2H4)

  15. Alkenes • Cleave next to double bond to form allylic fragments. (Double bonds can shift in M+.) • Homologous series is -2 (27, 41, 55, 69..) • Cyclohexenes do retro Diels Alder – give even mass fragment (M-C2H4, M-C3H6 etc.)

  16. Aromatics • No branching = strong M+. Often see M++. • m/z 77, 51 = phenyl (monosubst benzene) • Most prominent peaks are due to beta cleavage of branches to benzylic (really tropylium.)

More Related