Overview
This lecture explains the significance of the M+2 peak in mass spectrometry, its relationship to isotopes (especially chlorine and bromine), and demonstrates how to identify and interpret such peaks to deduce chemical structures.
Mass Spectrometry & Isotopes
- The M+2 peak appears two mass-to-charge (m/z) units higher than the molecular ion peak in a mass spectrum.
- The M+2 peak is due to the presence of atoms with isotopes differing in mass by two units, primarily chlorine and bromine.
- Isotopes are atoms of the same element with different numbers of neutrons (and thus different mass numbers), but the same number of protons.
Chlorine and Bromine Isotopes
- Chlorine has two main isotopes: ^35Cl (75% abundance) and ^37Cl (25% abundance).
- The presence of chlorine yields an M+2 peak roughly one-third the intensity of the M peak.
- Bromine has two main isotopes: ^79Br (51% abundance) and ^81Br (49% abundance).
- The presence of bromine results in M and M+2 peaks of nearly equal intensity.
Example: Chloroethane & Bromoethane
- In chloroethane, the M peak arises from ^35Cl, and the M+2 peak from ^37Cl.
- The intensity ratio of M to M+2 in chloroethane reflects the isotope ratio (3:1).
- In bromoethane, M and M+2 peaks correspond to ^79Br and ^81Br, with nearly equal intensities reflecting their similar abundances.
Structural Deduction Using Spectra
- If two parent ion peaks are seen and differ by two m/z units, the compound likely contains Cl or Br.
- Infrared spectroscopy can reveal functional groups (e.g., carbonyl at 1680–2000 cm⁻¹, broad O–H for carboxylic acid).
- The mass difference between characteristic peaks can help identify molecular fragments and potential structures.
- Comparing the calculated and observed peak masses helps determine the halogen present.
- The presence of a methyl ion (m/z 15) in fragmentation can distinguish isomers.
Key Terms & Definitions
- M+2 peak — A mass spectrum peak two units higher than the molecular ion, caused by isotopic variants.
- Isotope — Atoms of the same element with the same proton number but different neutron numbers.
- Molecular ion peak (M peak) — The peak representing the intact molecule’s mass in a mass spectrum.
- Relative abundance — The percentage occurrence of each isotope in nature.
Action Items / Next Steps
- Practice analyzing mass spectra for M+2 peaks and deducing possible halogen content.
- Review isotope abundance data for common elements.
- Complete any assigned problem sets on interpreting infrared and mass spectra.