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Molecular Ion Peak and Fragmentation Pattern Explained

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In a mass spectrum, the molecular ion peak (M+.) is the signal from the intact molecule that has lost a single electron during ionisation, and its mass-to-charge ratio (m/z) gives the molecule's relative molecular mass directly. In electron ionisation (EI, 70 eV), a high-energy electron knocks one electron out of the neutral molecule, forming a radical cation, the molecular ion, which may then break into smaller fragment ions.

The molecular mass is the single most powerful clue in structure determination: with the fragmentation pattern and isotope peaks, it often nails the molecular formula. Every organic chemist reads mass spectra, and questions on M+., M+1, M+2 and fragmentation are staples of JEE, GATE and CSIR-NET.

Key Concepts

  • Formation: M + e-(70 eV) gives M+. + 2e-; the molecular ion is an odd-electron (radical) cation.
  • Even-electron rule: odd-electron ions fragment into even-electron ions plus neutral radicals.
  • Nitrogen rule: an even nominal mass means an even number of nitrogens (or zero); an odd mass means an odd number.
  • M+1 peak: mostly 13C (1.1% abundance); intensity is about 1.1% times the carbon count, a quick carbon count.
  • M+2 peak: reveals chlorine (35Cl:37Cl ~3:1, M+2 about one-third of M) and bromine (79Br:81Br ~1:1, M+2 about equal to M).
  • Fragmentation modes: alpha-cleavage (bond next to a heteroatom or carbonyl breaks), McLafferty rearrangement (gamma-hydrogen transfer to a carbonyl), and the base peak (tallest peak, most stable fragment).

How It Works

  1. Find the molecular ion: the highest-m/z significant peak; check it makes chemical sense.
  2. Apply the nitrogen rule for the parity of the nitrogen count.
  3. Measure M+1 relative to M: %M+1 is about 1.1 times the carbon count; solve for it.
  4. Check M+2: ~33% of M suggests one chlorine; ~100% suggests one bromine.
  5. Propose a molecular formula matching M, the carbon count and the nitrogen rule.
  6. Explain the major fragments: alpha-cleavage, loss of small neutrals (CH3. = 15, H2O = 18, CO = 28), McLafferty rearrangement.

Worked Example

An unknown shows M+. at m/z 86 with M+1 at 5.5% of M and no significant M+2; major fragments at m/z 71, 58 and 43.

  1. Nitrogen rule: 86 is even, so zero or an even number of nitrogens; assume zero.
  2. Carbon count: n_C = 5.5/1.1 = 5 (5 x 12 = 60 mass units).
  3. Remaining mass = 86 - 60 = 26; one oxygen (16) leaves 10 hydrogens. Formula: C5H10O.
  4. Fragments: 86 - 15 = 71 (loss of CH3., alpha-cleavage); m/z 43 = CH3CO+ (acylium ion), consistent with a methyl ketone like pentan-2-one (MW 86).
  5. The McLafferty rearrangement of pentan-2-one explains m/z 58. The data converge on C5H10O, a methyl ketone.

Common Misconceptions

  • The tallest peak is the molecular ion. The tallest is the base peak (most stable fragment); the molecular ion is often small or absent in fragile molecules like alcohols.
  • M+1 is one heavier isotope of the molecule. It sums over all atoms: each carbon contributes ~1.1%, so a C20 compound shows ~22% M+1 though each molecule carries at most one 13C.
  • Fragmentation is random. The charge and radical site direct cleavage by strict rules, which is why spectra are reproducible.

Real-World Applications

  • Forensics: identifying drugs, poisons and explosives by library matching.
  • Environment: detecting pesticides and pollutants at trace levels (GC-MS).
  • Pharma: impurity profiling and product confirmation.
  • Proteomics: peptide sequencing by tandem MS.

The molecular ion gives the mass, the isotopes count carbons and halogens, and the fragments sketch the skeleton. Read a spectrum in that order, M+., M+1, M+2, fragments, and the molecular formula usually falls out in minutes.

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