Mass spectra: molecular ion and isotope patterns
A mass spectrum gives the molecular mass and shows chlorine and bromine through their isotope pattern. The nitrogen rule tells you whether a formula can fit at all.
Rules and values
- M⁺ uses the most common isotopes: C 12, H 1, N 14, O 16, ³⁵Cl 35, ⁷⁹Br 79
- Chlorine: M+2 about a third of M⁺ (³⁵Cl : ³⁷Cl ≈ 3 : 1)
- Bromine: M⁺ and M+2 almost equally high (⁷⁹Br : ⁸¹Br ≈ 1 : 1)
- M+1 from ¹³C: about 1.1% per carbon
- Nitrogen rule: odd molecular mass ↔ odd number of N atoms
Your turn
Three problems on this topic. Type whole numbers, choose splitting and structures.
Task
The mass spectrum of a compound shows the molecular ion region below (intensity relative to M⁺). Which halogen does it contain?
Mass spectrum
Halogen
Mass M of the molecule (lightest isotopes)
Type whole numbers, e.g. 3. NMR shifts are typical values (±0.3 ppm), splitting follows the n+1 rule.
Worked example
The mass spectrum of a compound shows the molecular ion region below (intensity relative to M⁺). Which halogen does it contain?
Mass spectrum
Solution
- 1.Chlorine: ³⁵Cl and ³⁷Cl about 3 : 1 → M+2 is about a third as high as M⁺.
- 2.Bromine: ⁷⁹Br and ⁸¹Br about 1 : 1 → M⁺ and M+2 almost equally high.
- 3.M+1 (4%) comes from ¹³C: about 1.1% per C atom, here 4 C.
- 4.Here M+2 at 97% → bromine; M = 136.
- 5.For example 1-bromo-2-methylpropane (C₄H₉Br).
Typical mistake: Taking the M+2 peak for the molecular ion or reading the M+1 line (¹³C) as a halogen hint. Halogens show at M+2.
The molecular ion is at m/z 58; there is no M+2 peak (no Cl or Br). Which formula fits?
Mass spectrum
Solution
- 1.C₂H₄O₂: M = 60
- 2.C₃H₉N: M = 59
- 3.C₃H₆O: M = 58 ✓
- 4.C₃H₅N: M = 55
- 5.Even mass → no or an even number of N atoms (nitrogen rule).
Typical mistake: Thinking an odd M⁺ is a slip. An odd molecular mass is the sign of nitrogen.
Common questions
Is the tallest peak always the molecular ion?
No. The tallest peak (base peak) is often a stable fragment. The molecular ion is the peak with the highest mass, apart from the isotope peaks M+1 and M+2.