Q7(c). Metastable ions in toluene; IR monitoring of an alkene isomerisation

Introduction. (i) Ions that fragment during their flight through the analyser, not in the ion source, give broad peaks at non-integral m/z. (ii) The isomerisation turns a 1,1-disubstituted alkene with a terminal =CH₂ into a symmetrical tetrasubstituted alkene, so the bands of =CH₂ disappear.

(i) Metastable ions

1. Definition. A metastable ion is an ion that survives the ion source and is accelerated as m₁⁺, but decomposes (m₁⁺→ m₂⁺ + neutral) in the drift region (no field) between the electric and magnetic sectors, after acceleration and before mass analysis. Its lifetime is of the order of microseconds.
2. Appearance. The daughter ion keeps the velocity of m₁ but only the mass m₂, so the magnet records it at an apparent mass m*=m₂²/m₁: a broad, weak, diffuse peak, usually at a non-integral m/z.
3. Use. A metastable peak proves that m₂ is formed directly from m₁ in one step.

Reading of the question: toluene's molecular ion is at m/z 92; the strong peak at 91 is the tropylium ion (M - H). The calculation uses 91 → 65 as printed.

m*=(m₂²)/(m₁)=((65)²)/(91)=(4225)/(91)

m* = 46.4

Figure: Sharp normal peaks and the broad metastable hump at 46.4 for 91 → 65 + 26.

1. The transition is C₇H₇⁺ → C₅H₅⁺ + C₂H₂: the tropylium ion loses acetylene (26 u). The metastable peak at 46.4 confirms that the cyclopentadienyl cation (65) comes directly from 91.

(ii) Following 2,3-dimethylbut-1-ene → 2,3-dimethylbut-2-ene by IR and Raman

Concentrated acid protonates the =CH₂ carbon (Markovnikov) to give a tertiary cation; loss of the tertiary C–H proton gives the more stable tetrasubstituted alkene (thermodynamic control).

1. Best marker. Record IR spectra of samples at intervals and follow the decrease of the strong =CH₂ wag at 880–900 cm⁻¹ (and of the 3080 and 1650 cm⁻¹ bands). When they have vanished, the conversion is complete.
2. Why the product shows almost nothing new in IR. 2,3-Dimethylbut-2-ene is centrosymmetric about its C=C. The C=C stretch causes no change in dipole moment, so it is IR inactive; it changes the polarisability strongly, so it is Raman active (rule of mutual exclusion). Raman spectroscopy shows the product's C=C band growing while the reactant's band falls.

Conclusion. (i) Metastable ions decompose in the drift region after acceleration and give broad peaks at m*=m₂²/m₁; for 91 → 65, m* = 46.4. (ii) Follow the disappearance of the reactant's =CH₂ bands (3080, about 1650 and 880–900 cm⁻¹) by IR, and the growth of the IR-inactive, Raman-active tetrasubstituted C=C stretch by Raman.

Application. Metastable peaks are used to map fragmentation pathways in structure work; in-line IR or Raman probes follow isomerisations and polymerisations in industrial reactors in real time.