Electrocyclic Reactions of Gas-Phase 1-Methyl- and 3-Methylcyclobutene Radical Cations

The facile electrocyclic ring opening of gas-phase 1-methyl- and 3-methylcyclobutene radical cations has been demonstrated by using collisional activation decomposition spectroscopy and Fourier transform mass spectrometry (FTMS). The effect of internal excitation on the isomerization was investigated by employing both electron- and charge-exchange ionization to form the ions with various internal energies. Both the collisional ionization spectra and the bimolecular reactivities of the methyl-substituted cyclobutene ions change with increasing excitation to reflect the onset of ring opening. The variations of reactivity in structurally specific ion-molecule reactions were probed by utilizing a new experimental approach involving pulsed introduction of reactants into the cell of an FTMS. The results of these experiments permit an approximate assignment of the activation energies for isomerization of 1-methylcyclobutene and 3-methylcyclobutene radical cations to be less than 16 and 4 kcal mol⁻¹, respectively. The activation energies are considerably lower than those of the corresponding electrocyclic reactions of neutral molecules, but the effects of substitution parallel those found for the neutral cyclobutenes.