Structure of the Intermediate Formed in the Reaction of the Styrene Radical Cation and Neutral Styrene

The structure of the ion-molecule adduct produced in the gas-phase reaction of the styrene radical cation with neutral styrene has been probed by collisionally stabilizing the adduct and then acquiring its collision-activated decomposition (CAD) spectrum with a tandem mass spectrometer. The CAD spectrum of the adduct is nearly identical with the CAD spectra of the cis-and trans-diphenylcyclobutane radical cations and with the product ion resulting from a 1,4-regiospecific water elimination from the 1,4-diphenylbutan-l-ol radical cation; therefore the radical cations from all four precursors possess the same structure. The ΔH_f of this radical cation is shown to be ≤239 kcal/mol; therefore it cannot have the trans-l,2-diphenylcyclobutane structure (ΔH_f = 247 kcal/mol). The results support a two-step mechanism for the [1 + 2] cycloaddition reaction.