Evidence of a double surface crossing between open- and closed-shell surfaces in the photodissociation of cyclopropyl iodide

Gas-phase photodissociations of cyclopropyl iodide were conducted at 266 and 279.7 nm, and the radical products were probed by multiphoton ionization, with imaging of the resulting ions and their corresponding electrons. Solution-phase photodissociations of cyclopropyl iodide were also conducted with TEMPO-trapping of the radical dissociation products. In both gas and solution phases, allyl radical was found to be a direct product of the cyclopropyl iodide photodissociation. CASSCF calculations indicate that the allyl radical could be formed directly from photoexcited cyclopropyl iodide by way of two surface crossings between open- and closed-shell potential energy surfaces. Each surface crossing represents a point Of potential bifurcation in the reaction dynamics. Thus, cyclopropyl iodide that is excited to a (1)(n,sigma*) state can remain on an open-shell surface and generate the cyclopropyl radical and an iodine atom or can cross to a closed-shell (ion-pair) surface. The cyclopropyl cation that results from the surface crossing Can undergo barrierless ring opening to the allyl cation before crossing back to an open-shell surface to generate allyl radical and an iodine atom. In this manner, both cyclopropyl radical and allyl radical can be formed as direct products of cyclopropyl iodide photodissociation.