Primary and Secondary Dissociation from Allyl Iodide Excited at 193 nm: Centrifugal Effects in the Unimolecular Dissociation of the Allyl Radical†

In the work presented here, we used photofragment translational spectroscopy and H atom Rydberg time-of-flight (HRTOF) spectroscopy to study the primary photofragmentation channels of allyl iodide excited at 193 nm and the ensuing dissociation of the nascent allyl radicals as a function of their internal energy. Two C-I bond fission channels were found to produce the allyl radical, one channel forming I(2P3/2) and the other forming I(2P1/2). The nascent allyl radicals are dispersed as a function of the translational energy imparted from the photolysis and therefore by their internal energy. Although all of the I(2P3/2) and a portion of the I(2P1/2) channel allyl radical products have enough internal energy to overcome the 60 kcal/mol barrier to form allene + H, the data showed that a substantial fraction of the allyl radicals from the I(2P1/2) channel that formed with internal energies as high as 15 kcal/mol above the 60 kcal/mol barrier were stable to H atom loss. The stability is due to centrifugal effects caused by significant rotational energy imparted to the allyl radical during photolysis and the small impact parameter and reduced mass characterizing the loss of an H atom from an allyl radical to form allene + H. A photoionization efficiency (PIE) curve identified the major C3H4 secondary dissociation products as allene. Comparison of the mass 40 signal in the TOF spectra at two photoionization energies showed that branching to H + propyne does not occur at near-threshold internal energies, indicating that the experimentally determined allyl f 2-propenyl radical isomerization barrier, which is lower than recent ab initio calculations of the barrier by 15 kcal/mol, is far too low