Centrifugal effects in the dissociation dynamics of allyl iodide and rotationally excited allyl radicals.

摘要

The dynamics of hot allyl radicals in their electronic ground state were studied through the photodissociation of allyl iodide near 200 nm. Two major channels were found: C-I bond fission producing ground state I (2P _3/2) products, and C-I bond fission producing spin-orbit excited I ( 2P_1/2) products. The dynamics of the nascent allyl radicals were studied as a function of internal energy using three different experimental techniques. The first technique, photofragment translational spectroscopy, was performed on two different apparatuses: one using photoionization and the other using electron impact ionization. A photoionization efficiency (PIE) curve determined that the major dissociation product of the nascent allyl radicals is allene. I (2P_1/2) products were selectively ionized, enabling the determination of the P(E_T) for the I (2P_1/2) formation channel. The nascent allyl radicals from this channel were found to be stable at internal energies much higher than the 60 kcal/mol barrier to allene + H formation. This was attributed to centrifugal effects produced through the significant partitioning of rotational energy to the allyl radical during the primary photolysis step. Photofragment translational spectroscopy using allyl-d₂ iodide confirmed the dominance of allene formation, although possible propyne contribution was seen. The allyl-d₂ radicals were also found to be stable at energies 2–3 kcal/mol higher than non-deuterated allyl radicals. This is most likely due to isotope effects and a lower impact parameter range available for the allyl-d₂ radical → allene + D dissociation.; The second technique used, H-atom Rydberg time-of-flight spectroscopy (HRTOF) characterized the allyl radical → allene + H channel as well as a primary H loss channel. Significant translational energy was found to be imparted in the allene + H channel.; Finally, velocity map imaging was used to characterize the I ( 2P_3/2) and I (2P_1/2) formation channels individually. Good agreement was found between the P(ET)s calculated from the images with those derived from the forward convolution fitting to photofragment translational spectroscopy spectra. The hypothesis that I ( 2P_3/2) formation is the major channel was confirmed, although it is not possible as of yet to determine a branching ratio.