Photodissociation of allyl-d2 iodide excited at 193 nm: Stability of highly rotationally excited H2CDCH2 radicals to C–D fission

The photodissociation of allyl-d2 iodide (H2C5CDCH2I) and the dynamics of the nascent allyl-d2 radical (H2CCDCH2) were studied using photofragment translational spectroscopy. A previous study found the allyl radical stable at internal energies up to 15 kcal/mol higher than the 60 kcal/mol barrier to allene1H formation as the result of a centrifugal barrier. The deuterated allyl radical should then also show a stability to secondary dissociation at internal energies well above the barrier due to centrifugal effects. A comparison in this paper shows the allyl-d2 radical is stable to allene1D formation at energies of 2–3 kcal/mol higher than that of the nondeuterated allyl radical following photolysis of allyl iodide at 193 nm. This is most likely a result of a combination of the slight raising of the barrier from the difference in zero-point levels and a reduction of the impact parameter of the dissociative fragments due to the decrease in frequency of the C–D bending modes, and therefore allene1D product orbital angular momentum, uLW u5muvWrelub . The integrated signal taken at m/e540 ~allene! and m/e541 (allene-d1 and propyne-d3) shows a minor fraction of the allyl-d2 radicals isomerize to the 2-propenyl radical, in qualitative support of earlier conclusions of the domination of direct allene1H formation over isomerization. © 2003 American Institute of Physics. @DOI: 10.1063/1.1596853#