Photodissociation of allyl-d_2 iodide excited at 193 nm: Stability of highly rotationally excited H_2CDCH_2 radicals to C-D fission

摘要

The photodissociation of ally-d_2 iodide (H_2C=CDCH_2I) and the dynamics of the nascent ally-d_2 radical (H_2CCCDCH_2) were studied using photofragment translational specctroscopy. A previous study found the allyl radical stable at internal energies up to 15 kcal/mol higher than the 60 kcal/mol barrier to allene+H 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 comparision in this paper shows the ally-d_2 radical is stable to allene+D 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 llevels and a reduction of the impact parameter of the dissociative fragments due to the decrease in frequencey of the C-D bending modes, and therefore allene+D product orbital angular momentum, |L|=mu|V_rel|b. The integrated signal taken at m/e=40(allene) and m/e=41(allene-d_1 and propyne-d_3) shows a minor fraction of the ally-d_2 radicals isomerize to the 2-propenyl radical, in qualitative support of earlier conclusions of the domination of direct allene+H formation over isomerization.