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

The photodissociation of allyl-d(2) iodide (H2C=CDCH2I) and the dynamics of the nascent allyl-d(2) 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 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 comparison in this paper shows the allyl-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 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 allene+D product orbital angular momentum, \(L) over right arrow\=mu\(v) over right arrow (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 allyl-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. (C) 2003 American Institute of Physics.