Photodissociation of allyl-d[sub 2] iodide excited at 193 nm: Stability of highly rotationally excited H[sub 2]CDCH[sub 2] radicals to C–D fission.

The photodissociation of allyl-d[sub 2] iodide (H[sub 2]C=CDCH[sub 2]I) and the dynamics of the nascent allyl-d[sub 2] radical (H[sub 2]CCDCH[sub 2]) 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[sub 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, |Lvector |=μ|vvector [sub rel]|b. The integrated signal taken at m/e=40 (allene) and m/e=41 (allene-d[sub 1] and propyne-d[sub 3]) shows a minor fraction of the allyl-d[sub 2] radicals isomerize to the 2-propenyl radical, in qualitative support of earlier conclusions of the domination of direct allene+H formation over isomerization. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]