The Dynamics of Allyl Radical Dissociation

Dissociation of the allyl radical, CH2CHCH2, and its deuterated isotopolog, CH2CDCH2, have been investigated using trajectory calculations on an ab initio ground-state potential energy surface calculated for 97 418 geometries at the coupled cluster single and double and perturbative treatment of triple excitations, with the augmented correlation consistent triple-zeta basis set level (CCSD(T)/AVTZ). At an excitation energy of 115 kcal/mol, corresponding to optical excitation at 248 nm, the primary channel is hydrogen loss with a quantum yield of 0.94 to give either allene or propyne in a ratio of 6.4:1. The total dissociation rate for CH2CHCH2 is 6.3 x 10(10) s(-1), corresponding to a 1/e time of 16 ps. Methyl and C2H2 are produced with a quantum yield of 0.06 by three different mechanisms: a 1,3 hydrogen shift followed by C-C cleavage to give methyl and acetylene, a double 1,2 shift followed by C-C cleavage to give methyl and acetylene, or a single 1,2 hydrogen shift followed by C-C cleavage to give methyl and vinylidene. In this last channel, the vinylidene eventually isomerizes to give internally excited acetylene, and the kinetic energy distribution is peaked at much lower energy (6.4 kcal/mol) than that for the other two channels (18 kcal/mol). The trajectory results also predict the v-J correlation, the anisotropy of dissociation, and distributions for the angular momentum of the fragments. The v-J correlation for the CH3 + HCCH channel is strongest for high rotational levels of acetylene, where v is perpendicular to J. Methyl elimination is anisotropic, with beta = 0.66, whereas hydrogen elimination is nearly isotropic. In the hydrogen elimination channel, allene is rotationally excited with a total angular momentum distribution peaked near J = 17. In the methyl elimination channel, the peak of the methyl rotational distribution is at J approximate to 12, whereas the peak of the acetylene rotational distribution is at J approximate to 28.