Ab initio classical trajectory study of the fragmentation of C3H4 dications on the singlet and triplet surfaces.

Ab initio molecular dynamics have been used to examine the fragmentation of allene, propyne, and cyclopropene dications on the singlet and triplet potential energy surfaces. Accurate energies and barrier heights were computed at the CBS-APNO level of theory. Classical trajectories were calculated at the B3LYP/6-31G(d,p) level of theory. To simulate vertical double ionizations of allene, propyne, and cyclopropene by short, intense laser pulses, the trajectories were started from the corresponding neutral geometries and were given ca. 240 kcal/mol excess kinetic energy; 200 trajectories were integrated for each case. Approximately one-third of the trajectories underwent extensive rearrangement before dissociation. Proton dissociation is the dominant pathway for all six cases, accounting for 50-75% of the trajectories. H(2)/H(2)(+) is produced in ca. 20% of the trajectories on the singlet propyne and cyclopropene dication surface. The calculated ratio of CH(+)/CH(2)(+)/CH(3)(+) compares favorably to that obtained in laser-induced Coulomb explosion of allene. The yield of CH(2)(+) is ca. 12% on the singlet and triplet allene dication surfaces compared to 6% or less in the other cases, whereas CH(+) is favored on the singlet propyne dication surface (12% vs 0-8% on the other surfaces). CH(3)(+) is formed primarily by direct dissociation on the triplet propyne dication surface (5% vs <1% on the other surfaces). The small amount of H(3)(+) seen experimentally for allene indicates that rearrangement can occur before dissociation. The dynamics simulations confirm that extensive isomerization occurs, even though the initial kinetic energy was too high to yield H(3)(+).