Untangling the chemical dynamics of the reaction of boron atoms, 11B(2Pj), with diacetylene, C4H2(X1Σg+)--a crossed molecular beams and ab initio study.

A crossed molecular beams experiment with ground state boron atoms, B((2)P(j)), and diacetylene, C(4)H(2)(X(1)Σ(g)(+)), was conducted at a collision energy of 21.1 ± 0.3 kJ mol(-1) under single collision conditions and combined with electronic structure calculations on the (11)BC(4)H(2) potential energy surface. Our combined experimental and computational studies indicate that the reaction proceeds without entrance barrier and involves indirect scattering dynamics. Three initial collision complexes, in which the boron atom adds to one or two carbon atoms, were characterized computationally. These intermediates rearranged via hydrogen shifts and/or successive ring-opening/ring closure processes on the doublet surface ultimately yielding a cyclic, C(s) symmetric (11)BC(4)H(2) intermediate. The latter was found to decompose via atomic hydrogen loss to yield a cyclic (11)BC(4)H(X(1)A') isomer; to a minor amount, the cyclic intermediate isomerized via ring-opening to the linear HCCBCCH(X(2)Σ(g)(+)) molecule, which in turn emitted a hydrogen atom to yield the linear HCCBCC(X(1)Σ(+)) molecule. The overall reactions to form these isomers were found to be exoergic by 55 and 61 J mol(-1), respectively, and involved rather loose exit transition states. On the basis of the energetics, upper limits of two energetically less stable species, the linear HBCCCC(X(1)Σ(+)) and BCCCCH(X(1)Σ(+)) species, were derived to be 12 and 2.2%, respectively. The dynamics of this reaction are also compared with the reaction of ground state boron atoms with acetylene studied earlier in our group.