Ab Initio Study on the Potential Energy Surfaces of B 4 Cluster

The singlet and triplet potential energy surfaces (PES) for the isomerization and dissociation reactions of B 4 isomers have been investigated using ab initio methods. Ten B4 isomers have been identified and of these 10 species, 4 have not been reported previously. The singlet rhombic structure 1 1 is found to be the most stable on the B 4 surface, in agreement with the results of previous reports. Several isomerization and dissociation pathways have been found. On the singlet PES, the linear 1 3b can rearrange to rhombus 1 1 directly, while 1 3c rearranges to 1 1 through two‐step reactions involving a cyclic intermediate. On the triplet PES, the capped triangle structure 3 2 undergoes ring opening to the linear isomer 3 3b with a barrier of 34.8 kcal/mol and 44.9 kcal/mol, and the latter undergoes ring closure to the square structure 3 1 with a barrier of 30.4 kcal/mol and 33.0 kcal/mol at the MP4/6–311+G(3df)//MP2/6–311G(d) and CCSD/aug‐cc‐pVTZ//MP2/6–311G(d) levels of theory, respectively. The direct decomposition of singlet B 4 yielding to B 3 +B is shown to have a large endothermicity of 87.3 kcal/mol (CCSD), and that producing 2B 2 to have activation energy of 133.4 kcal/mol (CCSD).