Theoretical study of isomerism, structure, and stability of dimers of C-doped aluminide clusters (C@Al12)2 and (C@Al12)(L@Al12) (L = Si, Ge)

The geometric, energetic, and vibrational characteristics of the dimers of carbon-doped aluminum clusters (C@Al₁₂)₂, (C@Al₁₂)(Si@Al₁₂), and (C@Al₁₂)(Ge@Al₁₂) have been calculated by ab initio density functional theory B3LYP method with the basis sets 6-31G* and 6-311+G*. As distinct from the silicon and germanium analogues (Si@Al₁₂)₂ and (Ge@Al₁₂)₂ with an AA lowest-lying structure, in which the Si@Al₁₂ and Ge@Al₁₂ blocks retain the shape of distorted icosahedra A, the symmetric AA structure is not typical of the (C@Al₁₂)₂ dimer (itcorresponds to a transition state) and the most favorable structures are two closely spaced BB and AD isomers, in which the C@Al₁₂ blocks have either a distorted icosahedral (A) or nonicosahedral geometry (B, a dicapped ten-vertex polyhedron; D, a marquee with a centered hexagonal base). The calculated energy of dissociation of the most favorable isomer AD into isolated C@Al₁₂ (I _h ) monomers is ～45 kcal/mol, which is 1.5–2 times as large as the corresponding dissociation energy of (Si@Al₁₂)₂ and (Ge@Al₁₂)₂. For mixed dimers of the (C@Al₁₂)(L@Al₁₂) type with L = Si or Ge, three types of low-lying isomers (AA, BA, and DA) are predicted in which the Si@Al₁₂ and Ge@Al₁₂ blocks retain their shape A and the C@Al₁₂ blocks have geometry A, B, and D, respectively. For all the isomers, the block-block bonds are comparable in length and energy to the bonds inside the blocks and have the same chemical nature. Specific features of the geometry, relative stability, ionization potentials, electron affinity, vibrational spectra, and other characteristics of doped aluminide dimers, which could be used for the identification of their isomers by spectroscopic methods, are discussed.