Structural Evolution of Carbon-Doped Aluminum Clusters AlnC– (n = 6–15): Anion Photoelectron Spectroscopy and Theoretical Calculations

Carbon-doped aluminum cluster anions, AlnC⁻ (n = 6-15), were generated by laser vaporization and investigated by mass-selected anion photoelectron spectroscopy. The geometric structures of AlnC⁻ (n = 6-15) anions were determined by the comparison of theoretical calculations with the experimental results. It is found that the most stable structure of Al₆C⁻ is a carbon endohedral triangular prism. The Al₇C⁻ anion is a magic cluster with high stability. The structures of Al₇₋₉C⁻ can be viewed as the additional aluminum atoms attached around the triangular prism Al₆C⁻. Two isomers of Al₁₀C⁻ have been detected in the experiments. The most stable one has a planar tetracoordinate carbon structure. The second one derives from Al₉C⁻ with the carbon atom located in a pentagonal bipyramid. The Al₁₁C⁻ anion has a bilayer structure composed of one planar tetracoordinate carbon and one aluminum-centered hexagon, in which the major interactions between two layers are multicenter bonds. The structures of Al₁₂₋₁₄C⁻ can be viewed as evolving from Al₁₁C⁻ by adding aluminum atoms to interact with the carbon atom. In Al₁₅C⁻, the carbon atom stays at the surface with a tetracoordinate structure, and an icosahedral Al₁₃ unit can be identified as a part of the geometric structure of Al₁₅C⁻.