Theoretical Study of Geometric Structures for Ground-state AlnC (n=2-7) Clusters

With the aid of the molecular orbital DMol(3) program, the energetics and electronic structures of several AlnC (n = 2-7) configurations have been searched and calculated by improved minimum energy paths (MEPs) by setting "imaging product". A new high symmetry, supervalence isomer of Al5C cluster, i.e., D5h-Al5C, at the local minimum in the MEPs is detected. Several parameters, such as binding energy, HOMO-LUMO energy gap, vertical electron detachment energy and electron affinity energy, are calculated to characterize and evaluate the stability of three Al5C configurations, i.e., D5h-Al5C, Cs-Al5C and Cl-Al5C. The results show that the D5h-Al5C cluster is the ground state structure instead of Cs-Al5C. Due to the formation of many central sigma bonds after polymerizing for D5h-Al5C, the decrease of the energy for HOMO orbit results in more territory for HOMO electrons of dislocation effect, then the energy difference between HOMO and LUMO is increasing to enhance the stability of molecules to produce such supervalence structure of Al5C cluster. The configuration evolution between D5h-Al5C, Cs-Al5C and Cl-Al5C and the synthesis preference in the mode of Al-5 + C -> Al5C reveals that the Cs-Al5C and Cl-Al5C configurations are permissive to coexist with D5h-Al5C structure in energetics.