Structure, thermodynamics, and energy content of aluminum-cyclopentadienyl clusters

We present quantum chemistry simulations of aluminum clusters surrounded by a surface layer of cyclopentadiene-type ligands to evaluate the potential of such complexes as novel fuels or energetic materials. Density functional theory simulations are used to examine the aluminum-ligand bonding and its variation as the size of the aluminum cluster increases. The organometallic bond at the surface layer arises mainly from ligand charge donation into the Al p orbitals balanced with repulsive polarization effects. Functionalization of the ligand and changes in Al cluster size are found to alter the relative balance of these effects, but the surface organometallic bond generally remains stronger than Al-Al bonds elsewhere in the cluster. In large clusters, such as the experimentally observed Al(50)Cp(12)*, this suggests that unimolecular thermal decomposition likely proceeds through loss of surface AlCp* units, exposing the strained interior aluminum core. The calculated heats of combustion per unit volume for these systems are high, approaching 60% that of pure aluminum. We discuss the possibility of using organometallic aluminum clusters as a means of achieving rapid combustion in propellants and fuels.