Ligand Effects in Aluminum Cluster based Energetic Materials
[Technical Report, Doctoral Thesis]
Naval Postgraduate School
Pagination or Media Count:
This dissertation examines the electronic structure and thermochemistry of low-valent aluminum clusters that may serve as precursors for new energetic materials. Clusters such as Al50Cp12 CpC5Me5 have theoretical heats of combustion more than twice that of common high explosives and potentially faster combustion kinetics than bulk metals. The tetrameric aluminum cluster Al4Cp4 is a prototypical monovalent aluminum compound, and a potential precursor for these larger metalloid clusters. The synthesis of Al4R4 RC5Me4Pr, C5Me4iPr, two clusters similar to Al4Cp4, was recently reported and the effect of their increased steric bulk is discussed here. Experimental results and density functional theory DFT analysis show that these clusters are enthalpically more stable than the Cp variant, due primarily to non-covalent interactions NCIs across ligand groups. These NCIs show how ligand steric bulk can add stability to tetrameric clusters in addition to low-valent metal bonding. Similar calculations are performed on seven other homoleptic Cp-related clusters with varying levels of steric bulk. DFT results are used to predict monomertetramer equilibrium for all clusters and show trends counter to expectations regarding the role of ligand bulk. This equilibrium could be an important component in determining the viability of a cluster as a precursor for larger clusters.
- Ammunition and Explosives