Endohedral and Exohedral Complexes of Polyhedral Oligomeric Silsesquioxanes (POSS): Endohedral Clusters of Si12 : a Theoretical Study
Pittman, Charles U., Jr
CommitteeMonts, David L.
Two distinct research projects were carried out in this dissertation. In the first project the results of first principle calculations on endo- and exohedral complexes of polyhedral oligomeric silsesqiuoxanes (POSS) with atomic and ionic species were carried out. Detailed studies were performed on structures, stabilities and electronic properties of these complexes. The stabilities of the endohedral Tn-POSS ( n = 8, 10 and 12) complexes depends on both the cage size and the nature of the endohedral species. Alkali metal ion encapsulation leads to cage contraction. Electron density was transferred from the cage to the alkali metal cations. Halide encapsulation caused the cages to expand. Electron density was transferred from the halides to the cage. Noble gas encapsulation has minimum effect on the cage structure. Electron transfer between cage framework atoms and He and Ne were negligible. However, a small amount of electron transfer between Ar and POSS cages occurred. Ionization potentials calculated for T10-POSS and T12-POSS endohedral complexes with alkali metals indicate that these complexes have "superalkali" behavior. Several transition metal encapsulations into the T8-POSS cage gave thermodynamically stable endohedral complexes. The HOMO-LUMO gaps for the transition metal endohedral complexes were reduced versus that of pure cage. In almost all cases, the exohedral Tn (n = 8, 10, 12) complexes were energetically more stable than their corresponding endohedral counterparts except for the complex with F-. The exohedral Fpenetrates directly into the Tn-POSS cage forming an endohedral complex. In the second project ab initio electronic structure calculations based on density functional theory were performed to study small silicon clusters containing an endohedral atoms or ions. The formation of endohedral clusters M@Si12 (Li0,1,-1, Na0,1,-1, K+, He, F- and Cl-) depended on the Si12 cage structure and the nature of the embedding species. Only Li0,1,-1, Na0,1,-1 and He form endohedral clusters with different Si12 cage isomers. All observed endohedral clusters are stable and have large HUMO-LUMO gaps (>1eV). The endohedral clusters Li-@Si12 and Na-@Si12 are thermodynamically more stable than their neutral and cationic counterparts. The stability order predicted for the alkali metal series was anionic clusters > neutral clusters> cationic clusters. Encapsulations of halides are completely unfavorable and halide insertions cause the Si12 cage rupture. Encapsulation of two Li atoms into the Si18 cage generates the endohedral Li2@Si18 complex. Encapsulating Na atoms into Si18 cage leads to an exohedral Na2Si18 cluster. Endohedral Si20@Li20 was also investigated and characterized.