Electronic Principles Governing the Stability and Reactivity of Ligated Metal and Silicon Encapsulated Transition Metal Clusters

Author

Marissa B. Abreu, Virginia Commonwealth UniversityFollow

DOI

https://doi.org/10.25772/G97J-YV62

Defense Date

2015

Document Type

Dissertation

Degree Name

Doctor of Philosophy

Department

Chemical Physics

First Advisor

Dr. Shiv N. Khanna

Second Advisor

Dr. B. Frank Gupton

Third Advisor

Dr. Joseph E. Reiner

Abstract

A thorough understanding of the underlying electronic principles guiding the stability and reactivity of clusters has direct implications for the identification of stable clusters for incorporation into clusters-assembled materials with tunable properties. This work explores the electronic principles governing the stability and reactivity of two types of clusters: ligated metal clusters and silicon encapsulated transition metal clusters. In the first case, the reactivity of iodine-protected aluminum clusters, Al₁₃I_x^- (x=0-4) and Al₁₄I_y^- (y-0-5), with the protic species methanol was studied. The symmetrical ground states of Al₁₃I_x^- showed no reactivity with methanol but reactivity was achieved in a higher energy isomer of Al₁₃I₂^- with iodines on adjacent aluminum atoms – complementary Lewis acid-base active sites were induced on the opposite side of the cluster capable of breaking the O-H bond in methanol. Al₁₄I_y^- (y=2-5) react with methanol, but only at the ligated adatom site. Reaction of methanol with Al₁₄^- and Al₁₄I^- showed that ligation of the adatom was necessary for the reaction to occur there – revealing the concept of a ligand-activated adatom. In the second case, the study focused heavily on CrSi₁₂, a silicon encapsulated transition metal cluster whose stability and the reason for that stability has been debated heavily in the literature. Calculations of the energetic properties of CrSi_n (n=6-16) revealed both CrSi₁₂ and CrSi₁₄ to have enhanced stability relative to other clusters; however CrSi₁₂ lacks all the traditional markers of a magic cluster. Molecular orbital analysis of each of these clusters showed the CNFEG model to be inadequate in describing their stability. Because the 3d_z2 orbital of Cr is unfilled in CrSi₁₂, this cluster has only 16 effective valence electrons, meaning that the 18-electron rule is not applicable. The moderate stability of CrSi₁₂ can be accounted for by the crystal-field splitting of the 3d orbitals, which pushes the 3d_z2 orbital up in energy. CrSi₁₄, on the other hand, has 18 effective valence electrons on Cr, minimal 3d-orbital splitting, and does follow the 18-electron rule. A repetition of these calculations with WSi_n (n=6-16) showed similar results, except WSi₁₂ shows all the markers of a magic cluster, due to the greater crystal-field splitting of 5d orbitals.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

5-4-2015