DESIGN AND CHARACTERIZATION OF POST-TRANSITION, MAIN-GROUP, HETEROATOMIC CLUSTERS USING MASS SPECTROMETRY, ANION PHOTOELECTRON SPECTROSCOPY AND VELOCITY MAP IMAGING

Open Access
Author:
Gupta, Ujjwal
Graduate Program:
Chemistry
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
November 04, 2009
Committee Members:
  • Albert Welford Castleman Jr., Dissertation Advisor
  • Albert Welford Castleman Jr., Committee Chair
  • Mark Maroncelli, Committee Member
  • John V Badding, Committee Member
  • Nitin Samarth, Committee Member
Keywords:
  • Photoelectron spectroscopy metal clusters velocity
Abstract:
Clusters are group of atoms combined together by covalent or van der Waals interactions, and are usually restricted to the sub-nanometer to nanometer range, where the number of atoms are defined accurately. Properties of clusters can be altered by changing the number of atoms or their composition or geometry. This presents a unique opportunity to create cluster assembled materials, in which bulk properties can be tuned based on the inherent property of the cluster building blocks. The main focus of the research work presented in this thesis is to use experimental observations to identify clusters that are stable and can be used as building blocks, and determine why they are stable. Once the cluster stability has been rationalized, the results are confirmed using first principles theoretical calculations performed by research collaborators at Virginia Commonwealth University. The clusters are synthesized using a laser vaporization cluster source, separated based on their mass/charge ratio using the time-of-flight mass spectrometer and characterized using the magnetic-bottle photoelectron spectrometer and a velocity-map photoelectron imaging spectrometer. It is illustrated, in a synergistic approach, that by using bismuth as a dopant to tin clusters, Zintl clusters with both aromatic and antiaromatic character can be designed. In all clusters, it is found that the aromatic character originates from the combination of the s-electrons, while the antiaromatic character evolves from the interaction of the p-electrons from the bismuth and tin atoms. The singly doped tin cluster Sn4Bi- is stable and aromatic in character with an electronic structure that mimics the Zintl cluster Sn52-. The agreement between vertical and adiabatic detachment energy from the negative ion photodetachment spectra and theoretical calculations confirm the ground state structures of the clusters. Studies of nine atom bismuth doped tin clusters established that highly charged Zintl ions, observed in the condensed phase, can be stabilized as isolated gas phase clusters through atomic substitution that preserves the overall electron count but reduces the net charge and thereby avoids instability due to Coulomb repulsion. Mass spectrometry studies reveal that Sn8Bi-, Sn7Bi2- and Sn6Bi3- exhibit higher abundances than neighboring species and photoelectron spectra show that all of these heteroatomic gas phase Zintl analogues (GPZAs) have high adiabatic electron detachment energies. Sn6Bi3- is found to be a particularly stable cluster, having a large HOMO-LUMO gap. Theoretical calculations demonstrate that the Sn6Bi3- cluster is isoelectronic with the well known Sn9-4 Zintl ion, however the fluxionality reported for Sn9-4 is suppressed by substituting Sn atoms with Bi atoms. Thus, while the electronic stability of the clusters is dominated by electron count, the size and position of the atoms affects the dynamics of the cluster as well. In another study, evidence is presented that the HOMO-LUMO gap can be tuned (1.12 eV-1.89 eV) by changing the Ga composition of Bi3Gay neutral and anionic clusters, some of which display special stability. The Bi3Ga2- cluster is very stable with a large calculated HOMO-LUMO gap of 1.89 eV, and can be viewed as a gas phase Zintl analogue of Sn52-, already synthesized in the solution phase. The stability of Bi3Ga2- is further attributed to the fact that it has 12 valence electrons and possesses a closo structure in agreement with Wade’s rules. It is also shown that In2Bi- is a triatomic cluster anion with a bent structure, where bismuth is the bridging atom between the two indium atoms. It is found that 6 p-electrons (2 π and 4σ) are delocalized over the cluster. The In-Bi bond lengths in In2Bi- are identical to bond lengths of In3Bi, which is an all-metal aromatic analogue to Al3Bi, and are shorter than the rest of InxBiy- clusters, showing the presence of a higher bond order. Therefore, the concept of delocalization of electrons in a chain structure can be applied to an all-metal system. The electronic and structural properties of neutral and negatively charged BixIny (x – 1-4, y = 1-6) clusters are also presented. The experimental and theoretical adiabatic and vertical detachment energies of the anionic species are reported. Among the BixIny series many clusters are found to exhibit a special stability and exhibit a large HOMO-LUMO gap in the range of 0.95 to 1.98 eV. This stability is rationalized by different mechanisms. Bi2In- is classified as a gas phase Zintl species despite only having three atoms, making it the smallest possible case. Bi3In2-, with 12 valence electrons and a closo structure in agreement with Wade’s rules, is similar to Bi3Ga2-, a gas phase Zintl analogue of Sn52-. Bi4In- and Bi4In2 are also identified as gas phase Zintl clusters. BiIn3 is a cyclic planar molecule similar to BiGa3 and BiAl3, all-metal aromatic systems. Additionally, an even odd oscillation of the HOMO-LUMO gap, formation energy and adiabatic electron affinities is found correlating with the open-shell/closed-shell nature of the clusters. In this thesis the identification of numerous stable clusters has been made. The experimental design has proven to be a powerful technique for this purpose, and these clusters can be considered as candidates for cluster assembled materials applications going forward. However, to encourage synthetic efforts, an understanding of the reasons for stability is crucial. For this reason, each stable cluster has been rationalized within a theoretical model. In particular, the model of gas phase Zintl analogues has been constructed to rationalize gas phase behavior by using established rules in condensed phase chemistry. The more common models of aromaticity and the Jellium model have also been used to explain the behavior of stable clusters experimentally observed.