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Structures and Charge Distributions of Cluster Anions Studied by Infrared Photodissociation Spectroscopy

Event Details

Event Dates: 

Thursday, November 13, 2014 - 10:00am

Seminar Location: 

  • JILA 10th Floor

Speaker Name(s): 

Benjamin J. Knurr

Speaker Affiliation(s): 

JILA, Weber group
Seminar Type/Subject

Scientific Seminar Type: 

  • JILA Thesis Defense

Event Details & Abstract: 

Anions are relevant in many different facets of chemistry. The study of anions, their interaction with other atoms and molecules and the effect of solvation on those interactions are of interest to further the understanding of chemical processes. Studies on cluster ions in vacuo allow for the generation of and excellent control over chemically interesting species. Experiments of this nature are ideal for studying the fundamental interactions of molecular partners by eliminating many of the complications of obtaining spectroscopic data on an individual species in the condensed phase. The clusters studied can also be viewed as model systems for more complex chemical processes that are otherwise difficult to understand at a molecular level.

In one part of this work, infrared spectra were obtained for [M(CO2)n]- (M = Au, Ag, Co, Ni and Cu) clusters. These studies were performed in an effort to assist in understanding the interactions resulting in the reduction of CO2. These cluster systems are viewed as model single atom catalysts for the purpose of CO2 reduction. Au, Ag and Cu were all found to form eta1 (monodentate) [MCO2]- complexes of the metal and one CO2 molecule with a structure akin to the formate anion. Study of the solvation environment of these eta1 complexes reveals that the excess charge on the core can be polarized onto the CO2 moiety with increasing efficiency from Au to Cu. Co, Ni and Cu all exhibit core ions of the form

[CO2MCO2]- where the CO2 ligands are bound in a bidentate (eta2) fashion.

The polarizability of the core species increases from Co to Cu. Cu is a member of both the first row transition metals and the coinage metals, and it shows interaction motifs characteristic for both groups of metals.

Additionally, two studies were performed on [CoO(CO2)n]- and [NiO(CO2)n]- clusters. It is shown that similar core structures are present in both CoO and NiO clusters and that the general behavior of the clusters is comparable to the analogous non-oxide cluster.

Two studies were performed on non-metal containing systems. A charge transfer reaction between a nitromethane anion and iodomethane was vibrationally induced. It is found that vibrational excitation of C-H stretches on either of the species in the complex leads to charge transfer from the nitomethane to the iodomethane and results in the formation of I- product anions. Ar tagging is used to study the energetics of the reaction and achieve quenching of the reactive channel. The reaction is discussed in the framework of a vibrationally induced SN2 reaction.

The infrared spectra of a naphthalene anion clustered with one to six water molecules were measured. The pi-system of the naphthalene accommodates the excess electron with a network of water molecules evolving on one side of the naphthalene. It was found that water-water H-bonds are generally more favorable than water-ion pi-H-bonds. With three or more water molecules present, the spectra indicate that there is more than one conformation of the water network present.