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45406-AC6
Solvent-Enabled Reactivity of Negative Ions

Andrei Sanov, University of Arizona

The outcomes of chemical reactions are subject to the multitude of interactions characteristic of condensed environments.  The research project supported by this ACS PRF grant examined the role of solvent molecules in photoinduced chemical processes, making use of cluster anions to attain a molecular view of chemistry.  The broad objective of this work, particularly relevant to the mission of the ACS Petroleum Research Fund, was to investigate solvent-enabled chemistry in anionic environments, spotlighting the reaction pathways that are either enhanced or made possible entirely by the presence of the solvent.  Specifically, we examined two types of effects:

(1) The formation of new chemical bonds under the effect of interactions with the environment; 

(2) Solvation-induced shift from the photoinduced electron detachment towards the breaking of chemical bonds.

To accomplish these objectives, we employed a combination of tandem time-of-flight mass-spectroscopy and photoelectron imaging.  Experiments were carried out on several cluster anion systems, including the pure and hydrated CO₂ of CS₂ cluster anions, NNO₂^-(N₂O)_n-1 and O^-(N₂O)_n cluster anions, as well as others.

First, using photoelectron imaging as a tool for examining the electronic structure and bonding motifs, we studied the a variety of cluster systems.  The intermolecular interactions in clusters often lead to the formation of new covalent bonds, in some important cases resulting in the competition and coexistence of the covalent dimer anion cluster cores and the monomer anion cluster cores.  This was found to be particularly true for the cluster anions of CO₂, CS₂, OCS, and others.  We have demonstrated that the balance between different cluster species is affected to a large degree by the homogeneous solvation and heterogeneous hydration.

The photochemistry is also affected by the intermolecular interactions with the environment. Absorption of light by a negative ion generally leads to electron detachment or promotion to excited anionic states, which serve as gateways to chemical transformations. Since the excess electron orbitals, due to their diffuse nature, are particularly sensitive to the surroundings, the excess electron is usually an active participant in chemical dynamics and a sensitive probe thereof.  In particular, the branching between the electron detachment and anion dissociation can be tuned using stepwise solvation. 

As a specific example, we used photoelectron imaging to investigate the coexistence of several isomers of (CS₂)₂^-.  The bands in the photoelectron images and spectra were assigned to the CS₂^-·CS₂ ion-molecule complex and two different dimer-anion structures with covalent bonding between the two monomers.  The isomer distribution and anionic reactivity were found to depend sensitively on the presence of water in the precursor gas mixture, which has important implications to chemistry in wet of humid environments.  One of our important findings was the evidence that the presence of water enhances the formation of the global-minimum dimer structure, rather than the metastable structure corresponding to the ion-molecule complex.  However, in the larger (CS₂)_n^- and (CS₂)₂^-(H₂O)_m clusters, the population of the covalent-dimer core structures diminishes drastically due to more favorable solvent interactions with the monomer-anion core.

In another PRF supported study, we investigated the structural properties and underlying reactivity of [O(N₂O)_n]^- cluster anions generated via electron bombardment of a pulsed supersonic expansion of pure N₂O gas.  The photoelectron images obtained in this study at 355 nm are shown in the Figure, where the laser polarization direction is set vertical in the plane of all images.  Depending on cluster size, the images give evidence of two dominant core-anion structures, corresponding to the NNO₂^-(N₂O)_n-1 and O^-(N₂O)_n cluster anions. In agreement with previous studies, the n = 1 anion has a covalently bound (Y-shaped) NNO₂^- structure. The striking new result that came out of our work is that the NNO₂^- core was found to persist only in the clusters with n up to 3. For n ³ 4 and up to at least n = 9, signatures of an O^- core were predominantly evident (see, for example, the sharp discontinuity in the overall image structures between n = 3 and 4).

These results provide prominent demonstrations of and insight into the role played by the solvent in controlling the structure of anionic compounds and their corresponding photochemistry. The results and conclusions drawn from our studies of solvent-enabled chemistry enhance our fundamental understanding of intermolecular interactions and their roles in chemistry of condensed environments. 

The work in this project was carried out by graduate students in the Sanov group.  These students were provided not only with support for their graduate education, but also with rich training opportunities in the multi-faceted experimental techniques and the process of scientific discovery.  Two of the students supported in part by this project (Luis Velarde and Terefe Habteyes) earned their Ph.D. degrees in 2008 and moved to postdoctoral positions in other research groups.

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