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 aims to examine the role of solvent molecules in photoinduced chemical processes, making use of cluster anions to attain a molecular view of chemistry. This work focuses on solvent-enabled chemistry in anionic environments, spotlighting the reaction pathways that are either made possible or greatly enhanced by the presence of the solvent.  Specifically, we examine two types of effects: (1) the formation of new chemical bonds under the effect of interactions with the environment; and (2) solvation-induced shift from the photoinduced electron detachment towards the breaking of chemical bonds.

To accomplish these objectives, we employ a combination of tandem time-of-flight mass-spectroscopy and photoelectron imaging.  In the first project year, we carried out experiments on the several cluster anion systems, including the pure and hydrated CO₂ of CS₂ cluster anions, as well as others.

Using photoelectron imaging as a tool for examining the electronic structure and bonding motifs, we studied the electronic and geometric structures of the (CS₂)_n^- and (CS₂)_n^-·H₂O clusters anions.  In this systems, the intermolecular interactions between individual monomers lead to the formation of new covalent bond, resulting in the competition and coexistence of the covalent dimer anion cluster cores and the monomer anion cluster cores.  The balance between different cluster species has been shown to be affected greatly by the homogeneous solvation and heterogeneous hydration, with the CS₂ and H₂O solvent molecules favoring different cluster core structures.

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. 

In the first project year, we studied these dynamics in the (CO₂)_n^-(H₂O)_m, (CS₂)_n^-, and (CS₂)_n^-(H₂O)_m cluster anions, with a particular emphasis on the effects of solvent configurations on the reactivity of the corresponding covalent dimer anions and the photodissociation pathways attributed to them.  Figure 1 illustrates our findings on the example of hydrated (CS₂)₂^-.  The Figure shows the photofragment mass-spectra obtained for (CS₂)₂^- and (CS₂)₂^-‌·H₂O.  All observed products, with the probable exception of S^-, are attributed to the covalent dimer anion, C₂S₄^-. The spectra show that C₂S₄^- can be “sliced” by visible‌/UV photons in (at least) three different ways, yielding C₂S₂^-, S₂^-, or CS₂^- as anionic fragments.  As also seen in the figure, the addition of just one (!) H₂O molecule changes the fragmentation pattern drastically.  Therefore, the water molecule acts as a “molecular switch”, turning off or suppressing some of the reaction channels, while enhancing the others.  For example, the C₂S₂^- fragments, very intense in 532 nm dissociation of bare C₂S₄^- (a), disappear almost completely in the corresponding C₂S₄^-·H₂O spectrum (d). Understanding the effect of hydration on channel competition requires unraveling the charge-switching character of the electronic states involved.  This work is now being finalized for publication.

This work is carried out by graduate students in the Sanov group, providing training opportunities in the multi-faceted experimental techniques and the process of scientific discovery.  The experimental apparatus used in this project has been built by graduate students, who are presently continuing the development of the apparatus, matching its capabilities to the needs of the research program.