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45789-AC6
Infrared Spectroscopy of Cluster Anions Containing Aromatic Molecules

J. Mathias Weber, University of Colorado (Boulder)

These first 20 months of the project have been very successful. After some work (optimizing the ion experiment for several target systems) had already been done before the start of the grant period, we were able to successfully perform a large portion of the proposed experiments. In a “sideline” experiment, we were also able to make the first steps into a new spectroscopic access to energy flow in hydrocarbon molecules.

In oxidative stress, cells are exposed to high levels of oxygen in the form of O₂ or superoxide O₂^-. This is relevant in the contexts of immune response, aging, and a variety of diseases. Consequently, there is considerable interest in the nature of the interactions of O₂ and O₂^- with biomolecules and models of biologically important functional groups. As a model system for aromatic groups, we chose O₂^-×benzene complexes. Experimentally, several transitions due to CH stretch fundamentals and various combination bands are observed in the 2700-3100 cm^-1 region. After completion of the experiments, we found out that the group of M. A. Johnson at Yale University had done similar and partially complementary experiments, corroborating our findings. We decided to publish our findings together, and brought K. D. Jordan (University of Pittsburgh) and E. M. Myshakin (NETL, Pittsburgh) into the team, who performed ab initio calculations to aid in the interpretation of the infrared spectra. The structure of the O₂-benzene complex is dominated by the geometry of the pi* orbital of the anion, and that the superoxide molecule binds preferentially to one CH group of benzene with one lobe of the p* orbital, while another lobe is weakly tethered to a neighboring CH group (see Figure 1). This is interesting, since the solvation with benzene does not occur in a symmetric fashion postulated previously. The results were published in J. Chem. Phys. in 2007.

Figure 1: Structure of O₂^-×benzene.

As proposed, we also investigated the structures of anionic hydrated fluorobenzenes. While the bonding of water to atomic and small molecular anions has been studied for many ions and in great detail, the hydration of larger charge distributions has not been studied at the same level. One class of interesting model systems contains an aromatic molecule with a small positive or even negative electron affinity, which is stabilized by the presence of a solvent molecule, such as water. This is interesting in the contexts of anion hydration, and of fluorination chemistry of aromatic molecules. The absorption bands show that only one isomer of each monohydrate complex is populated in our experiment, despite a multitude of low-lying isomers that we found in calculations. We can assign the observed bands to an isomer where water forms a weak double ionic hydrogen bond with two neighboring CF groups (see Figure 2). The spectroscopic motif of the binary complexes remains at lower fluorination levels. For dihydrated hexafluorobenzene anions, we observe hydrogen bonding between the water molecules laying the foundation for the formation of water networks. The results were published in 2007 in J. Chem. Phys.

Figure 2: Structure of C₆F₆^-×H₂O.

The most intriguing experiment was the structural investigation of fluorinated benzenes in complexes with chloride. This topic is interesting in the context of anion molecular recognition, as the supramolecular chemistry of aromatic scaffolds can be tailored by substitution. In chloride-benzene complexes, the anion binds to the benzene in bifurcated H-bonds, while it binds to the ring in chloride-hexafluorobenzene complexes, due to the opposite electrostatic makeup of the aromatic molecule (see Figure 3).

Figure 3: Electrostatic potentials around C₆H₆ and C₆F₆, and interaction with Cl^- ions.

The question we addressed was how many F substituents are needed to change the binding motif from interaction with the CH groups in the periphery of the fluorobenzene to the ring, investigating Cl^-×fluorobenzene complexes for all fluorination levels up to pentafluorobenzene and all stereoisomers. Interestingly, the complexes remain in the H-bonding configuration for all systems studied, implying that only chloride-hexafluorobenzene will exhibit a ring-bound structure. The reason for this behavior lies in the increasing acidity of the fluorobenzenes with increasing fluorination, resulting in additional stabilization of the H-bonded structure compared to a ring-bound isomer. The results were published in J. Am. Chem. Soc. in 2007.

            Funds from this grant were mainly used for the support of a graduate student (Holger Schneider). This has been the first grant during my time as an Assistant Professor, and has been extremely valuable. It has enabled a considerable amount of my research program. The student supported by this grant has been working enthusiastically in this project, and his participation in conferences has been secured by it. The results have led our group deeper into the field of anion molecular recognition, and we hope to attract more outside funding in this area.

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