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Structure and Reactivity of Gas Phase Ions

 

Carl Lineberger and J. Mathias Weber use laser spectroscopy to investigate the structure and reactivity of gas-phase ions.

Since joining the JILA faculty in 2006, J. Mathias Weber has undertaken studies of isolated mass-selected ions. Until very recently, the behavior of many complex molecular ions, such as DNA, polymer building blocks, and metal complexes has been studied primarily in solution. Because interactions with solvents can change the intrinsic properties of these molecules, Weber prefers to investigate them in isolation. His group accomplishes this by applying mass spectrometry and laser spectroscopy to the study of beams of mass-selected molecular ions. To investigate their properties, the group has used the fact that deposition of energy into targeted areas of these molecules can lead to their fragmentation or a change in their charge state. By carefully choosing the laser wavelength and targeting specific portions of the molecules, the group is investigating fragmentation pathways at different photon energies and looking for new insights into the photophysics and photochemistry of molecular ions ranging from small complexes to DNA fragments. His research focuses on the vibrational and electronic spectra of such molecular and cluster ions.

Carl Lineberger has spent many years "shedding light" on negative ions. He and his colleagues were the first investigators to use laser techniques to remove electrons from atomic or molecular anions in the gas phase. Over the years, they learned about thresholds for electron detachment and the binding energies that hold electrons inside chemical structures. Lineberger's work laid the foundation for an entire field of chemistry, in which researchers take advantage of the unique properties of negative ions to probe chemical reaction dynamics, molecular structure, forces that hold molecules together, and photodissociation reactions. His research has also revolutionized the spectroscopic study of gas-phase anions. He and his colleagues have determined precise values for the energy of electron attachment for most atoms and many open-shell molecules. These values are found in chemistry textbooks throughout the world.

Lineberger and his group are currently using ultrafast lasers to investigate the dynamics of partially solvated anions following dissociation, in particular long-range electron-transfer processes. Other ultrafast laser studies include investigations of reaction dynamics in size-selected anion clusters and the study of caging in large clusters. In these and other experiments, Lineberger works closely with leading theoreticians to gain a full understanding of what is being observed in the laboratory. His group currently collaborates with Anne McCoy (Ohio State), John Stanton (University of Texas), Wes Borden (University of North Texas), and Robert Parson (JILA).

A major focus of the Lineberger group is the use of nanosecond tunable lasers or continuous-wave lasers to obtain photoelectron spectra of negative ions. Such experiments yield direct information on key transient intermediates, transition states, and thermochemical properties of important reactive intermediates. In fact, negative ions have also proven to be an ideal vehicle for studying reactive processes. For instance, negative-ion photo detachment can be used to prepare neutral species in the midst of a reaction or in a geometry very close to a key "transition state" in a chemical reaction.

Lineberger and his colleagues used negative photoelectron spectroscopy to observe—for the first time—the oxyallyl diradical, believed to be a key reactive intermediate in a series of important organic transformations. While oxyallyl diradical had been predicted to exist (and had been the subject of many theoretical investigations), the Lineberger group’s experiments provided the first direct evidence for its existence. In addition to demonstrating oxyallyl diradical’s existence, the photoelectron studies provided detailed information on its electronic states and geometrical structure. In particular, the studies showed that the singlet ground state of oxyallyl diradical is the short-lived (<ps) transition state leading to ring closure, which forms cyclopropanone.

Carl Lineberger collaborates with Veronica Bierbaum and Barney Ellison (both at the University of Colorado) on studies of the properties of proxy radicals, which are unstable intermediates that play a key role in the chemistry of atmospheric pollutants. Once again, the combination of negative-ion photoelectron spectroscopy and gas-phase ion chemistry is providing direct information on structure, energetics, and electronic properties of highly unstable reactive intermediates. Recent studies included an investigation of the proxy acetyl radical, which is the key component in the formation of proxy acetyl nitrate (PAN). PAN is a major player in the cycle of urban air pollution. The collaboration’s studies showed that a key O–H bond is significantly less strong than previously thought. This finding may well affect atmospheric models.

The Lineberger group also investigates time-resolved photodissociation of anions, with the goal of understanding the mechanism for the transfer of electrons over long distances. The processes of electron transfer and charge separation are key components in energy storage, biological signaling, and solar energy use. Lineberger and his students recently demonstrated the ability of a single solvent molecule to facilitate an electron transfer on a subpicosecond time scale over distances as great as 7 Angstroms.

Cluster ions are the focus of another group project. Lineberger and coworkers use a cluster ion machine consisting of a cluster ion source and a tandem time-of-flight spectrometer together with a femtosecond modelocked laser system to probe the energy structures of ion clusters such as ICl-(CO2)n and IBr- (CO2)n. The group photo excites the ion clusters at two different wavelengths with a laser and investigates the photoproducts that are produced. These studies not only reveal important information about the electronic transitions during the photodissociation process, but also shed light on the role of the solvent in driving particular electronic processes such as spin-orbit relaxation and charge transfer. The cluster studies also provide information about the environment-induced recombination, or "caging" of photo-dissociated diatomic molecules. Comparative studies of the photodissociation dynamics of different dihalide solutes in the same gas-phase solvent are helping researchers understand the step-by-step process and dynamics of solvation.

 Vibrational Spectra of Molecular and Cluster Anions

J. Mathias Weber uses vibrational spectra to investigate cluster anions where a charged atom or molecule interacts with solvent molecules. As an example, the group has used the aromatic molecules both as ligands attached to anions and as charge carriers with ligands attached to study how aromatic molecules interact with negative charge.. Past experiments have focused on benzene’s special ring structure that allows some of its electrons to be shared among all 6 carbon atoms in the ring. For example, the group has adjusted the charge density in the ring by exchanging hydrogen atoms (H) in the ring with other atoms, such as fluorine (F), or with groups of atoms. Such substitutions change the charge pattern in the ring “seen” by neighboring molecules.

However, the Weber group discovered that interaction based on charge distribution is only part of what can happen. Anions (i.e., negatively charged atoms or molecules) can also link to a carbon-hydrogen (CH) group in a benzene molecule by hydrogen bonding. Hydrogen bonding is an attractive interaction between an H atom in one molecule and a negatively charged atom, such as oxygen, in another molecule.

During hydrogen bonding, electrons can be partially transferred from the negatively charged bonding partner to the molecule containing the H atom. The combination of charge-distribution effects and hydrogen bonding makes for very interesting benzene ring chemistry. Benzene ring chemistry is intriguing for chemists who want to use benzene-like parts of one molecule as binding sites for negatively charged groups on another molecule to make “supramolecular” structures. Such structures can assemble themselves into well-ordered thin films or nanomaterials with entirely new chemical properties.

Vibrational spectra also recently allowed the Weber group to probe the effect of solvent molecules on a catalytic reaction that adds chemical energy back to carbon dioxide (CO2) molecules. In this reaction, a gold atom with an extra electron transfers the electron to a CO2 molecule. This transfer could be an important first step in future industrial processes converting waste CO2 back into chemical fuels.

In the first step of the catalytic reaction, a CO2 molecule and a gold atom form a gold-CO2 complex. If the CO2 could acquire the extra electron and free itself from the gold atom, it would complete the first step of the conversion reaction. The newly formed, highly reactive (i.e., activated) CO2- ion would then be available for use in a series of additional steps to make recycled liquid fuels from waste CO2.

The Weber group found that as one to nine solvent CO2 molecules clustered around the CO2 end of the gold-CO2 complex, the activation of the complex-bound CO2 intensified, enabling the CO2 to grab more and more of the needed electron. The activation reaction was most favored when nine solvent molecules were in place around the complex.

But, adding even one more solvent molecule around the complex diminished the chances of activation. The reason was that there was no longer room around the complex-bound CO2 for more solvent. Additional solvent molecules ended up around the gold atom, where they pulled the electron nearer the gold atom, diminishing the chances for activation of the bound CO2.

This work has shown how solvation can both enhance and diminish the effects of a gold catalyst. It is expected to inform the design of future industrial systems aimed at creating carbon-neutral fuel cycles.

Transition Metal Chemistry

The J. Mathias Weber group is using spectroscopic tools to obtain a more detailed picture of transition metal chemistry such as the catalytic reaction of gold with CO2 described in the previous section. The group’s goal is to understand the role of negative charge in the context of transition metal-containing complexes. Transition metal complexes are important chemical species in processes ranging from homogeneous and heterogeneous catalysis to solar energy conversion. Gas-phase ion-molecule complexes can serve as model systems for studying ion-molecule interactions and yield important information about their analogues in condensed-phase chemistry. Mass spectrometry and photoelectron spectroscopy data have provided basic knowledge on cationic complexes, but there is a need for better spectroscopic experimental data.

The Weber group is interested in gaining a deeper insight into the electronic and geometric structures of ionic species and investigating the inter- and intramolecular forces in transition-metal-complex ions. The researchers use infrared and electronic photodissociation spectroscopy for these studies. The group anticipates that their experiments will uncover information on reaction intermediates in the chemistry of transition metal complexes. The interpretation of these complex experiments is aided by quantum chemical calculations.

In a recent experiment, the group performed photodissociation spectroscopy on chromate ester cluster ions in the gas phase. Researchers investigated both the properties of the ions and their fragmentation pathways. The goal was to discover information on the mechanisms involved in chromate photochemistry in nature. Such studies can help understanding the chemical behavior of toxic and mutagenic chromium compounds in rivers, lakes, and drinking water.

Future experiments will be concerned with molecules and mechanisms important for the conversion of carbon dioxide into chemical fuels.