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, Mathias Weber’s group has undertaken studies of both gas-phase ions and biomolecules. Until very recently, the behavior of molecular ions, DNA, proteins, polysaccharides, polymer building blocks, and nanoparticles 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 small molecules and DNA fragments. His research focuses on the vibrational spectra of molecular and cluster anions and transition metal chemistry.

For his part, 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 photodetachment 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.

Recently, 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.

In another research area, Lineberger collaborates with Veronica Bierbaum and Barney Ellison (both at the University of Colorado) on studies of the properties of peroxy 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 peroxy acetyl radical, which is the key component in the formation of peroxy acetyl nitrate (PAN). PAN is a major player in the cycle of urban air pollution. The collaboration’s studies showed that the strength of 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 co-workers use a cluster ion machine consisting of a cluster ion source and a tandem time-of-flight spectrometer together with a femtosecond mode-locked 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 photodissociated 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 containing aromatic molecules. The group has used the aromatic molecules both as ligands attached to anions and as charge carriers with ligands attached. Past experiments have focused on benzene’s special ring structure that allows some of its electrons to be shared among all six 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 benzenelike 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.

Additionally, the Weber group has used investigations of ultraviolet (UV) spectra of DNA nucleotides to better understand the mechanism by which UV light damages DNA. UV light, which is absorbed by the bases of DNA, causes chemical reactions that can break chemical bonds in two places in the nucleotides. First, the break can occur in the link between the phosphate and sugar groups. In an extended DNA chain, this sort of damage would lead to the breaking of the DNA strands. Second, the bond can break between the sugar and the base; this breakage can be followed by additional reactions and lead to the loss of genetic information from DNA. The processes responsible for this damage inside molecules are under investigation.

Vibrational spectra are also allowing the Weber group to investigate the conditions that favor the addition of water molecules (hydration) to particular locations in negative molecular ions such as fluorinated benzene ions (C₆H_(6-n)F_n^-), sulfur hexafluoride ion (SF₆^-), nitromethane ion (CH₃NO₂^-), and naphthalene ion (C₁₀H₈^-). With naphthalene, the anion only exists because of the interplay between the naphthalene scaffold and the hydration environment. The group’s microhydration studies are an important extension of the field of hydrated anions. Until now, very little has been known about the hydration environments of anions made of more than two atoms. Thus far, the researchers have determined that while the shape of the charge distribution is important, it is not the only factor that determines a particular hydration structure. In SF₆ hydration, for example, the polarizability of the extra electron is not only critical for the hydration structure, but also for the increased reactivity of the hydrated anion. This finding may be important in understanding a possible depletion mechanism (via electron attachment) for SF₆ in the atmosphere. SF₆ is currently one of the most potent greenhouse gases known; it is a purely anthropogenic pollutant and exhibits long atmospheric lifetimes. The Weber group is following up on its initial work on SF₆^- with a study of infrared-triggered reactions of SF₆^-•HCOOH complexes.

Transition Metal Chemistry

The J. Mathias Weber group is using spectroscopic tools to obtain a more detailed picture of transition metal chemistry and 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. Although anionic complexes are important in heterogeneous catalysis at metal oxide surfaces or as starting materials for metal nanoparticles in solution phase, most information gathered on gas-phase species thus far is on cationic complexes. Mass spectrometry and photoelectron spectroscopy data have provided basic knowledge on cationic complexes, but there is a need for better spectroscopic experimental data on anionic species that yield more information than these particular techniques can provide.

The Weber group is interested in gaining a deeper insight into the electronic and geometric structures of anionic species and investigating the inter- and intramolecular forces in transition metal complex ions. The researchers will use infrared and electronic photodissociation spectroscopy for these studies. The group anticipates that their experiments will uncover information on the electron donation/back donation in metal- and metal oxide-ligand complexes as well as electron-binding energies. The experiments will also shed more light on the electron structure of singly and multiply charged metalates as a function of coordination and size. The photodissociation spectra of charged metalates are expected to contain information that could suggest possible new routes for nanoparticle production. The interpretation of these complex experiments will be aided by quantum chemical calculations.