Special Issue Preface pubs.acs.org/JPCA
Tribute to A. W. Castleman, Jr. cesses. For example, Will’s seminal work on ionic clusters provided compelling evidence that ion-catalyzed reactions are critical precursors to heteromolecular nucleation processes. These fundamental studies were especially important for understanding heterogeneous reaction mechanisms occurring in the stratosphere. For example, large barriers exist for interactions between HCl and ClONO2 surfaces when both species are neutral, but Will showed quantitatively that electron-transfer processes involving anions allow reactions to proceed with little to no barrier. Building on these ion− molecule studies, Will and his group soon set out to determine the influences of solvation on chemical reactivity. Using isolated, size-selected gas-phase clusters as model systems, Will’s group provided numerous molecular-level insights into solvation phenomena. One exciting outcome of this work was the direct observation that HBr could be dissolved into its solvent-separated ion pairs by exactly five water molecules, thereby influencing reactivity. Will’s unique ability to identify and develop new experimental techniques to solve the problem at hand have also played an important role in shaping and advancing Physical Chemistry research. For example, his use of selected-ion flow tubes allowed him to approach ion thermochemistry with unprecedented size and compositional specificity and, in turn, yielded numerous breakthroughs for understanding heterogeneous chemistry and catalysis. Will was also one of the pioneers in using femtosecond time-resolved methods to study cluster dynamics, and he was indeed the first to use the Coulomb Explosion technique to study clusters. Will used Coulomb Explosion as a direct mechanistic probe for studying protontransfer reactions in model DNA base pairs. He then utilized clusters as a medium to understand how these fundamental processes are influenced by solvation. These groundbreaking studies laid the foundations for a field that has now expanded to include studies of ultrafast dynamics in proteins and other biologically relevant systems. Will has also advanced the field of cluster science by combining advanced experimental measurements with theoretical calculations. Through a number of important collaborations, Will’s research program has provided critical insights into how the electronic properties and reactivity of materials evolve as the species transitions from molecular to bulk levels. Will has made significant contributions to the rational design of functional catalysts. These contributions are the result of his using gas-phase metal and metal oxide clusters as model systems for investigating mechanisms of heterogeneous catalytic transformations. His early work on this subject involved photoionization and time-of-flight mass spectrometry to characterize the distributions of cationic metal oxide clusters formed through gas aggregation of vapors of alkaline earth metals. Will discovered that specific metal oxide clusters with missing oxygen atoms are stabilized through donation of
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t is with great pleasure that we join our many colleagues who contributed to this special issue of The Journal of Physical Chemistry A in dedicating it in honor of Professor A. W. “Will” Castleman, Jr. Will is a pioneer in the field of cluster science, and the far-reaching impact of his career is demonstrated by the broad range of topics covered in this issue. This diversity validates Will’s teaching that “fundamental research in cluster science allows you to explore any problem you want”. This ability to extend to other research areas comes because, as Will has shown both rigorously and elegantly, clusters are an intermediate state of matter that bridge the gap between molecular and bulk levels. Will’s excitement when unexpected phenomena present is unmistakable. Thankfully, Will has combined his enthusiasm for and deep understanding of clusters and Physical Chemistry to inspire many generations of students and colleagues. In this brief Tribute to Will’s career, we attempt to summarize some of his scientific contributions, which are detailed in more than 700 publications. Although Will’s work has always been rooted in cluster science, his research interests have evolved over the course of his career. Some of Will’s earliest accomplishments included molecular-level descriptions of atmospherically relevant pro© 2014 American Chemical Society
Special Issue: A. W. Castleman, Jr. Festschrift Published: September 18, 2014 8011
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catalysts resulted from the presence of vanadium oxide centers of a specific stoichiometry observed in the gas phase. Other clusters were found to be inert under these reaction conditions. Through collaboration with the theoretical group of Vlasta Bonačić-Koutecký, it was predicted that, in contrast to nonreactive clusters, Will’s highly reactive stoichiometric metal oxide clusters contained an oxygen radical center. These calculations indicated that the weakened metal−oxygen bond at the radical center made the stoichiometric clusters much more reactive toward the oxidation of hydrocarbons than other clusters. In addition, Will conducted energy-resolved experiments on the reactivity of mass-selected cationic vanadium oxide clusters with ethylene in an octopole collision cell, revealing an inverse dependence of the stoichiometric cluster’s reactivity on the kinetic energy of the ions. The reaction cross sections Will determined experimentally were consistent with those calculated using the Langevin formula for ion−molecule reactions. Following the success of this initial collaborative work, Will and Vlasta set out to understand better the structural features and energetic trends responsible for the differences in reactivity observed experimentally for specific gold oxide clusters. They showed that three distinct oxygen centers were responsible for reactivity: peripheral, bridging, and molecular oxygen motifs; the peripheral site was shown to be the most active toward the oxidation of carbon monoxide. They also showed that the mechanism of oxidation changed when the charge state of fixed-size gold oxide clusters was changed. More recent collaborative work between Will and Vlasta examined the influence of cluster size on the reactivity of oxygen radical centers in metal oxide clusters toward simple hydrocarbons and other atmospheric pollutants. The charge state of the clusters was also shown to influence the selectivity of oxygen radical centers in anionic clusters toward the oxidation of different molecules. Importantly, Will demonstrated experimentally that metal oxide clusters containing oxygen radical centers in both the cationic and anionic charge states may be regenerated by reaction with an oxidizing agent, thereby completing a full catalytic cycle in the gas phase. Most recently, Will examined the reactivity of cationic metal oxide clusters containing superoxide centers in a joint experimental and theoretical study with Vlasta, which revealed a strong dependence of the reactivity of the superoxide centers on cluster size. Throughout his career, Will pioneered the identification and characterization of clusters with enhanced stability and distinct reactivity that have the potential for eventual incorporation into cluster-assembled materials with tailored properties. One of his earliest and most important discoveries in this area was the identification of the M8C12+ metallocarbohedrene or “met-car” clusters. The high stability of these ionic clusters was attributed to a dodecahedral structure with high symmetry. This exciting observation launched a flurry of activity in Will’s group to characterize the reactivity, stability, electronic properties, and dynamics of these newly discovered clusters. Initial studies showed that met-cars were reactive toward molecules with large dipole moments or π-bonded systems but relatively unreactive with oxygen. Will began producing binary met-cars containing two different metals and examining their reactivity with oxygenated hydrocarbons and other molecules. The binary met-cars were found to be generally more reactive than the single metal clusters. Will also uncovered interesting electronic effects in the met-cars, including delayed ionization and delayed ionization/fragmentation phenomena. In addition, the relaxation dynamics of metal−carbon clusters, including the met-car,
electrons from the metal centers to the oxygen vacancy, similar to solid-state color centers on bulk metal oxides. Color centers are now known to play a critical role in immobilizing supported metal clusters on metal oxide surfaces as well as promoting reactivity through partial transfer of charge from the color center to the cluster. In this way, Will’s careful cluster-level studies have helped guide the development of catalytic materials. Realizing the important role that size and stoichiometry play in the reactivity of these materials, Will began studying reactions of mass-selected cationic metal clusters produced by laser vaporization in a selected ion flow tube. Using this technique, the bimolecular rate constants for reactions between size-selected cationic metal clusters and oxygen were determined at thermal conditions and found to vary considerably with the size of the clusters. This work was extended to anionic oxide clusters of transition metals generated using a hot filament source. The anionic clusters were reacted with molecules such as water, nitric oxide, and hydrochloric acid to determine the rate coefficients for these atmospherically relevant processes. The reaction processes observed via these experiments differed significantly, ranging from oxygen atom abstraction, hydroxide formation, and adduct formation, depending on cluster oxidation state. Will was able to confirm that the electronic shell of the metal atoms comprising the clusters determined the observed reactivity. Similar effects are now known to influence the optical and electronic properties of colloidal metal nanoclusters. Will continued to define the cutting edge for studying structure and reactivity of clusters by extending his studies to include the use of tandem mass spectrometry to study catalytically relevant systems. Experiments were conducted using a triple quadrupole instrument that enabled mass-selected clusters generated by laser vaporization to be reacted with gaseous molecules or, alternatively, fragmented through collision-induced dissociation. Will investigated oxide clusters of group V metals in detail with this technique. Initially, reactions of cationic vanadium oxide clusters with unsaturated hydrocarbons revealed products consistent with association, dehydration, cracking, and oxidation with the reactivity differing from cluster to cluster. This suggested that metal oxides may be tuned to promote a specific reaction depending upon the stoichiometry of the reactive centers. In addition, Will used fragmentation studies to determine the basic building blocks of the larger vanadium oxide clusters. Will expanded these mass-selected experiments to include cationic and anionic clusters of the heavier group V elements Nb and Ta as well as a broad array of catalytically relevant molecules such as halogenated hydrocarbons, alcohols, and aromatics. It is impossible to summarize the breadth of information obtained from these numerous studies; therefore, it is preferable to focus instead on Will’s contributions that aided the understanding of condensed-phase catalytic processes. It is well-known that bulk vanadia promotes the oxidation of hydrocarbons more effectively than niobia or tantala. Will’s gas-phase experiments demonstrated that only the stoichiometric cluster of vanadium oxide (V2O5)+ transferred oxygen to hydrocarbons, whereas the stoichiometric clusters of niobium and tantalum oxide were far less reactive. In addition, Will’s experiments with carbon tetrachloride indicated that this halogenated molecule degraded to phosgene via a proposed condensed-phase mechanism requiring two vanadium centers. Will also noted that the oxidative dehydrogenation of butadiene over bulk vanadia 8012
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photoelectron imaging to provide new insights into atom mimicry, which could quite possibly revolutionize the way functional materials are designed. Will continues to study the reactivity of metal clusters with a variety of molecules as a means of probing the role of electronic shell closing, spin accommodation, and s−p hybridization on the reactivity and stability of these species. Will’s contributions extend well beyond his research accomplishments. He has mentored numerous students − including 79 conferred Ph.D.s and postdocs who now hold positions in academia, national research laboratories, industry, and scientific publishing. Will gives tirelessly of his time and efforts for numerous committees and organizations that seek his help and advice. He has served on many advisory and award committees for the National Academy of Sciences, among others. Will has organized and/or chaired several conferences and symposia, including the first Gordon Research Conference to be held outside of the United States. Will has also served on the editorial boards for many journals, including 10 years as a Senior Editor to the Journal of Physical Chemistry, during which time he helped to raise its quality and reputation. Deservingly, Will has received numerous awards that recognize the importance of his contributions. These include the Irving Langmuir Award in Chemical Physics, seven Humboldt Awards, several with renewals, Invited Lectures at the Nobel Symposia on the Chemistry and Physics of Clusters and Femtochemistry, fellowships in the American Academy of Arts and Sciences and the American Physical Society, and election to the National Academy of Sciences, and many others. It is an honor for us to be a part of some portion of Will’s scientific life. We consider it a privilege to dedicate this special issue of The Journal of Physical Chemistry A to our colleague and friend, Will Castleman.
were investigated using femtosecond spectroscopy. These studies revealed increased free-electron behavior in the larger species. Will measured the ionization potentials of met-cars using photoionization and employed anion photoelectron spectroscopy to determine the electron affinity of metal− carbon clusters related to the met-car and actively pursued improved methods of met-car production for these gas-phase studies. For instance, Will determined that either cubic or metcar structures may be formed by varying the experimental conditions. This indicated the importance of both kinetic and thermodynamic factors in the preparation of these materials. In addition, Will undertook efforts to isolate the met-car in the condensed phase through plasma synthesis in an arc, laser ablation in solution, and soft landing of clusters produced by laser vaporization. Soft landing yielded promising results through characterization with electron microscopy. Will has been a leader in developing paradigms to explain the stability and reactive properties of metal clusters. In some of the earliest experiments on the reactivity of aluminum clusters, Will discovered an enhanced abundance of certain size clusters in the distribution during etching with oxygen in a fast-flow reactor. These abundant clusters are now known to correspond to species with a closed Jellium shell of electrons. This gave rise to the concept of Superatoms based on analogy to the United Atom and the mimicking of particular elements by certain atom groupings. Several years later, Will investigated the reactions of anionic aluminum clusters with hydrogen iodide and observed a pronounced increase of Al13I− in the mass spectrum. Through collaboration with the theoretical group of Shiv Khanna, it was predicted that the Al13− cluster adopted behavior similar to that of a superhalogen and formed an overall structure reminiscent of a polyhalide. These predictions were validated by calculations that showed that the Al13− framework is preserved in the presence of the iodine atom. Will understood the importance of these findings as they suggested that superhalogen clusters may have utility as building blocks in clusterassembled materials and, in fact, represent a third dimension of the periodic table. In a following study, Will and Shiv demonstrated that the Al13− cluster can substitute for conventional halogens in larger polyhalides. In addition, new compositions of polyhalides incorporating Al14− in a superalkali motif were observed that are not possible with traditional halogens. The concept of multivalent superatoms was introduced next using the Al7C− cluster as an example in which the Al7 cluster adopts different valence states. Will demonstrated that certain size aluminum hydrogen clusters have exceptional stability due to their large electron affinity and Homo−Lumo gap and, therefore, may serve as building blocks for cluster-assembled materials. In the spirit of bridging the gap between gas and condensed phases, he has recently shown that electronic spin also influences electron dynamics in colloidal Superatom nanoclusters. In a systematic study of the reactivity of different size anionic aluminum clusters with oxygen, Will and Shiv determined that the spin excitation energy controls the reactivity of these clusters. In another joint study, Will and Shiv showed that complementary active sites consisting of adjacent Lewis acid and base sites dictate the reactivity of anionic aluminum clusters with water. Although it has been nearly 3 decades since Will’s group started the work on aluminum clusters, which ultimately gave us the concept of Superatoms, it is quite possible his most exciting and creative work in this area lies ahead of us. Will’s group is currently using
Kenneth L. Knappenberger, Jr. Florida State University
Grant E. Johnson Pacific Northwest National Laboratory
Mostafa A. El-Sayed
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Georgia Institute of Technology
AUTHOR INFORMATION
Notes
The authors declare no competing financial interest.
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