Research

 

Return

 

I. Photoelectron Spectroscopy of Size Selected Nanoclusters

     1. Structures and reactivity of gold clusters

     2. Transition metal oxide clusters

     3. Clusters of boron and metal borides

 

II. Solution Chemistry in the Gas Phase

     1. Probing the unique properties of free multiply charged anions

     2. Probing the electronic structure of metal complexes and redox species

     3. Solvation and solvent stabilization of complex and multiply charged anions

     4. Photoelectron imaging of multiply charged anions

 



I.  Photoelectron Spectroscopy of Size Selected Nanoclusters (return to top of page)

 

 

 

Current Research Projects

 

1.  Structures and reactivity of gold clusters (return to top of page)

 

Gold is known to be the most inert metal, but small gold nanoparticles have been found to exhibit remarkable catalytic properties, a distinct nano-effect.  A prerequisite to elucidate the mechanisms of the catalytic effects of nanogold is to understand the structures and chemical reactivity of gold clusters as a function of size.

 

In collaboration with theoretical colleagues, we have been using photoelectron spectroscopy and theoretical calculations to probe the emerging structures and chemistry of size-selected gold clusters.  Because of the strong relativistic effects, gold is known to possess unusual properties relative to Cu or Ag.  This is also reflected in the gold clusters.  For example, small gold cluster anions are known to be planar up to 12 atoms.  We have found that Au20 forms a beautiful tetrahedral structure.  We have also discovered that the Au16 and Au17 clusters are hollow cages, which can be stuffed with a foreign atom, similar to the carbon cage clusters (the fullerenes).  We are also  investigating in general how a dopant atom can alter the electronic, chemical, and structural properties of a gold nanocluster.

 


 

Selected recent publications:

"Au20: A Tetrahedral Cluster" (J. Li, X. Li, H. J. Zhai, and L. S. Wang), Science 229, 864-867 (2003) .

“Unique CO Chemisorption Properties of Gold Hexamer: Au6(CO)n (n = 03)” (H. J. Zhai, B. Kiran, B. Dai, J. Li, and L. S. Wang), J. Am. Chem. Soc. 127, 12098-12106 (2005).

“Evidence of Hollow Golden Cages” (S. Bulusu, X. Li, L. S. Wang, and X. C. Zeng), Proc. Natl. Acad. Sci. (USA) 103, 8326-8330 (2006).

“Structural Transition of Gold Nanoclusters: From the Golden Cage to the Golden Pyramid” (W. Huang, S. Bulusu, R. Pal, X. C. Zeng, and L. S. Wang), ACS Nano 3, 1225-1230 (2009).

“Probing the 2D to 3D Structural Transition in Gold Cluster Anions Using Argon Tagging” (W. Huang and L. S. Wang), Phys. Rev. Lett. 102, 153401-1-4 (2009).

“Isomer Identification and Resolution in Small Gold Clusters” (W. Huang, R. Pal, L. M. Wang, X. C. Zeng, and L. S. Wang), J. Chem. Phys. 132, 054305-1-5 (2010).

“Probing the Interactions of O2 with Small Gold Cluster Anions (Aun, n = 17): Chemisorption vs. Physisorption” (W. Huang, H. J. Zhai, and L. S. Wang), J. Am. Chem. Soc. 132, 4344-4351 (2010).

Probing the Structural Evolution of Medium-Sized Gold Clusters: Aun (n = 27 to 35)” (N. Shao, W. Huang, Y. Gao, L. M. Wang, X. Li, L. S. Wang, and X. C. Zeng), J. Am. Chem. Soc. 132, 6596-6605 (2010).

 

2.  Transition metal oxide clusters (return to top of page)

Transition metal oxides constitute an important class of materials with broad applications in electronics and catalysis.  Early transition metal oxides are particularly widely used as catalysts in hydrocarbon transformations.  We study the electronic and structural properties of oxide clusters both as a function of size and composition.  The goal of this project is to use clusters as well-defined molecular models for the active sites of oxide catalysts.  In addition, the electronic and spectroscopic information obtained from our experimental investigations provides benchmarks to verify computational methods.

The flexibility of our cluster source allows us to produce oxide clusters with compositions that do not exist or are hard to make in the bulk.  Starting from a given metal cluster, Mx, we can produce oxide clusters, MxOy, with continuous oxygen content from O-poor composition (small y) to compositions beyond the bulk stoichiometry (large y).  The O-poor clusters may be viewed as models for the oxidation of bulk metal surfaces, whereas the O-rich clusters can be viewed as models for the bulk oxides.  Clusters with intermediate O-contents are where one may find structures that are good models for bulk defect sites or catalytic active centers.  The O-poor oxide clusters also provide unique opportunities to study metal-metal bonding.

 

Selected recent publications:

“Observation of d-Orbital Aromaticity” (X. Huang, H. J. Zhai, B. Kiran, and L. S. Wang), Angew. Chem. Int. Ed. 44, 7251-7254 (2005).

Experimental and Theoretical Characterization of Superoxide Complexes W2O6(O2) and W3O9(O2): Models for the Interaction of O2 with Reduced W Sites on Tungsten Oxide Surfaces” (X. Huang, H. J. Zhai, T. Waters, J. Li, and L. S. Wang), Angew. Chem. Int. Ed. 45, 657-660 (2006).

“Probing the Electronic Structure and Band Gap Evolution of Titanium Oxide Clusters (TiO2)n (n = 1-10) Using Photoelectron Spectroscopy” (H. J. Zhai and L. S. Wang), J. Am. Chem. Soc. 129, 3022-3026 (2007).

d-Aromaticity in Ta3O3” (H. J. Zhai, B. B. Averkiev, D. Y. Zubarev, L. S. Wang, A. I. Boldyrev), Angew. Chem. Int. Ed. 46, 4277-4280 (2007).

Probing the Electronic Structure of Early Transition Metal Oxide Clusters: Polyhedral Cages of (V2O5)n (n = 24) and (M2O5)2 (M = Nb, Ta)”( H. J. Zhai, J. Döbler, J. Sauer, and L. S. Wang), J. Am. Chem. Soc. 129, 13270-13276 (2007).

Probing the Electronic and Structural Properties of Chromium Oxide Clusters (CrO3)n and (CrO3)n (n = 1−5): Photoelectron Spectroscopy and Density Functional Calculations” (H. J. Zhai, S. G. Li, D. A. Dixon, and L. S. Wang), J. Am. Chem. Soc. 130, 5167-5177 (2008).

Structural and Electronic Properties of Reduced Transition Metal Oxide Clusters, M3O8 and M3O8 (M = Cr, W), from Photoelectron Spectroscopy and Quantum Chemical Calculations” (S. G. Li, H. J. Zhai, L. S. Wang, and D. A. Dixon), J. Phys. Chem. A 113, 11273-11288 (2009).

 

3.  Clusters of boron and metal borides (return to top of page)

Elemental boron is electron-deficient and boron compounds form interesting delocalized chemical bonds.  Atomic clusters of boron provide a new playground to discover new structures and novel chemical bonds.  Boron atoms possess strong chemical bonding capacities, resulting in refractory bulk materials and super-hard metal borides.  This project focuses on fundamental investigations of the chemical bonding and structures of boron and metal boride clusters.  Photoelectron spectroscopy is combined with theoretical calculations to probe the structures, stability, and chemical bonding of size-selected boron and boride clusters.  The goal is to understand the structures and stabilities of boron and boride nanoclusters as a function of size and lay the foundation for new forms of boron and boride nanostructures.  Guided by the cluster studies, the viability of synthesizing new boron-based nanostructures will be explored.

We have provided the first experimental evidence that small boron clusters are planar.  We have further shown that the planarity of the small boron clusters are due to the delocalization of pi electrons within the molecular plane.  Interestingly, these pi electrons are found to follow the Hückel rules for aromaticity and antiaromaticity, just like that in hydrocarbon molecules.  That has led us to propose the concept of hydrocarbon analogs of boron clusters.

 

The B9 molecular wheel and its three pi orbitals, which are similar to those of benzene.

 

Selected recent publications:

“Hepta- and Octa-Coordinated Boron in Molecular Wheels of 8- and 9-Atom Boron Clusters: Observation and Confirmation” (H. J. Zhai, A. N. Alexandrova, K. A. Birch, A. I. Boldyrev, and L. S. Wang), Angew. Chem. Int. Ed. 42, 6004-6008 (2003).

"Hydrocarbon Analogs of Boron Clusters: Planarity, Aromaticity, and Antiaromaticity" (H. J. Zhai, B. Kiran, J. Li, and L. S. Wang), Nature Materials 2, 827-833 (2003).

“Planar-to-Tubular Structural Transition in Boron Clusters: B20 as the Embryo of Single-Walled Boron Nanotubes” (B. Kiran, S. Bulusu, H. J. Zhai, S. Yoo, X. C. Zeng, and L. S. Wang), Proc. Natl. Acad. Sci. (USA) 102, 961-964 (2005).

“All-Boron Aromatic Clusters as Potential New Inorganic Ligands and Building Blocks in Chemistry” (A. N. Alexandrova, A. I. Boldyrev, H. J. Zhai, and L. S. Wang), Coord. Chem. Rev. 250, 2811-2866 (2006).

“A Photoelectron Spectroscopic and Theoretical Study of B16 and B162–: An All-Boron Naphthalene” (A. P. Sergeeva, D. Yu. Zubarev, H. J. Zhai, A. I. Boldyrev, and L. S. Wang), J. Am. Chem. Soc. 130, 7244-7246 (2008).

“Carbon Avoids Hyper Coordination in CB6, CB62–, and C2B5 Planar Carbon-Boron Clusters” (B. B. Averkiev, D. Yu. Zubarev, L. M. Wang, W. Huang, L. S. Wang, and A. I. Boldyrev), J. Am. Chem. Soc. 130, 9248-9250 (2008).

 “A Concentric Planar Doubly p Aromatic B19 Cluster”, (W. Huang, A. P. Sergeeva, H. J. Zhai, B. B. Averkiev, L. S. Wang, and A. I. Boldyrev), Nature Chem. 2, 202-206 (2010).

 

 

II.  Solution Chemistry in the Gas Phase (return to top of page)

There are many multiply-charged and complex anions in solution, ranging from simple oxo-species, such as SO42– or PO43–, to inorganic metal complexes to redox species and biologically-relevant molecules.  The electronic structure of these species is usually studied by UV-Vis absorption spectroscopy with the complication of solvent effects.  We have developed a technique to study the intrinsic electronic structures of solution phase anions outside the solution environment.  Electrospray ionization is used to transport anions from a solution sample to high vacuum, where they are interrogated by photoelectron spectroscopy.  This technique has allowed isolated multiply charged anions to be studied spectroscopically for the first time.

A second generation apparatus has been developed, in which ion temperatures can be controlled from 10 K to 350 K.  Cold anions give rise to better spectral resolution by eliminating vibrational hot bands, whereas temperature-dependent studies can yield information about conformation dynamics and isomer populations for weakly bonded complexes.

A photoelectron imaging system has been constructed and interfaced with our electrospray ion source.  Compared with the magnetic-bottle electron analyzer, the imaging method has the advantage of yielding angular distributions, as well as high electron collection efficiency.   Using photoelectron imaging, we have studied how the intramolecular Coulomb repulsion influences the outgoing electron trajectories.

 

 

Current Research Projects:

1.  Probing the unique properties of free multiply charged anions (return to top of page)

Multiply charged anions are common in the condensed phase and their existence has been generally taken for granted.   However, free multiply charged anions are fragile because of the strong intramolecular Coulomb repulsion and they are difficult to study in the gas phase.  We pioneered the experimental technique, using photodetachment photoelectron spectroscopy, electrospray, and ion-trap mass spectrometry, to probe free multiply charged anions in the gas phase.  Photodetachment photoelectron spectroscopy is ideal to probe the chemical and physical properties of free multiply charged anions.  Among the initial findings, the repulsive Coulomb barriers existing in multiply charged anions is directly observed.  The relationship between the intramolecular electron-electron repulsion and the potential barrier is elucidated, culminating in the first observation of a negative electron binding energy in a quadruply charged anion.  Electron tunneling effects through the repulsive Coulomb barriers are also observed and interpreted using a model Coulomb potential and the theory developed to understand alpha-decay in nuclear physics.  We are interested in the stability and dynamics of multiply charged anions and electrostatic interactions within complex anions.

Selected recent publications:

“Photodetachment Spectroscopy of A Doubly Charged Anion: Direct Observation of the Repulsive Coulomb Barrier”, (X. B. Wang, C. F. Ding, and L. S. Wang), Phys. Rev. Lett. 81, 3351-3354 (1998).

“Electron Tunneling through the Repulsive Coulomb Barrier in Photodetachment of Multiply Charged Anions” (X. B. Wang, C. F. Ding, and L. S. Wang), Chem. Phys. Lett. 307, 391-396 (1999).

“Observation of Negative Electron-Binding Energy in a Molecule” (X. B. Wang and L. S. Wang), Nature 400, 245-248 (1999).

“Probing Free Multiply Charged Anions Using Photodetachment Photoelectron Spectroscopy” (L. S. Wang and X. B. Wang), J. Phys. Chem. A 104, 1978-1990 (2000).

"Direct Experimental Probe of the Onsite Coulomb Repulsion in the Doubly Charged Fullerene Anion C702-" (X. B. Wang, H. K. Woo, X. Huang, M. M. Kappes, and L. S. Wang), Phys. Rev. Lett. 96, 143002-1-4 (2006).

“Negative Electron Binding Energies Observed in a Triply Charged Anion: Photoelectron Spectroscopy of 1-Hydroxy-3,6,8-Pyrene-Trisulfonate (HPTS3–)” (J. Yang, X. P. Xing, X. B. Wang, L. S. Wang, A. P. Sergeeva, and A. I. Boldyrev), J. Chem. Phys. 128, 091102-1-4 (2008).

“Probing the Electronic Stability of Multiply Charged Anions: Sulfonated Pyrene Tri- and Tetra-Anions” (X. B. Wang, A. P. Sergeeva, X. P. Xing, M. Massaouti, T. Karpuschkin, O. Hampe, A. I. Boldyrev, M. M. Kappes, and L. S. Wang), J. Am. Chem. Soc. 131, 9836-9842 (2009).

“Photoelectron Spectroscopy of Multiply Charged Anions” (X. B. Wang and L. S. Wang), Annu. Rev. Phys. Chem. 60, 105-126 (2009).

 

2.  Probing the electronic structure of metal complexes and redox species (return to top of page)

We are interested in studying the intrinsic electronic structures of inorganic metal complexes, in particular, redox speices.  The gas phase data can be used directly to compare with theoretical calculations.  For redox species, the intrinsic reorganization energies can be directly obtained.   Biologically-relevant molecules, such as Fe-S clusters and nucleotides are also of interest.

Selected recent publications:

“Photodetachment of Free Hexahalogenometallate Doubly Charged Anions in the Gas Phase: [ML6]2, (M = Re, Os, Ir, Pt; L = Cl and Br)” (X. B. Wang and L. S. Wang), J. Chem. Phys. 111, 4497-4509 (1999).

“Photodetachment of Multiply Charged Anions — The Electronic Structure of Gaseous Square-Planar Transition Metal Complexes PtX42 (X = Cl, Br)” (X. B. Wang and L. S. Wang), J. Am. Chem. Soc. 122, 2339-2345 (2000).

“Probing the Electronic Structure and Metal-Metal Bond of Re2Cl82 in the Gas Phase” (X. B. Wang and L. S. Wang), J. Am. Chem. Soc. 122, 2096-2100 (2000).

“Probing the Electronic Structure of Redox Species and Direct Determination of Intrinsic Reorganization Energies of Electron Transfer Reactions” (X. B. Wang and L. S. Wang), J. Chem. Phys. 112, 6959-6962 (2000).

“Probing the Intrinsic Electronic Structure of the Cubane [4Fe-4S] Cluster: Nature’s Favorite Cluster for Electron Transfer and Storage” (X. B. Wang, S. Niu, X. Yang, S. K. Ibrahim, C. J. Pickett, T. Ichiye, and L. S. Wang), J. Am. Chem. Soc. 125, 14072-14081 (2003).

“Photoelectron Spectroscopy of the Doubly-Charged Anions [MIVO(mnt)2]2 (M = Mo, W; mnt = S2C2(CN)22).  Access to the Ground and Excited States of the [MVO(mnt)2]- Anion”  (T. Waters, X. B. Wang, X. Yang, L. Zhang, R. A. J. O'Hair, L. S. Wang, and A. G. Wedd), J. Am. Chem. Soc. 126, 5119-5129 (2004).

“Direct Experimental Observation of the Low Ionization Potentials of Guanine in Free Oligonucleotides Using Photoelectron Spectroscopy” (X. Yang, X. B. Wang, E. R. Vorpagel, and L. S. Wang), Proc. Natl. Acad. Sci. (USA) 101, 17588-17592 (2004).

 “De novo Synthesis of the H-Cluster Framework of Iron-Only Hydrogenase” (C. Tard, X. Liu, S. K. Ibrahim, M. Bruschi, L. D. Gioia, S. Davies, X. Yang, L. S. Wang, and C. J. Pickett), Nature 433, 610-613 (2005).

“Probing the Intrinsic Electronic Structure of the bis(dithiolene) Anions [M(mnt)2]2 and [M(mnt)2]1 (M = Ni, Pd, Pt; mnt = 1,2-S2C2(CN)2) in the Gas Phase Using Photoelectron Spectroscopy” (T. Waters, H. K. Woo, X. B. Wang, and L. S. Wang), J. Am. Chem. Soc. 128, 4282-4291 (2006).

“Electrospray Ionization Photoelectron Spectroscopy: Probing the Electronic Structure of Inorganic Metal Complexes in the Gas Phase” (T. Waters, X. B. Wang, and L. S. Wang), Coord. Chem. Rev. 251, 474-491 (2007).

“Evidence of Significant Covalent Bonding in Au(CN)2” (X. B. Wang, Y. L. Wang, J. Yang, X. P. Xing, J. Li, and L. S. Wang), J. Am. Chem. Soc. 131, 16368-16370 (2009).

“Covalent Gold” (L. S. Wang), Phys. Chem. Chem. Phys. 12, xxx (2010)

 

3.  Solvation and solvent stabilization of complex and multiply charged anions (return to top of page)

Many textbook multiply charged anions, such as SO42– or CO32–, are not stable in the gas phase due to the tremendous intramolecular Coulomb repulsion.  But they are stabilized in solution by the solvent.  We are interested in solvation effects and solvent stabilization of multiply charged anions.  For example, we have found that both SO42– and C2O42– require a minimum of three water molecules to be stabilized in the gas phase.  Solvated clusters provide molecular models to understand solvation in the bulk.  We have developed a low temperature apparatus, which allows us to probe conformation changes and isomer populations as a function of temperature.

 

 

Selected recent publications:

“Electronic Instability of Isolated SO42 and Its Solvation Stabilization”, (X. B. Wang, J. B. Nicholas, and L. S. Wang), J. Chem. Phys. 113, 10837-10840 (2000).

“Bulk-Like Features in the Photoemission Spectra of Hydrated Doubly-Charged Anion Clusters" (X. B. Wang, X. Yang, J. B. Nicholas, and L. S. Wang), Science 294, 1322-1325 (2001).

“Probing Solution Phase Species and Chemistry in the Gas Phase” (X. B. Wang, X. Yang, and L. S. Wang), Int. Rev. Phys. Chem. 21, 473-498 (2002).

“Photodetachment of Hydrated Oxalate Dianions in the Gas Phase, C2O42(H2O)n (n = 340) – From Solvated Clusters to Nano Droplet” (X. B. Wang, X. Yang, J. B. Nicholas, and L. S. Wang), J. Chem. Phys. 119, 3631-3640 (2003).

“Solvent-Mediated Folding of A Doubly Charged Anion” (X. Yang, Y. J. Fu, X. B. Wang, P. Slavicek, M. Mucha, P. Jungwirth, and L. S. Wang), J. Am. Chem. Soc. 126, 876-883 (2004).

"Observation of Weak C-H...O Hydrogen-Bonding by Unactivated Alkanes” (X. B. Wang, H. K. Woo, B. Kiran, and L. S. Wang), Angew. Chem. Int. Ed. 44, 4968-4972 (2005)

First Steps Towards Dissolution of NaSO4 by Water” (X. B. Wang, H. K. Woo, B. Jagoda-Cwiklik, P. Jungwirth, and L. S. Wang), Phys. Chem. Chem. Phys. 8, 4294-4296 (2006).

"Observation of Cysteine Thiolate and S…H-O Intramolecular Hydrogen Bond” (H. K. Woo, K. C. Lau, X. B. Wang, and L. S. Wang), J. Phys. Chem. A 110, 12603-12606 (2006).

Microsolvation of the Dicyanamide Anion: [N(CN)2](H2O)n (n = 0–12)” (B. Jagoda-Cwiklik, X. B. Wang, H. K. Woo, J. Yang, G. J. Wang, M. F. Zhou, P. Jungwirth, and L. S. Wang), J. Phys. Chem. A 111, 7719-7725 (2007).

“Observation of Entropic Effect on Conformation Changes of Complex Systems under Well-Controlled Temperature Condition” (X. B. Wang, J. Yang, and L. S. Wang), J. Phys. Chem. A 112, 172-175 (2008).

“Photoelectron Spectroscopy of Cold Hydrated Sulfate Clusters, SO42(H2O)n (n = 4–7): Temperature-Dependent Isomer Populations” (X. B. Wang, A. P. Sergeeva, J. Yang, X. P. Xing, A. I. Boldyrev, and L. S. Wang), J. Phys. Chem. A 113, 5567-5576 (2009).

“Stepwise Hydration of the Cynaide Anion: A Temperature-Controlled Photoelectron Spectroscopy and Ab Initio Computational Study of CN(H2O)n (n = 25)” (X. B. Wang, K. Kowalski, L. S. Wang, and S. S. Xantheas), J. Chem. Phys. 132, 124306-1-10 (2010).

 

4.  Photoelectron imaging of multiply charged anions (return to top of page)

The excess charges in multiply charged anions impart many unique properties to this class of important ions in the gas phase because of the strong intramolecular Coulomb repulsion.  The intramolecular Coulomb repulsion makes many multiply charged anions unstable as isolated species, yet it produces a repulsive Coulomb barrier that gives dynamic stability to these species, even leading to the observation of negative electron binding energies.  The intramolecular Coulomb repulsion is also expected to have significant influence on the dynamics of the outgoing electrons.  We have constructed a photoelectron imaging system, which allows the angular distributions of photoelectrons to be recorded.  We have observed in a series of linear dicarboxylate dianions, , O2C(CH2)nCO2 (n = 3–11), that photoelectrons are preferentially emitted along the axis of the dianions because of the Coulomb repulsion from the remaining charge on the opposite end of the dianions.  The imaging detector is more sensitive to slow electrons.  We are interested in taking advantage of this sensitivity to obtain high resolution photoelectron spectra for low energy electrons and for singly charged anions.

 

Selected recent publications:

“Imaging Intramolecular Coulomb Repulsions in Multiply Charged Anions” (X. P. Xing, X. B. Wang, and L. S. Wang), Phys. Rev. Lett. 101, 083003-1-4 (2008).

“Photoelectron Angular Distribution and Molecular Structure in Multiply Charged Anions” (X. P. Xing, X. B. Wang, and L. S. Wang), J. Phys. Chem. A 113, 945-948 (2009).

“Photoelectron Imaging of Multiply Charged Anions: Effects of Intramolecular Coulomb Repulsion and Photoelectron Kinetic Energies on Photoelectron Angular Distributions” (X. P. Xing, X. B. Wang, and L. S. Wang), J. Chem. Phys. 130, 074301 (1-6) (2009).

“Photoelectron Imaging of Doubly Charged Anions, O2C(CH2)nCO2 (n = 2–8): Observation of Near Zero-eV Electrons due to Secondary Dissociative Autodetachment” (X. P. Xing, X. B. Wang, and L. S. Wang), J. Phys. Chem. A 114, 4524-4530 (2010).