Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

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Chemical Kinetics and Dynamics at Interfaces Cluster Model Investigation of Condensed Phase Phenomena Xue-Bin Wang Chemical & Materials Sciences Division, Pacific Northwest National Laboratory, P.O. Box 999, MS K8- 88, Richland, WA 99352. E-mail: xuebin.wang@pnnl.gov Program Scope This program is aimed at obtaining a microscopic understanding of solution chemistry and condensed phase phenomena using gas phase clusters as model systems. Ionic and molecular (including hydrated) clusters with molecular specificity, selected sizes, and controlled compositions are ideal model systems to understand condensed phases processes and obtain molecular-level understanding of environmental materials, solution chemistry, atmospheric aerosols, and biological functions involving these species. Our primary experimental technique is a cold and temperature-controlled photoelectron spectroscopy (PES) coupled with electrospray ionization (ESI), which is used to produce complex anions, including multiplycharged anions, and solvated clusters with multiple compositions from solution samples. The precise control of ion temperatures in the range from 10 K to room temperature has facilitated not only the enhancement of the photoelectron spectroscopic resolution and precision, but also the investigation of new temperature-dependent phenomena. Experiments and ab initio calculations are synergistically combined to • Obtain a molecular-level understanding of the solvation and stabilization of complex singly- and multiply-charged anions important in condensed phases • Probe ion specific interactions with biological implications • Study temperature-dependent conformation changes and isomer populations of complex solvated clusters • Understand the molecular processes and initial steps of dissolution of salt molecules in polar solvents • Investigate intrinsic electronic structures of environmentally and catalytically important species and reactive diradicals. The central theme of this research program lies at obtaining a fundamental understanding of environmental materials and solution chemistry important to many primary DOE missions (waste storage, subsurface and atmospheric contaminant transport, catalysis, etc.), and enhances scientific synergies between experimental and theoretical studies towards achieving such goals. Recent Progress Study of Ion Specific Interactions of Alkali Cations with Dicarboxylate Dianions. Alkali metal cations (M + ) often show pronounced ion specific interactions and selectivity with macromolecules in biological processes, colloids, and interfacial sciences, but a fundamental understanding about the underlying microscopic mechanism is still very limited. The ion-pair interaction between M + and carboxylate has been a particular topic of great interest, partly due to the important biological implications of the remarkable metal selectivity (Na + versus K + ion channels) present in normal functioning of life. We reported a direct probe of interactions between M + and dicarboxylate dianions, – O2C(CH2)nCO2 – (Dn 2- ) in the gas phase by combined PES and ab initio electronic structure calculations on nine M + –Dn 2- complexes (M = Li, Na, K; n = 2, 4, 6). PES spectra show that the electron binding energy (EBE) decreases in the 199
order of Li + > Na + > K + for complexes of M + –D2 2- , whereas the order is changed to Li + < Na + ≈ K + when M + interacts with a more flexible D6 2- dianion. Theoretical modeling suggests that M + prefers to interact with both ends of the carboxylate –COO – groups by bending the flexible aliphatic backbone, and the local binding environments are found to depend upon backbone length n, carboxylate orientation, and the specific cation M + . The observed variance of EBEs reflects how well each specific dicarboxylate dianion accommodates each M + . This work demonstrates the delicate interplay among several factors (electrostatic interaction, size matching, and strain energy) that play critical roles in determining the structures and energetics of gaseous clusters as well as ion specificity and selectivity in solutions and biological systems. Probing Electronic Structures of Diradicals: meta-Benzoquinone and (CO)4. Electron affinities (EAs) and electronic structures of benzoquinone molecules (BQs) are crucial information in understanding a wide range of applications involving these molecules from biological photosynthesis to energy conversion processes. We carried out a systematic spectroscopic probe on the electronic structures and EAs of all three isomers (o-, m-, and p-BQs) employing PES and ab initio electronic structure calculations. Similar spectral pattern was observed in the o- and p-BQ ●– spectra, each revealing a broad ground state feature and a large band gap, followed by well-resolved excited states peaks. In contrast, the spectrum of m-BQ ●– is distinctly different from its two congeners, with no clear band gap and a much higher EA. Accompanying theoretical calculations confirmed experimental EAs and band gaps, and further unraveled a triplet ground state for m-BQ in contrast to singlet ground states for o- and p- isomers. The diradical nature of m-BQ, consistent with its non-Kekulé structure, is primarily responsible for the observed high EA and also explains its nonexistence in bulk materials. On the other hand, despite a seemingly simple and Kekulé appearance, cyclobutanetetraone (C4O4) has been predicted to have a very low-lying triplet state among four low-lying electronic states. Determining the energetic ordering of these states and the ground state of C4O4 – theoretically has been proven to be considerably challenging. The most reliable theoretical calculations by Borden and co-workers predicted surprisingly a triplet ground state for (CO)4. But such prediction remained to be confirmed experimentally. We recently conducted a low temperature (20K) PES approach to obtain electronic structure of (CO)4 molecule. Well resolved spectra were obtained at both 193 and 266 nm. Combined with the theoretical studies by Borden and co-workers, our PES study reveals that (CO)4 indeed has a triplet ground state with diradical character. The two low-lying excited states of C4O4 are also determined to be 1 A1g (8π), and 1 B2u with the term values of (6.27 ± 0.5 kJ/mol) and 1 B2u (13.50 ± 0.5 kJ/mol), respectively. The obtained spectroscopic information is valuable to benchmark high-level ab initio methods. Hydrogen Bonded Arrays: The Power of Multiple Hydrogen Bonds. Many enzymes catalyze a wide variety of chemical processes by using two or even three hydrogen bonds to a single oxygen atom in what is commonly referred to as an oxyanion hole. The energies of these hydrogen bonding interactions are important in catalysis but are not well understood. In collaboration with Kass and co-workers at University of Minnesota, we used PES to directly probe the energetic consequences of hydrogen bond arrays in small covalently bound model compounds, and the experimental results are compared to computational predictions. We find that in a series of monodeprotonated polyhydroxyalcohols (i.e., polyols) the strengths of the hydrogen bond arrays systematically increase with the number of hydroxyl groups from 1 to 3 and for 6. That is, an oxygen anion center is stabilized most effectively by up to 3 hydrogen bond donors, but in the process the donor groups become better hydrogen bond acceptors. The resulting hydrogen-bonded network can provide a large energetic stabilization that may lead to catalytic rate enhancements and greater acidities and basicities than those measured in water. Photoelectron Spectroscopic Investigation on Atmospherically Important Halogen Oxide Radicals: BrO2,3 and IO2-4. Higher halogen oxides (XOn, n ≥ 2) are important species involved in the ozone 200
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Eighth Condensed Phase and Interfac
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Potential energy landscape of the (
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Agenda
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Session III Chair: Jay A. LaVerne,
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Table of Contents
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Transition Metal-Molecular Interact
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Ultrafast Electron Transport across
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CPIMS Principal Investigator Abstra
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VUV LDPI image of a polymer photore
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Publications based on DOE support (
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entirely quenched, presumably due t
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Future Plans In addition to the TiO
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The relative path indexes of the ur
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We have recently carried out molecu
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have been obtained with γ-rays sug
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Two novel plasma devices have been
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(Figure 2) can be adsorbed exclusiv
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3. Future Plans Our planned work de
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Figure 1: Snap shots from molecular
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6. Varilly, P., A. J. Patel and D.
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hypothesis that delocalized charges
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Efforts will be made to observe ele
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Photochemistry and Solvation Dynami
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complementary to work we have carri
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The Co3O4 films for the optical pum
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chamber will allow us to heat the s
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calculations. Additional PMF calcul
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the gas solute first needs to cross
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laser system, which is an infrared
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Publications (10/1/2009-present) fo
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interest are: (i) What is the diffu
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and anilino groups attached to the
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had been formed. This coincides wit
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Figure 1. (A) Simplified bR photocy
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Figure 4. Photocurrent density of b
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single-file diffusion and cannot ca
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PUBLICATIONS SUPPORTED BY USDOE CHE
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arrangement. However, the hydrogen
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(5) “Water Dynamics in Salt Solut
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from high- energy x-ray diffraction
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concentrated aqueous solutions. The
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molecule’s center of mass, or alt
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Moreover, SHG studies by Saykally
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The effective fragment potential (E
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10. D.D. Kemp, J. Rintelman, M.S. G
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temperature dependent, equilibrium
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mimicking real organic photovoltaic
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100 kHz amplifier, SE-FSRS was succ
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photovoltaics will be explored via
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for laser surface reactions to thes
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This is a significant advantage of
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the STM, which might underlie the u
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Future Plans: Figure 3. One and two
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methane molecule and a lattice atom
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eactive than the Ni, experiment sug
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Fig. 1. PE spectra of M3Oy � (M =
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each of the intermediates obtained
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a regime where each solvent molecul
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observables that arise from subtle
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total potentials. One can see from
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Our calculations have shown that ch
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our understanding of structure and
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viscosity, η, are inversely relate
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photodissociation of the metal - NO
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from a Ru 2p3/2 orbital to an orbit
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Integrated O 2 PSD (ML) Integrated
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anisotropy in the structure of the
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XPS studies have been performed on
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Publications with BES support since
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MLCT (metal to ligand charge transf
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molecule-metal interface without a
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observe the predicted effects, whic
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(4*) Lymar, S. V.; Schwarz, H. A. J
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(τ .820 ns), but we have not been
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B. Photodissociation spectroscopy o
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oth BLYP and PBE. Our results have
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Publications from BES support (2010
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understanding of factors that contr
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Our talk will discussion our initia
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Figure 1 shows the band structure o
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likely to also occur in low coordin
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Page 167 and 168: method recently developed by Prende
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Page 173 and 174: "Probing the Hydration Structure of
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Page 177 and 178: 10) C. Zhang, M.E. Grass, A.H. McDa
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Page 185 and 186: Selected publications acknowledging
Page 187 and 188: 2. Optical Stark study of PtF (Acce
Page 189 and 190: Figure 4. The Q(3.5) line of the [1
Page 191 and 192: In marked contrast to the selective
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Page 197 and 198: approach on a model phenol-amine pr
Page 199 and 200: We performed pump-probe experiments
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Page 203 and 204: may be initially produced in a non-
Page 205 and 206: 4. Inelastic scattering of hydrogen
Page 207 and 208: Using QM/MM free energy methods we
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Page 217 and 218: 2. MM Meyer, XB Wang, CA Reed, LS W
Page 219 and 220: Technical Approach and Progress to
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Page 225 and 226: Studies of structure and reaction d
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Page 229 and 230: 9. S. Pandelov, J. C. Werhahn, B. M
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Page 233 and 234: Figure 1 (Left) The HOMO and LUMO e
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Page 237 and 238: Participant List Dr. Musahid Ahmed
Page 239 and 240: Professor Kenneth Eisenthal Columbi
Page 241 and 242: Dr. Ireneusz Janik Notre Dame Radia
Page 243 and 244: Professor Eric Potma University of
Page 245: Dr. James Wishart Brookhaven Nation
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Eighth Condensed Phase and Interfacial Molecular Science (CPIMS)

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