Frontiers in Chemical Physics & Analysis
2011
Douglas Doren
Douglas Doren
Professor, Chemistry and Biochemistry
Associate Dean, Arts and Sciences
University of Delaware
Tuesday, May 17, 2011
EMSL Auditorium
10:00AM
High temperature aqueous solutions are central to many natural and industrial processes but their properties can only be measured accurately over a limited range of temperature and density. I will describe a computational approach that allows simulations of thermodynamic properties of such systems from first principles. This approach is a QM/MM method that combines configurational averaging from molecular dynamics or Monte Carlo simulations with accurate quantum chemistry calculations of energies. The method was initially developed for understanding solvation in high temperature water, and it is especially efficient in such systems. Applications to supercritical water and solvation of sodium chloride at high temperatures will be described, followed by more recent work on low-temperature aqueous aerosols. Several comparisons to experiment as well as internal validation methods confirm the reliability of the approach. In conditions where accurate experimental measurements are not possible, this computational method provides a new source of reliable data.
George Schatz
George Schatz
"Plasmon-enhanced Optical Phenomena"
Editor-in-Chief, Journal of Chemical Physics
Morrison Professor of Chemistry, Northwestern University
Monday, March 14, 2011
EMSL Auditorium
11:00AM
Silver and gold nanoparticles have strong absorption and scattering in the visible and near-infrared as a result of plasmon excitation. The optical properties of these particles can be tuned by varying nanoparticle size (in the few nm to few hundred nm range) and shape, and generally these properties can be effectively modeled using classical electromagnetic theory. However, there are aspects of these optical properties where the classical picture fails, and then it is necessary to incorporate quantum effects into the theoretical description. This talk describes our latest work with understanding surface-enhanced Raman scattering from silver and gold nanoparticles and nanostructures, with emphasis on data from the Van Duyne and Mirkin groups concerned with dimer structures that have a small gap (~1 nm) between 100 nm particles. I will also describe the use of silver-coated nanoparticles for plasmon enhancement in dye-sensitized solar cells.
Einar Uggerud
Einar Uggerud
"Structural Rearrangements and Proton Transfer within Water Clusters of Atmospheric Relevance"
Head of Mass Spectrometry Lab
University of Oslo
Friday, February 25, 2011
EMSL Auditorium
11:00AM
Hydrogen/deuterium exchange induced in reactions of various protonated or deprotonated water clusters, (X)(Y)(H2O)n (1 ≤ n≤ 30; containing ammonia, pyridine and/or hydrogen sulfate) with heavy water has been studied experimentally at near-thermal collision energies. The interpretation has been facilitated by quantum chemical calculations, including extensive Born-Oppenheimer molecular dynamics simulations. Lifetimes of the collision complexes have been estimated using RRKM theory. The experiments reveal unique and surprising details of the structure and dynamics of water clusters. For a given cluster size, both the total cross-section for isotope exchange and the relative amount of single and double substitution of H by D depends on the chemical properties of the core molecules X and Y. The experimental observations can be rationalized on the basis of the basicity and solubility of the core molecules X and Y. It appears that single H/D exchange is proton catalyzed, while double substitution has contributions from this mechanism as well as from ligand exchange. The tendency for a proton to migrate within a water cluster is determined by a fine balance between the relative proton affinities of the various basic sites. Solute molecules may slow down proton migration by having high proton affinity. On the other hand, water solvation and local hydrogen bond patterns may modify the proton affinity of a given site significantly.
Thom H. Dunning Jr, Ph.D.
Thom H. Dunning, Jr, Ph.D.
"Main Group Chemistry Beyond First Row: The Remarkable Chemistry of the Late p-Block Elements"
Director, National Center for Supercomputing Applications;
Professor, Distinguished Chair for Research Excellence in Chemistry
Thursday, February 24, 2011
EMSL Auditorium
2:30PM
Chemists have employed computational modeling to understand the structures, spectra, energetics and reactivities of the first row elements (Li-F) with great success. But, it has long been recognized that the chemistry of the main group elements in the second and subsequent rows of the p-block of the Periodic Table differs dramatically from that of their first row counterparts. This "first row anomaly" is well recognized in inorganic chemistry, and many reasons have been advanced to explain the difference.
One of the most striking differences between the first and subsequent row elements is the ability of the latter row elements to "expand their valence shell," forming hypervalent molecules such as PCl5, SF4/SF6 and ClF3/ClF5. Recent studies of the fluorides of phosphorus, sulfur and chlorine revealed that (i) hypervalency was the result of a new type of bonding, recoupled pair bonding, that results when a pair of electrons in a lone pair orbital are recoupled to form a bond with a ligand using one of the electrons in the pair, (ii) recoupled pair bonding is common in non-hypervalent molecules, e.g., recoupled pair bonding leads to bound, low-lying excited 4S-, 2S- and 2D states in SF and the 3B1 and 3A2 excited states in SF2, and (iii) recoupled pair bonding has a dramatic effect on the structure, spectra and energetics of the XFn species The ability of electronegative ligands to recouple the electrons in the lone pair orbitals of second and higher row late p-block elements is a major factor in the anomalous behavior of these elements, including their unusual reactivities.
The seminar will introduce the basic features of recoupled pair bonding and illustrate how recoupled pair bonding affects the structure, spectra and bond energies of second row compounds.
Richard Haglund
Professor Richard Haglund
Research Highlight: Molecular Magician
Department of Physics and Astronomy
Vanderbilt University
Thursday, February 3, 2011
EMSL Auditorium
11:00AM
Vanadium dioxide, first synthesized in bulk crystalline form half a century ago, undergoes a first-order phase transition from a semiconductor to a metal at approximately 70°C that can be triggered electrically, optically or thermally. The mechanism of the phase transition appears to involve both electronic (metal-insulator) and structural (monoclinic-to-tetragonal) components, and the detailed dynamics of the transition depends crucially on the mode of excitation. Recent experiments with time (length) resolution down to tens of femtoseconds(nanometers) are opening up a truly microscopic understanding of the ways in which materials synthesis, sample morphology and dimensionality are interconnected with the kinetics and dynamics of the phase transformation. This understanding, in turn, is opening up interesting applications of vanadium dioxide in plasmonics, metamaterialsand silicon photonics.
Konrad Thürmer, Ph.D.
Konrad Thürmer, Ph.D.
"Deciphering the morphology of ice films on metal surfaces"
Research Highlight: Molecular-level Insights into the Formation of Ice
Material Physics Department
Sandia National Laboratories
Thursday, January 27, 2011
EMSL Auditorium
2:15PM
Although extensive research has been aimed at the structure of ice films, questions regarding basic processes that govern film evolution remain. Recently we discovered how ice films as many as 30 molecular layers thick can be imaged with STM. The observed morphology yields insights about water-solid interactions and how they affect the structure of ice films. This talk gives an overview of this progress for crystalline ice films on Pt(111). STM reveals a first molecular water layer very different from bulk ice: besides the usual hexagons, it also contains pentagons and heptagons. Slightly thicker films (~1 nm, at T>120K) are comprised of ~3-nm-high crystallites, surrounded by the one-molecule-thick wetting layer. These crystals dewet by nucleating layers on their top facets. Measurements of the nucleation rate as a function of crystal height provide estimates of the energy of the ice-Pt interface. For T>115K surface diffusion is fast enough that surface smoothing and 2D-island ripening is observable. By quantifying the T-dependent ripening of island arrays, we determined the activation energy for surface self-diffusion. The shape of these 2D islands varies strongly with film thickness. We attribute this to a transition from polarized ice at the substrate towards proton disorder at larger film thicknesses. Despite fast surface diffusion ice multilayers are often far from equilibrium. For example, ice grows between ~120 and ~160 K in its cubic variant rather than in its equilibrium hexagonal form. We found this to be a consequence of the mismatch in the atomic Pt-step height and the ice-bilayer separation and propose a mechanism of cubic-ice formation via growth spirals around screw dislocations.