Frontiers in Chemical Physics & Analysis
2012
Welch Professor, Department of Chemistry & Biochemistry
Texas Tech University
Tuesday, November 6, 2012
BSF Crick
10:00AM
Direct dynamics have made the application of classical chemical dynamics simulations possible for a broad range of problems. However, since classical dynamics is approximate it is important to understand when the use of classical dynamics is appropriate. In this talk the following will be addressed: (1) the methodology of direct dynamics simulations; (2) accuracy of classical dynamics simulations, with emphasis on unimolecular and intramolecular dynamics; (3) post-transition state dynamics; and (4) gas-phase SN2 reaction dynamics. The VENUS/NWChem software package is used for the simulations and access to high-performance computing is critical for the research.
"It's All About Energy!. The Impact of Theory and Computation"
Professor, Chemistry and Physics
University of California-Davis
Tuesday, September 25, 2012
EMSL Auditorium
10:00AM
Professor Galli will discuss problems involved in understanding and controlling energy conversion processes, including photo-electrochemical and thermoelectric conversion. Using recent examples from ab initio and atomistic simulations done by her and her team at the University of California-Davis, she will discuss two intertwined issues:
- What is the impact of theory and computation?
- How do we take up the challenge of building much needed tighter connections between theory and experiment?
"Water on a Knife-Edge: The Subtle Balance of Forces in Clusters, Ice and Liquid"
London Centre for Nanotechnology & Thomas Young Centre
University College London, UK
Friday, April 27, 2012
EMSL Auditorium
9:30AM
Water has probably been studied more comprehensively than any other substance, but its molecular-scale energetics remains surprisingly elusive. Parameterized interaction models, including some developed at PNNL, can be remarkably successful. But widely used electronic-structure methods based on conventional density-functional theory (DFT) struggle to reproduce the properties of water clusters, ice structures and the bulk liquid, for reasons that are still controversial. I will summarize some new approaches that we are pursuing at UCL in collaboration with colleagues at Cambridge and Bristol, focusing particularly on our recent work with quantum Monte Carlo (QMC) [1] and Gaussian Approximation Potentials (GAP) [2]. I will show that QMC is much more accurate than DFT for the energetics of clusters [3] and ice structures [4], and that it can also supply useful benchmarks for statistical samples of configurations of the bulk liquid. We are using QMC and correlated quantum chemistry techniques to analyze the sources of error in DFT approximations and to quantify the accuracy of GAP corrections to DFT. It is becoming clear from this work that conventional DFTs have a hard time describing water systems because they misrepresent the subtle balance between 2-body (dispersion) and beyond-2-body (polarization) parts of the energy.
"Influence of Wet Electron States on the Water-Oxygen Surface Chemistry of TiO2."
London Centre for Nanotechnology & Thomas Young Centre
University College London, UK
Wednesday, April 25, 2012
EMSL Auditorium
11:00AM
Oxygen vacancies on metal oxide surfaces have long been thought to play a key role in the surface chemistry. For the (110) surface of TiO2, this view was recently challenged by work which suggested that a quasiparticle bandgap state involved in the reactivity arises from near-surface Ti interstials. Here I describe some recent experiments that tested this idea. We find that vacancies provide the dominant contribution to the band gap state. As for the reaction with molecules, such processes have been directly visualised in the case of the model photocatalyst surface TiO2(110) in reactions with water and dioxygen. These vacancies have been assumed to be neutral in calculations of the surface properties. However, by comparing experimental and simulated scanning tunneling microscopy images and spectra, we show that oxygen vacancies act as trapping centres and are negatively charged. We demonstrate that charging the defect significantly affects the reactivity by following the reaction of dioxygen with surface hydroxyl formed by water dissociation at the vacancies. Calculations with charged hydroxyl favour a condensation reaction forming water and surface oxygen adatoms, in line with experimental observations. This contrasts with simulations using neutral hydroxyl where hydrogen peroxide is found to be the most stable product. An interesting question remains as to the relevance of these results to the liquid water—TiO2 interface. Surface X-ray diffraction measurements after dipping the (110) surface in water reveal that the surface is hydroxylated in the first layer of the interface, with ordering of water in the second layer. This is in contrast to the results of recent calculations of the perfect surface, which suggest no dissociation at the liquid/TiO2(110) interface.
Professor David Manolopoulos
"Competing Quantum Effects in Liquid Water"
Physical and Theoretical Chemistry Laboratory
The University of Oxford
Tuesday, March 20, 2012
EMSL Auditorium
9:00AM
I will begin this talk with a brief review of the ring polymer molecular dynamics (RPMD) method for condensed phase quantum dynamics. I will then use this method to investigate the properties of a flexible water model that has been parameterized to agree with a wide variety of experimental measurements in quantum mechanical (path integral) simulations. I shall show in particular that there is a competition between two opposing quantum mechanical effects in the dynamics of the room temperature liquid. The zero point motion in the intramolecular OH stretch increases the average OH bond length and the average dipole moment of the water molecules, leading to stronger intermolecular interactions and a more viscous liquid. However, this is offset by the zero point motion in the intermolecular modes, which disrupts the hydrogen bonding network and makes the liquid more labile again. The net result is that there is only a very small overall quantum mechanical effect in the self-diffusion and orientational relaxation of the liquid. If time permits at the end of the talk, I shall go on to discuss some related studies by other groups that support this picture.
Professor Takayuki Ebata
Department of Chemistry
Graduate School of Science
Hiroshima University, Japan
Thursday, March 1, 2012
EMSL Auditorium
11:00AM
Laser spectroscopic and theoretical study will be presented for elucidating the structures and complexation mechanism of gas phase host-guest complexes formed in supersonic jets and electrospray ionization cold 22 ion-trap. The examined hosts include crown ethers and calix[4]arene, and for the guest species various neutral molecules and alkali metal cations are chosen. We measure the electronic spectra by laser-induced fluorescence (LIF), mass-selected resonance enhanced multiphoton ionization (REMPI) and ultraviolet-ultraviolet hole-burning (UV-UV HB) spectroscopy. The vibrational spectra are measured by infrared-ultraviolet double resonance (IR-UV DR) and fragment detected infrared photodissociation (IRPD) spectroscopy. For the ionic complexes, ultraviolet photodissociation (UVPD) and IR-UV DR spectroscopy has been applied. The obtained results are analyzed by density functional theory and first principles electronic structure calculations. We discuss how the host molecule changes its conformation or which conformer is preferred for forming stable encapsulation complex as well as the key interactions, leading to the molecular recognition.
Michiel Sprik
Professor Michiel Sprik
"Density Functional Theory Modeling of the Metal Oxide Water Interface"
Department of Chemistry
University of Cambridge
Monday, February 27, 2012
EMSL Auditorium
10:00AM
The interface between a metal oxide and an aqueous solution is a complex environment controlled by the exchange of charge between solid and electrolyte. Coordination with the positive metal ions increases the acidity of adsorbed water which at high pH leads to deprotonation and the buildup of negative charge. Alternatively basic oxygen sites on the surface can become protonated at low pH. Metal oxides can also exchange electronic charge with the electrolyte if the metal ions are redox active. Examples are transition metal ions. Modeling these processes is a major challenge for electronic structure calculation. In this talk, Professor Sprik will outline the DFT-based molecular dynamics methodology that was developed to meet this challenge using the calculation of the pH of zero proton charge, the potential at zero electronic charge and electric double layer capacitance of a small set of structurally related semiconductor oxides as validation. The key tool in the approach is a scheme for a molecular dynamics normal hydrogen electrode. As an application, Professor Sprik will discuss the thermochemistry of the creation of hydroxyl radicals at the TiO2/water interface. All these calculations have been carried out using the CP2K package. The recent implementation of efficient methods for the evaluation of exact exchange in extended systems has been in particular critical for the application to semiconductor oxides as reported in this talk.
Klaus Müller-Dethlefs
Professor Klaus Müller-Dethlefs
Founding Director of The Photon Science Institute
School of Chemistry
The University of Manchester
Monday, February 6, 2012
EMSL Auditorium
11:00AM
Bose-Einstein Condensation (BEC) was first achieved in the liquid phase in helium a century and, for gas phase atoms, a decade ago. The question arises if there could be a third BEC of a solid, crystalline, state. A possible pathway towards such a new state of matter is a quantum plasma for which the de Broglie wavelength becomes larger than the mean distance between particles. For the electrons in an ultra-cold ion-electron plasma this condition is fulfilled for a temperature below 0.1K and a density above 1015 cm3. We produce such an ultra-cold Rydberg plasma by laser threshold ionization of NO molecules in the high-density expansion region of a supersonic jet close to the nozzle. This plasma has an extremely long lifetime of milliseconds, and it shows the compressibility of a "sponge like" ultra-soft solid. An explanation is the formation of an electron Wigner crystal, which according to A A Abrikosov should also lead to the formation of a lattice of the cations. A possible cooling mechanism for the molecular cations (such as 14N16O+ Bosons) towards quantum degeneracy, i.e. a molecular Bose-Einstein Condensate, will be discussed.