Frontiers of chemical dynamics

CO₂ reduction to syngas and N₂ reduction to ammonia are processes of fundamental interest to energy science. In this talk, theory is used to provide new directions to this research in two areas. The first part of the talk discusses plasma enhanced dry reforming, and photocatalytic conversion of N₂ to ammonia. Dry reforming is a process wherein CH₄ and CO₂ react to give syngas and/or liquid fuels. Dry reforming is normally done under high temperature and pressure conditions, with a Ni catalyst, however it has recently been discovered that if a plasma is also present near the catalyst, then it is possible to get this reaction to go under modest conditions close to room temperature and atmospheric pressure. The role of the plasma in this process is poorly understood. The talk focuses on the use of electronic structure studies and Born-Oppenheimer molecular dynamics to describe gas-surface reactions that arise from plasma species. The plasma is known to fragment the reacting gases, especially CH₄ into CH₃ + H and CH₂ + 2H, so a highlight of this work concerns reaction of atomic hydrogen and CH₂ with adsorbed CO₂ and CO to give CO, water, formaldehyde, methanol and other products. The results include comparisons with experiments from the Koel group at Princeton, and with other groups, and it is found that hot-atom and Eley-Rideal mechanisms play an important role. The second part of this talk considers the reaction mechanism underlying recent work from the Kanatzides group at Northwestern in which it was discovered that iron-sulphur clusters present in gel materials can participate in the photocatalytic conversion of N₂ to ammonia under ambient conditions. The theoretical studies use broken symmetry density functional theory to reveal a mechanism for this process that is related to what happens with the nitrogenase enzyme, but with important differences that arise from photon-induced delivery of electrons to the iron-sulphur clusters.

Professor George C Schatz, Northwestern University, USA

Professor George C Schatz, Northwestern University, USA

George C. Schatz is the Morrison Professor of Chemistry at Northwestern University. He received his undergraduate degree at Clarkson University and PhD at Caltech.  He was a postdoc at MIT, and has been at Northwestern since 1976. Schatz is a member of the National Academy of Sciences, the American Academy of Arts and Sciences, and he has been Editor-in-Chief of the Journal of Physical Chemistry since 2005. Schatz is a theoretician specializing in electronic structure methods, dynamical processes, electrodynamics, and statistical mechanics, who studies the optical, structural and thermal properties of nanomaterials, including plasmonic nanoparticles, catalysts, DNA and peptide self-assembled nanostructures, and carbon-based materials, with applications  to chemical and biological sensing, electronic and biological materials, heterogeneous catalysis and solar energy.

Heterogeneously catalyzed processes are of large importance: the production of the majority of chemicals involves catalysis at some stage. Heterogeneously catalyzed processes consist of several elementary reactions. Accurately calculating their rates requires the availability of accurate barriers for the rate controlling steps. Unfortunately, currently no first principles methods can be relied upon to deliver the required accuracy. To solve this problem, in 2009 a novel implementation of the specific reaction parameter approach to density functional theory (SRP-DFT) was formulated. This allowed reproducing experiments for H₂ reacting on copper surfaces, and to determine barrier heights for H₂-Cu systems, with chemical accuracy. The original procedure used was not extendable to reactions of molecules heavier than H₂ with surfaces, because the metal surface was treated as static. This problem has now been solved through a combination of SRP-DFT with Ab Initio Molecular Dynamics (AIMD). This method was applied to the dissociative chemisorption of methane on a Ni surface, a rate-limiting step in the steam reforming reaction. Experiments on CHD₃ + Ni(111) were reproduced with chemical accuracy, and a value of the reaction barrier height was derived that is claimed to be chemically accurate. Even better results were obtained for CHD₃ + Pt(111), suggesting that the SRP density functional for methane interacting with Ni(111) is transferable to methane interacting with other group X metal surfaces. Even more interestingly for applications to catalysis, the SRP functional derived for methane reacting with Ni(111) also gives a very accurate description of molecular beam sticking experiments on CHD₃ + Pt(211).

Professor Geert-Jan Kroes , Leiden University, The Netherlands

Professor Geert-Jan Kroes , Leiden University, The Netherlands

Geert-Jan Kroes obtained his PhD in Chemistry in 1990 at the University of Amsterdam (The Netherlands), working under the supervision of Professor RPH Rettschnick. He worked as a post-doc with David Clary from 1990 to 1992 (Cambridge, UK), and with Marc van Hemert and Ewine van Dishoeck from 1992 to 1993 (Leiden, The Netherlands). He worked as a KNAW-fellow at the Free University of Amsterdam and at Leiden University from 1993-1998. At Leiden he became Assistant Professor in 1998, and Full Professor in 2003. His present research is focused on reactive scattering of molecules from metal surfaces.

The Cl+CH₄→HCl+CH₃ reaction has been the subject of extensive experimental and theoretical investigations due to its crucial role in the Cl/O₃ destruction chain mechanism in the stratosphere, and has also become a prototype for studying mode specificity and bond selectivity in polyatomic reactions with a late barrier. Earlier quantum dynamics studies on a high-quality potential energy surface (PES) constructed by Czako and Bowman (CB) revealed that there is a distinctive peak in the total reaction probabilities for the total angular momentum J=0 at collision energy of about 3 kcal/mol, which was inferred to be related to a dynamics resonance in the reaction. In this talk, Professor Zhang will present a quantum dynamics study of the reaction on a new PES by using the reduced dimensionality model of Palma and Clary by restricting the non-reacting CH₃ group in a C3V symmetry. The calculated total reaction probabilities for the total angular momentum J=0 on the new PES, which is of a quantitative level of accuracy, exhibit a clear peak structure as on the CB PES. Detailed dynamics analysis uncovered that the peak structure does not originate from a dynamics resonance, it is a very reaction probability oscillation associated with the heavy-light-heavy (HLH) nature of the reaction. State-to-state quantum dynamics calculations revealed that the HLH oscillation in the reaction has important influence to product rotation distributions, and also leave a clear peak in the backward scattering direction which can be detected by a cross-molecular beam experiment.

Professor Dong Hui Zhang, Dalian Institute of Chemical Physics, China

Professor Dong Hui Zhang, Dalian Institute of Chemical Physics, China

Dong Hui Zhang received his BS degree in physics from Fudan University, China, in 1989. He earned a PhD in chemical physics from New York University in 1994. After working as a postdoctoral research fellow in New York University and the University of Chicago, he joined the National University of Singapore in 1997. He moved to Dalian Institute of Chemical Physics, CAS, in 2006, and was elected to the Chinese Academy of Sciences in 2017. Now he is the director of the state key laboratory of molecular reaction dynamics in the institute. His primary research area is in theoretical and computational studies of chemical reactions in the gas phase and on surfaces.

In the textbook experiment of Young, quantum interference between two distinct trajectories leads to oscillations in the observed scattering of an atomic beam through two slits. Similar interferences patterns can be observed as oscillations in the intensities of scattering of a molecular beam from a given initial rotational state into various final states. This quantum interference structure becomes especially rich when the initial and final states are coupled by two electronic potential energy surfaces. This field will be reviewed with particular attention to work done in collaboration with Professor Clary.

Professor Millard H Alexander, University of Maryland, USA

Professor Millard H Alexander, University of Maryland, USA

Professor Millard Alexander obtained a BA from Harvard College in 1964, a D. ès Sciences from the University of Paris in 1967 and was a postdoctoral fellow at Harvard University between 1967 and 1970. Professor Alexander is currently a Distinguished University Professor in the Department of Chemistry and Biochemistry and Institute for Physical Science and Technology, University of Maryland, USA. His research interests are molecular collision and reaction dynamics, non-adiabatic coupling in chemical dynamics, simulation of molecular collisions in planetary and interstellar environments, photodissociation of atmospheric molecules. Professor Alexander’s work has provided a framework for the understanding of non-adiabatic effects in molecular collisions, in weakly bound complexes and in molecular photodissociation. By combining state-of-the art techniques in ab initio quantum chemistry and new methods in quantum collision theory, he has pushed forward the frontier in the understanding of how electronic and nuclear motion are coupled in elementary inelastic and reactive collisions. Professor Alexander’s awards and honors include: Fellow American Physical Society; John Simon Guggenheim Memorial Fellow; Dr Lee’s Fellow, Oxford; Kirwan Research Prize, University of Maryland; Hillebrand Award, Washington Section, American Chemical Society; past President, Telluride Science Research Center; Fellow, American Association for the Advancement of Science.

Alkene ozonolysis is a primary oxidation pathway for alkenes emitted into the troposphere and also an important source of atmospheric hydroxyl radicals. Alkene ozonolysis takes place on a reaction path with multiple minima and barriers along the way to OH products. In particular, a key reaction intermediate, known as the Criegee intermediate, R₁R₂COO, had eluded detection until very recently. In this laboratory, the simplest Criegee intermediate CH₂OO and methyl-, dimethyl-, ethyl-, and vinyl-substituted Criegee intermediates have been generated by an alternative synthetic route, detected by VUV photoionization, and characterized on a strong π*←π transition. Recent studies have focused on vibrational activation of Criegee intermediates in the vicinity of and at energies much below the barrier associated with hydrogen transfer that leads to OH radical products. Infrared action spectra of the Criegee intermediates are obtained, along with time-resolves rates for appearance of OH radical products following vibrational activation under collision-free conditions. Complementary theoretical calculations are carried out to predict the energy-dependent unimolecular decay rates of the Criegee intermediates. Quantum mechanical tunneling through the barrier is shown to make a significant contribution to the decay rates. The results are extended to thermally averaged unimolecular decay of stabilized Criegee intermediates under atmospheric conditions.

Professor Marsha I Lester, University of Pennsylvania, USA

Professor Marsha I Lester, University of Pennsylvania, USA

Marsha I Lester is the Edmund J Kahn Distinguished Professor in the Department of Chemistry of the University of Pennsylvania. She is the Editor of The Journal of Chemical Physics and Chair of the Penn Forum for Women Faculty. Previously, she served as Chair of the Department of Chemistry at Penn. Lester has received many honors and awards, including her election to Fellowship in the National Academy of Sciences and the American Academy of Arts & Sciences, the Garvan-Olin Medal of the American Chemical Society, the Bourke Lectureship of the Faraday Division of the Royal Society of Chemistry, a John Simon Guggenheim Memorial Foundation Fellowship, Fellow of the American Association for the Advancement of Science, the American Chemical Society, and the American Physical Society, an Alfred P Sloan Research Fellowship, and the Dreyfus Teacher-Scholar Award.

David Clary’s pioneering adiabatic capture calculations on low temperature barrierless reactions provided a strong motivation for experiments in low temperature gas kinetics, which lead to the discovery of whole classes of neutral-neutral reactions that remain rapid or even become faster as the temperature is decreased down to 10 K or even below. An overview will be given of the importance of determining temperature dependent rate constants and product branching ratios for elementary chemical reactions and collisional energy exchange for understanding the creation and destruction of molecules in space. In particular attention will be focused on studies of the reactivity of carbon-containing radical species with organic co-reagents leading to efficient molecular growth even at the low temperatures of dense interstellar clouds (10—20 K) and circumstellar envelopes or of the atmospheres of planets and their moons, such as that of Titan (70—180 K). We employ both pulsed and continuous flow CRESU (Cinétique de Réaction en Ecoulement Supersonique Uniforme, or Reaction Kinetics in Uniform Supersonic Flow) apparatuses, combined with a variety of laser photochemical techniques, as well as, most recently, synchrotron photo-ionization mass spectrometry and chirped pulse mm-wave rotational spectroscopy.

Professor Ian Sims, University of Rennes 1, France

Professor Ian Sims, University of Rennes 1, France

Ian Sims studied as an undergraduate at Cambridge, and followed Ian WM Smith to Birmingham for a PhD (1989) in gas-phase kinetics and dynamics, including low temperature measurements inspired by David Clary’s theoretical predictions. He spent two postdoctoral periods of two years each at Caltech (with Ahmed Zewail) and Rennes (with Bertrand Rowe and Ian WM Smith). In Rennes he developed the application of the CRESU (Reaction Kinetics in Uniform Supersonic Flow) technique for the study of neutral-neutral reactions at very low temperatures. A 5-year Advanced Fellowship at the School of Chemistry of the University of Birmingham followed and he was appointed Lecturer (1998) and Senior Lecturer (2001). In 2003 he moved to a chair in physics at the University of Rennes 1, where he is now a Distinguished Professor.

The domain of velocity-map imaging has expanded rapidly from highly detailed quantum-state resolved studies of small molecules to investigations into the photofragmentation dynamics of much larger molecules, including model systems of direct relevance to atmospheric chemistry, astrochemistry, photobiology, and organic photochemistry. Such studies present a number of challenges. The numerous fragmentation channels available to a polyatomic molecule generally make it unfeasible to attempt to record state-resolved images for each and every fragment. Instead, universal (or near-universal) ionization schemes are employed to ionize and therefore detect all products simultaneously. This allows the scattering distribution for each fragment to be recorded, though at the expense of quantum-state resolution. New imaging sensors capable of detecting individual particles with nanosecond time resolution allow images for all fragments to be acquired simultaneously within a single experiment, with covariance analysis of the data set revealing the correlated scattering distributions of pairs of photofragments. This talk will review recent technical advances in the arena of multi-mass imaging, as well as exploring examples of their application to a number of chemical systems.

Professor Claire Vallance, University of Oxford, UK

Professor Claire Vallance, University of Oxford, UK

Claire Vallance is a Professor of Physical Chemistry in the Department of Chemistry at the University of Oxford, and Tutorial Fellow in Physical Chemistry at Hertford College, Oxford.  She grew up in the UK and New Zealand, and holds a BSc(hons) and PhD from the University of Canterbury (Christchurch, NZ).  Her current research interests include chemical reaction dynamics, particularly the development and implementation of high speed detectors for multi-mass velocity-map imaging; the use of optical microcavities in chemical sensing applications; and the development of spectroscopic and imaging techniques for cardiovascular and neurosurgical applications.  In addition to a range of teaching and outreach activities, she is a member of the editorial advisory boards of the Journal of Physical Chemistry and Annual Reviews of Physical Chemistry and the international organising committees for the MOLEC and iCOMET conferences, and is currently President Elect of the Faraday Division of the Royal Society of Chemistry.

It is long known theoretically that geometric phase effect is important in chemical reactions with conical intersections. Even though there has a number of theoretical studies for this effect since 1970s, experimental observation of such effect proves to be extremely difficult and fruitless. Recently, we have constructed a new crossed beams imaging machine and observed the geometric phase effect in the H+HD→H₂+D reaction at collision energy near the conical intersection of this reaction using high resolution threshold ion imaging technique. In addition, we have also observed experimental evidence of geometric phase effect at collision energy significantly lower than conical intersection energy using high resolution H-atom Rydberg tagging technique. These new experiments allows us to probe this important effect in chemical reaction at the most fundamental level.

Professor Xueming Yang, Dalian Institute of Chemical Physics, CAS, China

Professor Xueming Yang, Dalian Institute of Chemical Physics, CAS, China

Xueming Yang obtained his PhD in chemistry at University of California at Santa Barbara in 1991. He did postdocs at Princeton and UC Berkeley from 1991 to 1995. He then became an associate research fellow at the Institute of Atomic and Molecular Sciences in Taipei and was promoted to be a tenured full research fellow in 2000. He moved to Dalian Institute of Chemical Physics, Chinese Academy of Sciences in 2003 and is now a distinguished research fellow. His main research focus is in the area of physical chemistry, especially dynamics and spectroscopy in the gas phase and at the interfaces. He has developed a new generation of molecular beam instruments for quantum state resolved reaction dynamics studies, especially quantum resonances in chemical reactions. In the recent years, he has developed new experimental tools to investigate surface photocatalysis that allows us to understand photocatalysis at the most fundamental level. He has published more than 300 research papers, including 11 in Science and one in Nature. Professor Yang has received many research awards such as the Humboldt Research Award, Tan Kah Kee Science Award and National Natural Science Award. He is a fellow of American Physical Society and Royal Society of Chemistry. He has also been a member of Chinese Academy of Sciences since 2011.