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.

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).

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.

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.

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.

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.

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.

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.