Richard Zare of Stanford University is supported by an International Collaborations in Chemistry (ICC) award from the Chemical Structure and Mechanisms program in the Chemistry division to carry out detailed studies of the quantum dynamics of four-atom bimolecular reaction, in collaboration with Stuart Althorpe of Cambridge University in the UK. The proposed research builds on a recent series of collaborations between Zare's experimental research group and the theoretical group of S.C. Althorpe in which they studied the simplest chemical reaction (H + H2 -> H2 + H and isotopic variants) at an unprecedented level of detail and rigor, both experimentally and theoretically, using time-dependent wave packets to interpret the experimental scattering data. Surprisingly, this research has revealed a variety of unusual mechanistic effects that result from the quantum properties of the hydrogen nuclei, and which have implications for a wide range of other chemical reactions. The current project extends these studies to one of the simplest four-atom reactions: H + H2O -> H2 + OH. The significance of the extra atom is that it allows for a variety of effects that a three-atom reaction such as H + H2 is obviously too simple to capture, such as stereodynamical processes, and the effect of mode-selectivity on the wave function at the transition state. The theoretical part of this collaboration extends to four atoms the "plane wave packet method" of Althorpe and co-workers, in which the scattering into space of the products of a reaction is described by time-evolving wave packets which can be mapped directly onto experimentally measured state-to-state differential cross sections. The cross sections are measured by the Zare group, using an extension of a newly constructed instrument for imaging the three-dimensional velocity distribution of photo-ionized products from photo-initiated chemical reactions. An important feature of this research is that both highly accurate theory and novel and challenging experimental techniques are required; it is truly cutting edge research.
This fundamental research, like the prior work on the simplest three atom reaction, is expected to lead to surprising results that will influence our perceptions of reaction mechanisms and reaction dynamics. The work gives a deeper understanding of the role that quantum mechanics plays in many reactive processes. The work is also likely to find its way into textbooks on physical chemistry, and influence how students and their teachers think about chemical reaction dynamics. Several graduate students, in the US and the UK are being trained in the leading-edge theoretical and experimental techniques as part on an international collaboration in which there is a high level of synergy between theory and experiment.
At the heart of chemistry is the transformation of one compound to another, from reactants to products. For such processes, the simplest occur in the gas phase and the most information can be learned by studying chemical transformations, one collision at a time. This is achieved in ultra-low pressure gases inside vacuum chambers. We have been studying the simplest neutral chemical reaction in which a hydrogen atom (H) collides with a hydrogen molecule (H2) and collisionally excites it or interchanged hydrogen atoms in a reactive scattering process. To allow us to easily distinguish the collision partners we use heavy hydrogen (deuterium, D) in the form of D2 as one of the collision partners. We study in detail the inelastic scatter process: (H + D2 -->D2 (excited) + H) and the reactive scattering process (H + D2 -->HD + D). This process is not simple experimentally but it is regarded theoretically as the simplest reaction because the reaction system contains only three electrons and therefore can be modeled to high accuracy. We have been pursuing this reaction to compare theory against experiment and to gain insight into what features might apply to more complex collision processes. While in general theory and experiment closely agree, there remain some puzzling discrepancies, which is the subject of ongoing research. Moreover, experiment has been able to find features in the description of the collision process that were quite unexpected. These include that vibrational excitation of the D2 molecule involves extension of the D-D bond rather than its compression, caused by the incipient but unsuccessful formation of the HD product. In the reactive scattering we have found purely quantum effects arising from a fleeting HDD intermediate that lives a very short period of time. We have also discovered that the general rule that head-on collisions cause little rotational excitation and lead to backward scattering in the center-of-mass frame whereas glancing collisions cause more rotational excitation and lead to sideways scattering in the center-of-mass frame has exceptions which become evident when the amount of energy available to be disposed into internal motions of the product are restricted by conservation of energy.
