International Collaboration in Chemistry: Quantum Dynamics of 4-Atom Bimolecular Reactions

The proposed research will build on a recent series of collaborations between the experimental group of R.N. Zare (Stanford University) and the theoretical group of S.C. Althorpe (University of Cambridge). In this work, we have 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 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 aim of this proposal is to extend these studies to one of the simplest 4-atom reactions: H + H2O -> H2 + OH. The significance of the extra atom is that it allows for a variety of effects that a 3-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 will extend to 4-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 wave packets will be computed using a new 4-atom reactive scattering code, developed recently under the aegis of CCP6 (UK Collaborative Computational Project on Molecular Quantum Dynamics). The cross sections will be measured by the Zare group, using an extension of a newly constructed instrument for imaging the three-dimensional velocity distribution of photoionized products from photoinitiated chemical reactions. By bringing together theory and experiment in this way, we will be able to probe much more deeply into the dynamics than would be possible if either were applied independently. In particular, the experimental data will be used to scan the cross sections, so that the expensive computations can focus on those final states that are most likely to correspond to interesting dynamical effects. The exchange of information between experimental data will also be used to reduce the number of calculations that need to be carried out (by interpolation), and the theoretical results will in turn allow more precise measurements of the angular dependence to be extracted from the raw experimental data. We are confident that this strategy will result in the first time-dependent wave functions describing the scattering into space of products from a 4-atom reaction, at a much lower computational cost than could be achieved by theory acting alone.

We cannot guarantee that this curiosity-led research will lead to new findings, but on the basis of our previous work on 3-atom reactions, we think that this is highly likely. The latter has resulted in 4 papers in general Science journals (Nature, Science and PNAS), and some of the new things learnt about reaction dynamics from these studies (e.g. the behaviour of time-delays at transition-state thresholds) are now being taught in UK Undergraduate Chemistry courses (in Oxford, Bristol, Durham and Cambridge); they are also the subject of international talks such as the plenary lecture given recently (by Zare) at the IUPAC conference in Glasgow, Scotland. This 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. Another major impact of this work will be the training given to the two PhD students, who will have an exciting opportunity to be part of an international collaboration between leading theoretical and experimental research groups.