Preface

Gas phase unimolecular reactions are central to the complex chemistry of numerous natural, anthropogenic and technological processes, such as those occurring in the atmospheres of Earth and other planets, combustion for transportation and power generation, and industrial manufacturing of advanced chemicals and materials. Improved understanding of the fundamental chemistry of these processes is a pressing concern not only in the context of atmospheric pollution and climate change, but also designing more efficient industrial processes and fuels that can help attenuate this impact. The rate coefficients for unimolecular reactions have a complex dependence on temperature, composition and pressure, and the Lindemann mechanism has been central to the analysis and interpretation of this dependence for a century. The central feature of the mechanism, the delay between reactant activation and reaction, remains at the heart of modern thinking, but our mechanistic understanding has been significantly extended in a number of ways since Lindemann first presented his seminal ideas. These extensions have allowed ever more complex systems to be investigated experimentally and theoretically, giving greater insight into the unimolecular process, but there remain many unresolved issues. These issues concern various facets of the Lindemann mechanism and include the definition of rate coefficients at high temperatures, better ways to calculate accurate state counts, identification of key parameters to investigate experimentally, better understanding of the energy transfer process and the impact of quantum mechanical effects on reaction rates.

Thus, it seemed fitting, one hundred years since the publication of the Faraday Discussion at which Lindemann advanced the idea of collision as the basis for a mechanism of unimolecular reaction, to hold a discussion to examine the state of the field and its future direction. The meeting, and the papers selected for discussion, was structured so as to reflect the basic elements of the Lindemann mechanism, so there are papers on energy transfer, on the reaction step, and the combination of the two in the form of the master equation. A fourth section was added to allow for a wider interpretation of the mechanism. As is clearly evident from the papers in this volume, the subject of unimolecular reactions not only remains one of great interest and endeavour, but it has also advanced in ways that would be scarcely recognisable to the attendees of that first meeting – it is to be remembered that at the time of the 1921 Discussion the phenomenon of rate coefficient fall-off had yet to be observed.

Acknowledgements

Like so many scientific committee chairs that have gone before me, I am truly grateful to the dedication and professionalism of the staff of the RSC for helping to make this meeting the success it was, so my sincere thanks to Susan Weatherby, Stuart Govan, Katie Ackermann, Rachel Cole, Sarah Latham, and Claire Springett. In the collection of questions and comments as well as the preparation of this volume I am indebted to Ellis Crawford and Ellie Griffiths. I, perhaps more than most chairs, relied on my colleagues on the scientific committee and so it is with great gratitude that I acknowledge the considerable contributions made by Judit Zádor, Claire Vallance, Andrew Orr-Ewing, György Lendvay and Paul Seakins in the planning of the meeting as well as their scientific scrutiny. My sincere thanks also go to Stephen Klippenstein for an outstanding opening lecture that truly captured the breadth and depth of the field and set the stage for the discussion to follow and to William Green for his equally impressive expert summary of the meeting and assessment of the future directions of the subject in his concluding lecture. The unique feature of a Faraday Discussion is, of course, discussion and this needs to be led, so my thanks go to David Clary, Dwayne Heard and Nicolas Green, as well as the members of the scientific committee, who chaired the sessions and kept the discussion moving along. I thank the Poster committee of Arkke Eskola, Matthias Olzmann, and David Osborn for their diligent review of the posters and selection of the winner (Magdalena Salzburger from Universität Innsbruck). Finally, I should like to thank all the speakers and participants for all of their contributions and for making this a stimulating and memorable Faraday Discussion.

 

Struan H. Robertson

Cambridge, July 2022

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