Complex Chemistry and Chemical Activation

Lead Research Organisation: University of Leeds
Department Name: Sch of Chemistry

Abstract

Models of complex chemical processes such as combustion or atmospheric chemistry assume that the molecules taking part are thermalized, that is that their energy is characterized by the temperature of the system. Chemical activation (CA) occurs when the energy released by a reaction is channelled into the products and they have an energy greater than would be thermally predicted. How does the reactivity of these activated species compare with their thermalized equivalents? What is the significance of CA? How can CA be incorporated into chemical models of complex systems? These are the questions at the heart of our project: Complex Chemistry and Chemical Activation (C3A).

Aspects of CA have been known about for more than 100 years, indeed 2022 marks the centenary of the Lindemann Mechanism, the first theory proposed to explain the pressure dependence of some chemical reactions. Models of CA have grown in sophistication, yet uncertainties in key processes (energy transfer, calculation of densities of states) limit the accuracy of kinetic and thermodynamic predictions from such systems. Addressing the uncertainties in these aspects of current models through new experimental data and developments in fundamental models is one strand of C3A.

More recently, work in this group and elsewhere has shown that systems which were thought to be adequately modelled by thermalized reagents, such as abstraction reactions (e.g. OH + HCHO), do need to considered in the context of chemical activation. In a 2018 review, Klippenstein states: 'These studies ultimately led us to the realization that at combustion temperatures, the foundational assumption of thermalization prior to reaction is not always valid, and further that its breakdown significantly affects key combustion properties' (Proceedings of the Combustion Institute, 36, p77). These phenomena are not limited to combustion; plasma chemistry and the atmospheric chemistry of Earth and other planets provide other important examples of applications.

C3A is a collaboration between leading groups from Leeds and Oxford, both with interests in experiments and theory. C3A will generate a wealth of new experimental data, which in combination with theoretical interpretation, will allow us to assess the significance of CA in real systems and provide the tools to allow CA to be accurately incorporated into chemical models of of these processes. The impact of C3A to industry will be facilitated by collaborations with Shell, Dassault Systemes and AirLabs.

Such models are essential tools for understanding important questions from current highly practical issues (how can combustion systems be optimized to minimize CO2 emissions and improve air quality) to future questions (biofuels for aviation, novel methods of renewable energy storage such as ammonia generation and combustion) to important, fundamental questions such as modelling the atmospheres of hot-Jupiter exo-planets or the interstellar medium. The accurate assessment and incorporation of CA into such models will significantly enhance their reliability and predictive value.

Publications

10 25 50
 
Description Influence of Remote Groups on Structure Activity Relationship (SAR) Correlations.
Esters, and particularly the formate esters, are potential candidates for chemical activation (a major focus of the project). Our studies on formates have shown that the site specific rate coefficient for abstraction of an H atom at the formate site is dependent on the nature of the R group on the other side of the ester grouping. In conventional SAR analysis, only near neighbours contribute to activity at a site. This work suggests that more sophisticated SAR are required. This work has been presented at the International Conference on Chemical Kinetics (Heifei, July 2023) and the AGU (San Francisco, Dec 2023).
Exploitation Route Too early to say at this point, but SAR are something of a hot topic both because they are required to estimate rate coefficients, but also because of the role that AI can play in their construction.
Sectors Energy

Environment

 
Description Contributed talk to AGU on Structure Activity Relationships 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Contributed talk given by PhD student Niamh Robertson at the American Geophysical Union conference in San Francisco, December 2023.
Year(s) Of Engagement Activity 2023
 
Description Invited lecture 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Invited, plenary lecture at the International Conference on Chemical Kinetics, Heifei, July 2023
Year(s) Of Engagement Activity 2023
 
Description Plenary Talk at the International Symposium on Gas Kinetics, Rennes, France, September 2022 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Plenary Talk at the International Symposium on Gas Kinetics, Rennes, France, September 2022. Presented material on the reaction of OH + methyl-formate which is one of the workpackages in this project.
Year(s) Of Engagement Activity 2022
 
Description Poster Presentation at Faraday Discussion on Unimolecular Reactions. 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Poster presentation at the Faraday Discussion on Unimolecular Reactions to mark the centenary of the Lindemann Mechanism of Unimolecular Reactions. Keble College, Oxford, June 2022.
Year(s) Of Engagement Activity 2022
 
Description Poster Presentation by Dr Robin Shannon 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Poster presentation on the role of trajectory calculations on determining the fraction of product that can undergo chemically activated decomposition. Presented at the International Symposium on Gas Kinetics, Rennes, France, September 2022.
Year(s) Of Engagement Activity 2022
 
Description Presentation to the European Combustion Symposium 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach International
Primary Audience Professional Practitioners
Results and Impact Short talk, conference paper and poster presentation by Dr Lavinia Onel at the European Combustion Symposium April 2023.
Year(s) Of Engagement Activity 2023