Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - NAMDIA (NonAdiabatic Molecular Dynamics of organic Intermediates in Atmospheric chemistry)

Teaser

\"Our atmosphere is a seemingly tranquil mixture of inert gases like nitrogen. However, it actually behaves more like a massive chemical reactor, as the small portion of carbon-based molecules contained in our atmosphere can react and form new molecules in an astonishingly...

Summary

Our atmosphere is a seemingly tranquil mixture of inert gases like nitrogen. However, it actually behaves more like a massive chemical reactor, as the small portion of carbon-based molecules contained in our atmosphere can react and form new molecules in an astonishingly complex network of chemical reactions. Atmospheric chemists have developed models of these chemical networks, a tool that would help humankind predict the future composition of our atmosphere. Development of these tools is beyond a pure scientific curiosity, as human activities are changing the chemical composition of our atmosphere, impacting both climate and air quality. Because the elementary mechanistic details of volatile organic compound (VOC) – a family of carbon-based molecules – oxidation are often beyond the reach of experiment, molecular studies based on theoretical and computational chemistry are increasingly used to construct chemical models. A surprising observation, though, is that these models do not always adequately account for reactions of VOCs with sunlight. As a matter of fact, atmospheric molecules can sometimes absorb sunlight energy, bringing them into an \'excited state\' that can trigger new families of \'photochemical reactions\', often exotic in comparison to the normal chemistry of such molecules.

This project proposed to use state-of-the-art techniques in theoretical chemistry to start answering the question: \"What is the importance of photochemistry in the reaction mechanisms of atmospheric VOC intermediates?\" The tools developed during this project would help theoretical chemists and atmospheric modelers to calculate how likely a VOC will absorb light and what would be the outcome of a photochemical reaction, providing insight into how excited-state dynamics impact atmospheric chemistry on global and regional scales.

This project confirmed the need for theoretical photochemistry to support atmospheric chemists. By focusing on critical atmospheric molecules and by collaborating with experimentalist groups, our work led to the creation of new tools for theoretical photochemistry of VOCs and triggered at least four new research themes. Importantly, these themes need further development in the future, as improving the accuracy of atmospheric models and our fundamental understanding is paramount to solve an urgent societal problem with significant health and economic impacts. The fundamental insight provided by NAMDIA will improve the accuracy of VOC oxidation mechanisms and atmospheric models, ultimately aiding societal efforts to develop strategies that mitigate the impacts of non-CO2 species on climate change.

Work performed

The project NAMDIA was divided into different subtasks: (i) Theoretical study of a typical molecule, HPALD, and its photochemical reactions upon light absorption; (ii) Develop and benchmark a protocol to determine if a given VOC molecule is likely to absorb light under sun irradiation; (iii) Develop new theoretical methodologies for the simulation of the light-absorption process and the generation of simple kinetic models. Finally, a central goal of the NAMDIA project was to bridge theory and experiment around the question of VOC photochemistry.

The VOC molecule HPALD-C6 is a close parent of an important VOC molecule and is stable enough to be studied experimentally. HPALD molecules have been proposed to release OH radicals upon light irradiation, and could explain the larger concentration of OH radicals often measured in specific regions such as on top of tropical forests as compared to predicted theoretical value. We performed an in-depth characterization of the photochemistry of HPALD-C6 by simulating its different electronic states, how they are coupled together, and simulate all-atom dynamics of the molecule after light absorption. A clear limit of the methodology employed is the level of theory to describe the electronic structure of HPALD-C6, a constant issue with theoretical photochemical studies. To alleviate this problem, we developed a model for HPALD (based on Zhu-Nakamura theory and implemented in the software Mesmer) in a reduced number of nuclear dimensions, selected based on our all-atom simulations, and we performed high-level electronic structure calculations to feed the model. Such model indicates that specific excited-state effects (`nonadiabatic effects\') increases the lifetime of the excited molecule by a factor of at least two, and highlighted an exciting mechanism known as \"diabatic trapping\". Additional calculations are currently ongoing to validate our results.

An important question that arose from project (i) is: how to simulate sunlight absorption explicitly? We developed a new theoretical method, coined XFAIMS, that includes explicitly the effect of light during the dynamics of a molecule, naturally promoting it in the excited states (J. Chem. Phys., 2016). A second project emanated during discussions with the spectroscopy group at the University of Bristol (Prof. Andrew Orr-Ewing) on the mechanism of \'intersystem crossings processes\'. These discussions led to a successful experimental/theoretical collaboration, during which we studied the process of intersystem crossings for a cobalt complex (Ang. Chem. Int. Ed., 2017).

We benchmarked most of the available strategies to determine the absorption spectra of VOC molecules, and compared their result to experimental ones. We developed different protocols based on the nature of the studied molecule and applied those protocol to predict the absorption spectra HPALD-C6 (which is not available experimentally). In collaboration with two experimental groups (Prof. Andrew Orr-Ewing and Dr. Max Mcgillen), we employed these strategies to predict the absorption spectra of molecules produced by the reaction of a Criegee\'s intermediate with simple alcohols (ACS Earth Space Chem., 2017).

Computing accurately and efficiently the electronic configuration of molecules of the size of atmospheric VOCs constitutes a critical bottleneck for nowadays theoretical methods. We therefore developed a new strategy to push the limits of the gold-standard method called \'EOM-CCSD\'. The technique was tested on a typical atmospheric molecule, acrolein, and was efficient enough to study it in isolation as well as in the aqueous phase (J. Phys. Chem. Lett, 2017).

Final results

The NAMDIA project achieved its central goal: stimulating the development and the application of new methods in theoretical photochemistry to VOC intermediates, in direct collaborations with atmospheric chemists. The two new theoretical techniques developed during this project are available in the electronic structure code Molpro and can be used by researchers to study photoexcitation processes of VOCs at an unprecedented level of theory. The in silico photoabsorption calculation protocol developed during this programme has been successfully applied to atmospheric molecules, impacting atmospheric chemists by informing them on the potential photochemistry of VOC intermediates and leading to the incorporation of these theoretical results in a molecular mechanism for atmospheric modeling.

The results from the NAMDIA projects, as well as their positive receptions by atmospheric chemists, validated the original assumptions of the project program: there is a clear need for \"in silico atmospheric photochemistry\" to support atmospheric chemists in their construction of more detailed chemical mechanisms.