Sunlight-Induced Nonadiabatic Dynamics of Atmospheric Molecules

Project Information

SINDAM

Grant agreement ID: 803718

DOI

10.3030/803718

Project closed

EC signature date 17 August 2018

Start date 1 January 2019

End date 31 December 2024

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 499 457,00

EU contribution

€ 1 499 457,00

Coordinated by

UNIVERSITY OF BRISTOL
United Kingdom

Periodic Reporting for period 4 - SINDAM (Sunlight-Induced Nonadiabatic Dynamics of Atmospheric Molecules)

Reporting period: 2023-09-01 to 2024-12-31

Summary of the context and overall objectives of the project

While our atmosphere may appear as a rather inert environment, it should rather be pictured as a large chemical reactor. Volatile organic compounds (VOCs) are small organic molecules present in our atmosphere either naturally or as a result of human activities. These molecules are highly active and will react with either light or radicals in the atmosphere, triggering a very complex network of chemical reactions in the troposphere. To understand the composition of our atmosphere and its evolution over time, atmospheric chemists and modelers have derived chemical models, compiling all possible chemical reactions for primary and secondary VOCs as well as their associated rates – based on both experimental and computational data. These models offer a powerful tool to understand the evolution of VOCs in our atmosphere, of key importance to inform political and societal decisions in the context of air pollution and its mitigation. Surprisingly though, chemical reactions that are triggered by the absorption of light – so-called photochemical reactions – for transient VOCs are mostly missing in such chemical mechanisms. The short lifetime of some transient VOCs explains this lack of experimental and spectroscopic data to characterize their sunlight-induced photolysis processes. There is therefore an urgent to develop strategies in theoretical/computational chemistry and apply them to investigate the importance of sunlight-triggered photochemical reactions of transient VOCs in our troposphere - central for improving the predictive power of the chemical models described earlier.
The SINDAM project proposes to study the importance of photochemical processes for transient VOCs by developing and using state-of-the-art strategies in computational photochemistry. The principal aims of this project are as follows: (1) establish the importance of photochemical processes in the chemical mechanisms of VOCs, in gas phase and diverse aqueous conditions; (2) determine missing theoretical values for photolysis rate constants and wavelength-dependent quantum yields to create more accurate atmospheric models; (3) develop a widely applicable yet affordable theoretical methodology that accounts for all potential nonradiative mechanisms important for atmospheric; (4) produce an open-access software for atmospheric modelers allowing for the identification of potentially photoreactive VOCs.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The SINDAM Team developed a theoretical protocol that allows researchers to calculate fully in silico the different components of a photolysis rate constant for a given VOC. This protocol was tested and employed to investigate the photolysis rate constants of various VOCs, e.g. pyruvic acid, 2-hydroperoxy-propanal, CF3COCl, or thiophenone.
We also conducted a series of joint theory/experiment studies on the gas-phase photochemistry of various molecules. For example, we studied the photochemistry of pyruvate and thiophenone by combining computational photochemistry and photoelectron spectroscopy (as well as ultrafast electron diffraction). All these examples showed the maturity of the methods used in computational photochemistry and support their use to predict photolysis quantities for transient VOCs.

Based on its interactions with atmospheric photochemists, the SINDAM team focused on the refinement of strategies for calculating photoabsorption cross-sections. The SINDAM team developed an automated tool, coined AtmoSpec, that uses a user-friendly interface and takes as an input a simple representation of the VOC of interest (SMILES code, xyz geometry) and automates the calculation of photoabsorption cross-sections based on the nuclear ensemble approach. AtmoSpec is open-source and freely available.

The SINDAM team also worked on the development of new strategies to predict the photochemistry of molecular systems. After developing a set of stringent benchmark systems to challenge excited-state dynamics methods, the SINDAM team developed a hierarchy of methods based on the multiple spawning, which offers the quality of an ab initio multiple spawning simulation at nearly the computational cost of surface hopping.

The SINDAM team has been very active in disseminating the results of the action to the computational photochemistry and atmospheric communities. For example, the team worked with atmospheric modelers to test the possibility of developing structure-activity relationships based on the theoretical photoabsorption cross-sections. The team also co-organized a workshop in March 2024 in Lausanne, gathering theoreticians and spectroscopists interested in atmospheric photochemistry and leading to the publication of a perspective article summarizing the main recent developments in the field and future challenges. The SINDAM team also co-led a theoretical challenge for the computational photochemistry community: predicting the photochemistry of a VOC (cyclobutanone) before a time-resolved experiment is performed on the molecule and comparing the prediction with the experimental observables as part of a special issue of the Journal of Chemical Physics. The SINDAM team also disseminated the results of SINDAM to a broader audience via the 'Scientist Next Door' initiative, which uses a web platform and Zoom calls to connect scientists to school-aged children aiming at supporting parents with homeschooling, originally during COVID-19 lockdowns in the UK and extended to support less favored socioeconomic areas.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The SINDAM project led to the development of a protocol for the calculation of photolysis rate constant and a new series of methods for excited-state dynamics that offer a compromise between ab initio multiple spawning and trajectory surface hopping. Our research group successfully applied the protocol for photolysis quantities to study and characterize the gas-phase photochemistry of the molecules 2-HPP, pyruvic acid, pyruvate, thiophenone, cyclobutanone, CF3COCl, o-nitrophenol, and B-N compounds. Other computational photochemistry groups have already adopted our protocol to determine photolysis rate constants of small organic and inorganic molecules. We also worked alongside spectroscopists to validate the photochemical pathways determined theoretically and identify specific unexpected photoproducts. For example, our theoretical work combined with photoelectron spectroscopy revealed that, upon irradiation in the UVA region, gas-phase pyruvate photodecomposes to form a CO2, CO, and an unexpected methide anion. Combining our theoretical work with time-resolved ultrafast electron diffraction, we could unravel the early-time formation of photoproducts upon irradiation of thiophenone.
Another key development of the SINDAM project is the theoretical determination of photoabsorption cross-sections for VOCs. This quantity enters the photolysis rate constant equation and provides a first key indication on the timescale of photolysis for a given VOC. Our group develops a protocol based on accurate sampling for the determination of photoabsorption cross-section for flexible molecules like VOCs. In addition, we released an open-source and freely available software, AtmoSpec, that automates the process of calculating photoabsorption cross-sections for VOCs. AtmoSpec does not require the user to be an expert in computational chemistry and can be used to streamline the decision process of including or not the photolysis of transient VOCs in chemical mechanisms.

Graphical abstract highlighting the central photolysis rate constant equation