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Sunlight-Induced Nonadiabatic Dynamics of Atmospheric Molecules

Periodic Reporting for period 2 - SINDAM (Sunlight-Induced Nonadiabatic Dynamics of Atmospheric Molecules)

Reporting period: 2020-07-01 to 2022-02-28

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.
During this first part of the project, the SINDAM team has been working on three main topics. The following summary is organized around such topics for clarity.

-Topic 1: Mechanistic study of excited-state processes in the atmospheric chemistry of VOC intermediates

Models for chemical reactions in the atmosphere rely on the determination of photolysis rate constants to account for possible photochemical reactions of VOCs. In the context of this first topic, one key objective was to develop a theoretical protocol that would allow us to calculate fully in silico the different components of a photolysis rate constant for a given VOC. We have developed and tested such a protocol for the case of tert-butyl hydroperoxide, a VOC for which experimental data exists. Our protocol not only allowed us to closely reproduce the experimental data available (photoabsorption cross-section and quantum yield at 248 nm), but it also permitted to predict the wavelength dependence of the quantum yield – a key quantity revealing the different photochemical processes a molecule can suffer when different light wavelengths are absorbed. We are currently developing a refined protocol for the calculation of photoabsorption cross-section (also useful for an upcoming topic of the SINDAM project) and for the excited-state dynamics simulations required to determine the quantum yields.

We then employed this protocol to investigate the photolysis rate constants of two important VOCs: pyruvic acid and 2-hydroperoxy-propanal (2-HPP). Pyruvic acid is an important VOC in our troposphere and its photochemistry is strongly affected by the presence of water molecules. 2-HPP is a member of an extended family of multichromophoric VOCs for which no experimental data are available. The simulations conducted allowed to reproduce some of the known deactivation channels for pyruvic acid, highlighting in particular the importance of the dynamical effects following the deactivation. The dynamics of 2-HPP shed lights on the influence of triplet states in the excited-state dynamics of this family of molecules and reveal some intriguing photodynamical processes called diabatic trappings – originally postulated for molecules with no connection to atmospheric chemistry. These two studies also allowed us to highlight a severe flaw of one of the most commonly used methods for electronic structure when studying excited states: ADC(2).

We also studied other organic molecules in gas phase to probe the quality of the excited-state methods employed, and also to investigate the importance of the ground-state dynamics following the deactivation of an excited molecule. Non-statistical effects were observed during such dynamics, meaning that products would be formed on timescales that are much faster than what transition state theory would predict. Simulating the dynamics of VOCs in the ground state after deactivation is therefore of prime importance to understand the kinetics of formation of its photoproducts.

-Topic 2: Deciphering the influence of water molecules on the photochemistry of key atmospheric species: from microsolvation to interface and bulk properties

Based on the work of pyruvic acid in vacuum, we are now exploring the effect of microsolvation and solvation on the photodynamics of this important VOC. We are focusing our attention on the enhancement of intersystem crossing processes – that is, the transfer from a singlet to a triplet state mediated by spin-orbit coupling - suffered by pyruvic acid when solvated. These calculations involve the use of continuum model for the solvent as well as explicit water molecules treated quantum-mechanically. One important aspect of this study is to account for the possible new species formed when pyruvic acid is in water. We are also investigating the adsorption of pyruvic acid at the surface of ice.

-Topic 3: A unified theoretical framework for the nonadiabatic dynamics of VOCs.

Simulating the excited-state dynamics of molecular systems like VOCs can only be performed nowadays with a handful of methods, two of the most common ones being trajectory surface hopping (TSH) and ab initio multiple spawning (AIMS). TSH is the method we employed for the simulations performed in the context of Topic 1 above. AIMS is perceived as a more reliable method but can suffer from its computational cost. We have developed a benchmark strategy for excited-state dynamics methods, allowing for a comparison of their performances for the photodynamics of molecular systems. Such a benchmark allows us to determine which method – between TSH and AIMS – is the most suitable for a given photochemical problem. We have then developed a strategy coined stochastic-selection ab initio multiple spawning (SSAIMS), which offers the quality of an AIMS simulation at nearly the computational cost of TSH. While SSAIMS is a promising strategy, it relies on a new set of parameters that the user has to determine before launching a simulation. To alleviate this issue, we proposed the method called ab initio multiple spawning with informed stochastic selections (AIMSWISS), which is even cheaper than SSAIMS, does not require any new user-defined parameters, and provides excited-state dynamics as accurate as SSAIMS.
The main progress beyond the state of the art that we wish to highlight at this stage of the project is 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. The protocol we developed has already been adopted by other computational photochemistry groups to determine photolysis rate constants of small organic molecules.
We expect to be able to release soon a series of theoretical studies on the photochemistry of 2-HPP, pyruvic acid, and other transient VOCs. We are currently working alongside spectroscopists to validate some of our results on pyruvic acid in gas phase and in solution, and with atmospheric chemists to employ our protocol for the determination of photoabsorption cross-sections and wavelength-dependent quantum yields of carbonyl-containing VOCs.
Another key development of the SINDAM project will be the release of an automated software for the determination of photoabsorption cross-sections based on the protocol developed in Topic 1.
Graphical abstract highlighting the central photolysis rate constant equation