On the Theoretical Determination of Photolysis Properties for Atmospheric Volatile Organic Compounds

Abstract

Volatile organic compounds (VOCs) are ubiquitous atmospheric molecules that generate a complex network of chemical reactions in the troposphere, often triggered by the absorption of sunlight. Understanding the VOC composition of the atmosphere relies on our ability to characterize all of their possible reaction pathways. When considering reactions of (transient) VOCs with sunlight, the availability of photolysis rate constants, utilized in general atmospheric models, is often out of experimental reach due to the unstable nature of these molecules. Here, we show how recent advances in computational photochemistry allow us to calculate in silico the different ingredients of a photolysis rate constant, namely, the photoabsorption cross-section and wavelength-dependent quantum yields. The rich photochemistry of tert -butyl hydroperoxide, for which experimental data are available, is employed to test our protocol and highlight the strengths and weaknesses of different levels of electronic structure and nonadiabatic molecular dynamics to study the photochemistry of (transient) VOCs.

Calculating Photoabsorption Cross-Sections for Atmospheric Volatile Organic Compounds

“…For methylhydroperoxide, the 6-311+G* basis set was employed since diffuse basis functions are needed to describe the valence-Rydberg mixing in peroxide excited states. 54 …”

A Simple and Efficient Method for Simulating the Electronic Absorption Spectra of Criegee Intermediates: Benchmarking on CH2OO and CH3CHOO

“…The overarching aim is to devise a simple yet versatile technique for computing the absorption spectra that may be used to calculate the UV absorption spectra of CIs of varying molecular complexity. The nuclear ensemble method has previously been successfully applied to simulating the electronic absorption spectra of other atmospherically relevant systems. − …”

Nonadiabatic Kinetics in the Intermediate Coupling Regime: Comparing Molecular Dynamics to an Energy-Grained Master Equation

“…Each trajectory can stochastically switch between electronic states in regions of strong NA coupling. While FSSH has known shortcomings (including overcoherence and neglect of tunneling and interference effects), it often provides an accurate and scalable method that is now widely used to explore photochemical and photophysical phenomena, also for atmospheric chemistry. − Given that FSSH typically has a sub-femtosecond integration time step, pushing the simulation into the nanosecond regime necessitates compromises with respect to the electronic structure method and number of trajectories. It has been suggested that this bottleneck might be overcome through machine-learned energies and couplings …”