Calculating Photoabsorption Cross-Sections for Atmospheric Volatile Organic Compounds

Abstract

Characterizing the photochemical reactivity of transient volatile organic compounds (VOCs) in our atmosphere begins with a proper understanding of their abilities to absorb sunlight. Unfortunately, the photoabsorption cross-sections for a large number of transient VOCs remain unavailable experimentally due to their short lifetime or high reactivity. While structure–activity relationships (SARs) have been successfully employed to estimate the unknown photoabsorption cross-sections of VOCs, computational photochemistry offers another promising strategy to predict not only the vertical electronic transitions of a given molecule but also the width and shape of the bands forming its absorption spectrum. In this work, we focus on the use of the nuclear ensemble approach (NEA) to determine the photoabsorption cross-section of four exemplary VOCs, namely, acrolein, methylhydroperoxide, 2-hydroperoxy-propanal, and (microsolvated) pyruvic acid. More specifically, we analyze the influence that different strategies for sampling the ground-state nuclear density—Wigner sampling and ab initio molecular dynamics with a quantum thermostat—can have on the simulated absorption spectra. We highlight the potential shortcomings of using uncoupled harmonic modes within Wigner sampling of nuclear density to describe flexible or microsolvated VOCs and some limitations of SARs for multichromophoric VOCs. Our results suggest that the NEA could constitute a powerful tool for the atmospheric community to predict the photoabsorption cross-section for transient VOCs.

A Theoretical Perspective on the Actinic Photochemistry of 2-Hydroperoxypropanal

“…The flexibility of some atmospheric VOCs means that a harmonic Wigner distribution may not always lead to a satisfactory representation of the ground-state probability density, and other alternatives like ab initio molecular dynamics with a quantum thermostat are sometimes required—such effects appear negligible for 2-HPP. 30 …”

“…For atmospheric molecules that are not too flexible and for which a proper level of electronic structure theory can be selected, the NEA combined with Wigner sampling reproduces reasonably well the positions of the different bands forming a photoabsorption cross-section as well as their shape and corresponding intensity. 26 − 30 Importantly, the NEA cannot reproduce vibronic progressions.…”

“…The flexibility of some atmospheric VOCs means that a harmonic Wigner distribution may not always lead to a satisfactory representation of the ground-state probability density, and other alternatives like ab initio molecular dynamics with a quantum thermostat are sometimes required�such effects appear negligible for 2-HPP. 30 Last but not least, the excited-state dynamics simulations have some implicit limitations and rely on significant approximations. A first limitation may come from the use of the mixed quantum/ classical method TSH.…”

“…We note that the level of theory may also influence the overall intensity of the first band, as discussed recently for the theoretical determination of a photoabsorption cross-section for 2-HPP within DFT and LR-TDDFT. 30 Both levels of theory appear to indicate that the 2-HPP photoabsorption cross-section may be slightly altered by intramolecular effects but, perhaps, not as much as suggested by the scaling factor employed in the SARs cross-section.…”

Simulation of Nuclear Quantum Effects in Condensed Matter Systems via Quantum Baths

“…This method is general and versatile and offers a wide choice for the friction and random force kernels; nonetheless, it requires a fine tuning of the characteristic matrices, which might depend on the system under consideration. The QT method has been made available in the open-source i-PI suite [35], which has enabled its wider use [36][37][38].…”