Photo-initiated ground state chemistry: How important is it in the atmosphere?

Abstract

Abstract. Carbonyls are among the most abundant volatile organic compounds in the atmosphere. They are central to atmospheric photochemistry as absorption of near-UV radiation by the C=O chromophore can lead to photolysis. If photolysis does not occur on electronic excited states, non-radiative relaxation to the ground state will form carbonyls with extremely high internal energy. These “hot” molecules can access a range of ground state reactions. Up to nine potential ground state reactions are investigated at the B2GP-PLYP-D3/def2-TZVP level of theory for a dataset of 20 representative carbonyls. Almost all are energetically accessible under tropospheric conditions. Comparison with experiment suggests the most significant ground state dissociation pathways will be concerted triple fragmentation in saturated aldehydes, Norrish type III dissociation to form another carbonyl, and H2-loss involving the formyl H atom in aldehydes. Tautomerisation, leading to more reactive unsaturated species, is also predicted to be energetically accessible and is likely to be important when there is no low-energy ground state dissociation pathway, for example in α,β-unsaturated carbonyls and some ketones. The concerted triple fragmentation and H2-loss pathways have immediate atmospheric implication to global H2 production and tautomerisaton has implication to the atmospheric production of organic acids.

Evaluating the contribution of the unexplored photochemistry of aldehydes on the tropospheric levels of molecular hydrogen (H<sub>2</sub>)

“…The glycolaldehyde TF energy barrier is even lower than that of propanal (295 kJ mol −1 ), which has been shown to have a H 2 quantum yield of ∼8% (Kharazmi, 2018). Thus, both methylglyoxal and glycolaldehyde are the aldehydes that are theoretically most likely to have a H 2 channel based on the calculations from Rowell et al (2021) and (for glycolaldehyde at least) may have a H 2 quantum yield greater than the 1% tested in our simulations. The experimental determination of H 2 quantum yields from a TF channel for methylglyoxal and glycolaldehyde should therefore be prioritised.…”

“…While we did not distinguish here between different types of aldehydes, Rowell et al (2021) explain that the aldehydes that most likely yield H 2 in the troposphere are those with a triple fragmentation (TF) channel with energies below 350 kJ mol −1 . Sufficiently low TF energy barriers have been calculated for both methylglyoxal (330 kJ mol −1 ) and glycolaldehyde (229 kJ mol −1 ) (Rowell et al, 2021). Glycolaldehyde has the lowest energy barrier for the TF channels of any of the aldehydes calculated by Rowell et al.…”

“…We assigned the 1% quantum yield found by Harrison et al (2019) for acetaldehyde to the selected aldehydes tested in GEOS-Chem, analogously to what was done for the box modelling (Section 2.1). The 1% from acetaldehyde was taken as the reference quantum yield to test given that measurements for the rest of the aldehydes are not available, but the energy barriers for the production of H 2 from aldehyde photolysis indicates that the dissociation channels are accessible (Rowell et al, 2021). For acetaldehyde, glycolaldehyde, HPALD and RCHO, a branching ratio on the existing photolysis channels was added to account for the primary production of H 2 in addition to the existing photolysis products.…”

“…Recent laboratory findings by Harrison et al (2019) identified a previously unknown H 2 channel for acetaldehyde yielding H 2 at a 1% quantum yield. This finding by Harrison et al (2019) was complemented by aldehyde ground state calculations that show that the direct H 2 channel is also possible for other aldehydes (Rowell et al, 2021). Here, we assessed the impact of the We configured the box model at three sites (London, Cape Verde and Borneo) to explore the production of H 2 under distinctive atmospheric conditions and constrained each box model simulation with measurements.…”

Deciphering Methylation Effects on S2(ππ*) Internal Conversion in the Simplest Linear α,β-Unsaturated Carbonyl

“…AC and its methylated derivatives are all present in the atmosphere as volatile organic compounds, being both the precursors and intermediates in several chemical processes. − Despite their small size, characterizing the electronic structure of their excited states has proven challenging. An early experimental study on AC from Walsh assigned absorption bands at 412, 387, and 193.5 nm to S 0 → T 1 ( nπ *), S 0 → S 1 ( nπ *), and S 0 → S 2 ( ππ *) transitions, respectively .…”