# Tropospheric Photochemistry of 2-Butenedial: Role of the Triplet States, CO and Acrolein Formation, and the Experimentally Unidentified Carbonyl Compound—Theoretical Study

## TL;DR

This study explores the chemical reactions of 2-butenedial in the atmosphere, focusing on the formation of unknown compounds and carbon monoxide using theoretical methods.

## Contribution

The study identifies a possible pathway for an experimentally unidentified carbonyl compound and explains CO and acrolein formation via triplet states.

## Key findings

- Water-assisted isomerizations lead to a cyclic carbonyl compound, though the pathway is energy-intensive.
- An anhydride formed via ketene-enol and carboxylic acid/enol addition is proposed as the unidentified carbonyl product.
- Triplet states play a key role in the formation of carbon monoxide and acrolein from 2-butenedial.

## Abstract

Solar irradiation of 2-butenedial in the lower troposphere mainly produces isomeric ketene-enol (a key intermediate product), furanones, and maleic anhydride, the formation pathways of which were investigated in a previous study. The other main products were carbon monoxide and an experimentally unidentified carbonyl compound. This was the subject of the present study. The oxidative reaction mechanisms were studied using DFT calculations. Water intervention is found essential. Its addition and subsequent water-assisted isomerizations (an ene-gem-diol/enol and a carboxylic acid/enol form), followed by cyclization, lead to an interesting cyclic carbonyl compound, but this pathway appears to be rather energy demanding. An alternative implies water cooperation in a ketene-enol + carboxylic acid/enol addition that gives the relevant anhydride. The anhydride is proposed as a candidate for the experimentally unidentified carbonyl product. Regarding CO and acrolein formation, the role of the triplet states, as defined by the probability of intersystem crossing from the excited singlet state S1 to T2 and T1, is discussed. The T1 photolysis pathway connecting butenedial to propenal + CO was then defined.