# Formation of Acetaldehyde on CO-rich Ices

## TL;DR

This study uses density functional theory to simulate radical interactions on CO ice surfaces, demonstrating that acetaldehyde can form spontaneously through radical recombination, highlighting a potential pathway for complex organic molecule formation in space.

## Contribution

It provides detailed computational evidence that radicals on CO ice can form acetaldehyde via barrier-less reactions, emphasizing the importance of configurational sampling and vibrational effects.

## Key findings

- Radicals HCO and CH3 bind to CO ice with broad energy distributions.
- Radical pairs can form acetaldehyde or other products depending on orientation.
- Complex organic molecules can form via radical recombination on CO surfaces.

## Abstract

The radicals HCO and CH$_3$ on carbon monoxide ice surfaces were simulated using density functional theory. Their binding energy on amorphous CO ice shows broad distributions, with approximative average values of 500 K for HCO and 200 K for CH$_3$. If they are located on the surface close to each other (3 to 4 \AA), molecular dynamics calculations based on density functional theory show that they can form acetaldehyde (CH$_3$CHO) or CH$_4$ + CO in barrier-less reactions, depending on the initial orientation of the molecules with respect to each other. In some orientations, no spontaneous reactions were found, the products remained bound to the surface. Sufficient configurational sampling, inclusion of the vibrational zero point energy, and a thorough benchmark of the applied electronic structure method are important to predict reliable binding energies for such weakly interacting systems. From these results it is clear that complex organic molecules, like acetaldehyde, can be formed by recombination reactions of radicals on CO surfaces.