Formation of Acetaldehyde on CO-Rich Ices

The radicals HCO and CH3 on carbon monoxide ice surfaces were simulated using the 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 CH3. If they are located on the surface close to each other (3–4 A), molecular dynamics calculations based on the density functional theory show that they can form acetaldehyde (CH3CHO) or CH4 + CO in barrier-less reactions, depending upon the initial orientation of the molecules with respect to each other. In some orientations, no spontaneous reactions were found and 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, such as acetaldehyde, can be formed by recombination react...