Reduction of CO functional groups through H addition reactions: a comparative study between H2CO + H, CH3CH2CHO + H and CH3OCHO + H under interstellar conditions

Abstract

H-addition reactions on the icy interstellar grains may play an important role in the formation of complex organic molecules. In the present work we propose a comparative study of H₂CO + H, CH₃CH₂CHO + H and CH₃OCHO + H solid state reactions at 10 K under interstellar conditions in order to characterize the main reaction pathways involved in the hydrogenation of a CHO functional group. We show that the most probable mechanism for the formation of alcohols under non-energetic conditions through the saturation of the CHO group corresponds to the attachment of the H atom to the CH group with noticeable variations of the energy barriers for each studied reaction. These energy barriers have been calculated to be 8.3, 14.6 and 32.7 kJ mol⁻¹ for H₂CO + H, CH₃CH₂CHO + H and CH₃OCHO + H, respectively. The coupling of the experimental and theoretical analysis proves that while the simplest aldehyde, formaldehyde, is easily reduced to methanol, methylformate and propanal behave differently under H-bombardments but they cannot be a source of alcohol formation through H-addition reactions. Consequently, for the formation of alcohols larger than CH₃OH, other chemical pathways should be taken into account, probably energetic processing such as the photolysis of interstellar ice analogues containing C-, H- and O-bearing compounds or the coupling of the H-addition reaction and photon-irradiation on species with a CHO functional group.