Irradiation of water ice by C(+) ions in the cosmic environment.

We present a first-principles MD (FPMD) study of the interaction of low-energy, positively charged carbon (C(+)) projectiles with amorphous solid water clusters at 30 K. Reactions involving the carbon ion at an initial energy of 11 and 1.7 eV with a 30-molecule cluster have been investigated. Simulations indicate that the neutral isoformyl radical, COH(•), and carbon monoxide, CO, are the dominant products of these reactions. All of these reactions are accompanied by the transfer of a proton from the reacting water molecule to the ice, where it forms a hydronium ion. We find that COH(•) is formed either via a direct, "knock-out", mechanism following the impact of the C(+) projectile upon a water molecule or by creation of a COH2(+) intermediate. The direct mechanism is more prominent at higher energies. CO is generally produced following the dissociation of COH(•). More frequent production of the formyl radical, HCO(•), is observed here than in gas-phase calculations. A less commonly occurring product is the dihydroxymethyl, CH(OH)2(•), radical. Although a minor result, its existence gives an indication of the increasing chemical complexity that is possible in such heterogeneous environments.

Organic synthesis in the interstellar medium by low-energy carbon irradiation.

We present a first principles molecular dynamics (FPMD) study of the interaction of low-energy neutral carbon projectiles with amorphous solid water clusters at 30 K. Reactions involving the carbon atom at an initial energy of 11 and 1.7 eV with 30-molecule clusters have been investigated. Simulations indicate that the formation of hydroxymethylene, an intermediate in formaldehyde production, dominates at the higher energy. The reaction proceeds by fragmenting a water molecule, binding the carbon to the OH radical, and saturating the C valence with a hydrogen atom that can arise from the originally dissociated water molecule, or through a chain of proton transfer events. We identified several possible pathways for the formation of HCOH. When the initial collision occurs at the periphery of the cluster, we observe the formation of CO and the evaporation of water molecules. At the lower energy water fragmentation is not favorable, thus leading to the formation of weakly bound carbon-water complexes.