Ab Initio Quantum Chemical Studies of Reactions in Astrophysical Ices: 2. Reactions in H 2CO/HCN/HNC/H 2O Ices

Abstract

Theoretical electronic structure calculations were used to investigate reactions between formaldehyde (H 2CO) and both hydrogen cyanide (HCN) and isocyanide (HNC) in search of other favorable reactions such as ammonia–formaldehyde addition, which was found in a recent theoretical study to be strongly enhanced when it occurs within cold ices (D. E. Woon 1999, Icarus 142, 550–556). Reaction components were characterized in clusters composed of the reactants and up to two explicit, catalytic waters and then embedded in a continuum polarization field to incorporate the bulk solvation effects of ice. Intriguingly, reactions between H 2CO and HCN or HNC exhibit isomerization during the reaction: H 2CO+HCN yields HOCH 2NC (isocyanomethanol), while H 2CO+HNC yields HOCH 2CN (glyconitrile). As a direct consequence of the greater stability of the -CN bond over the -NC bond, H 2CO+HNC has a lower reaction barrier and is substantially more exothermic. However, the barrier for isomerization of HOCH 2NC to HOCH 2CN is comparable with the initial barrier and may yield the more stable nitrile if conditions are favorable. Although both reactions are enhanced by active and passive interactions with water in the ice, neither barrier is reduced to the point where the reaction is likely to proceed at very cold temperatures without another source of energy. If ammonia were also present in the ice, heat from its reaction with formaldehyde is predicted to be sufficient to initiate H 2CO+HNC reactions and may also drive less favorable H 2CO+HCN reactions. Three-body reactions that yield very small polyoxymethylene polymers terminated with -CN and -NC groups were also studied, as well as reactions between HCN and ammonia or water.