Ab Initio Quantum Chemical Studies of Reactions in Astrophysical Ices: 2. Reactions in H₂CO/HCN/HNC/H₂O Ices

Theoretical electronic structure calculations were used to investigate reactions between formaldehyde (H₂CO) 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, Icarus142, 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 H₂CO and HCN or HNC exhibit isomerization during the reaction: H₂CO+HCN yields HOCH₂NC (isocyanomethanol), while H₂CO+HNC yields HOCH₂CN (glyconitrile). As a direct consequence of the greater stability of the -CN bond over the -NC bond, H₂CO+HNC has a lower reaction barrier and is substantially more exothermic. However, the barrier for isomerization of HOCH₂NC to HOCH₂CN 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 H₂CO+HNC reactions and may also drive less favorable H₂CO+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.