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 reac tions between formaldehyde (H2CO) and both hydrogen cyanide (HCN) and isocy anide (HNC) in search of other favorable reactions such as ammonia-formalde hyde 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 r eactants and up to two explicit, catalytic waters and then embedded in a co ntinuum 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 H2C O + HNC yields HOCH2CN (glyconitrile). As a direct consequence of the great er stability of the -CN bond over the -NC bond, H2CO + HNC has a lower reac tion 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. Althoug h 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 energ y. If ammonia were also present in the ice, heat from its reaction with for maldehyde is predicted to be sufficient to initiate H2CO + HNC reactions an d 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. (C) 2001 Academic Press.