De. Woon, Ab initio quantum chemical studies of reactions in astrophysical ices - 2.Reactions in H2CO/HCN/HNC/H2O ices, ICARUS, 149(1), 2001, pp. 277-284
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.