Ab initio quantum chemical studies of reactions in astrophysical ices. 4. Reactions in ices involving HCOOH, CH₂NH, HCN, HNC, NH₃, and H₂O

Ice-bound condensed-phase reactions involving formic acid (HCOOH), methylenimine (CH₂NH), hydrogen cyanide (HCN), hydrogen isocyanide (HNC), and ammonia (NH₃) were investigated in order to characterize possible pathways to larger organic species that are efficient at the cold temperatures prevalent in cometary nuclei and the interstellar medium. Previous laboratory and computational modeling has demonstrated that reactions between some closed-shell species can be significantly enhanced when they occur within a matrix of water ice. Certain key reactions can occur at temperatures under 100 K, in spite of having gas phase barriers that may be 30 kcal/mol or higher. The present study considered one- and two-step reactions of HCOOH and NH₃ to yield formamide (NH₂CHO), reactions between CH₂NH and HCN, HNC, NH₃, and H₂O, and the reaction of HCOOH and CH₂NH to yield glycine (NH₂CH₂COOH). The most favorable process identified in this work is production of NH₂CH(OH)₂, the intermediate in the two-step pathway from HCOOH and NH₃ to NH₂CHO, which is enhanced considerably when it occurs within ice but not to the extent that it is likely to occur unassisted at temperatures below 100 K.