Ab Initio quantum chemical studies of reactions in astrophysical ices - Reactions involving CH₃OH, CO₂, CO, and HNCO in H ₂CO/NH₃/H₂O ices

While reactions between closed shell molecules generally involve prohibitive barriers in the gas phase, prior experimental and theoretical studies have demonstrated that some of these reactions are significantly enhanced when confined within an icy grain mantle and can occur efficiently at temperatures below 100 K with no additional energy processing. The archetypal case is the reaction of formaldehyde (H₂CO) and ammonia (NH ₃) to yield hydroxymethylamine (NH₂CH₂OH). In the present work we have characterized reactions involving methanol (CH ₃OH), carbon dioxide (CO₂), carbon monoxide (CO), and isocyanic acid (HNCO) in search of other favorable cases. Most of the emphasis is on CH₃OH, which was investigated in the two-body reaction with one H₂CO and the three-body reaction with two H₂CO molecules. The addition of a second H₂CO to the product of the reaction between CH₃OH and H₂CO was also considered as an alternative route to longer polyoxymethylene polymers of the -CH₂O- form. The reaction between HNCO and NH₃ was studied to determine if it can compete against the barrierless charge transfer process that yields OCN ^- and NH₄⁺. Finally, the H₂CO + NH₃ reaction was revisited with additional benchmark calculations that confirm that little or no barrier is present when it occurs in ice.