Formamide formation on forsterite and Mg(OH)_2·H_2O

Formamide formation in the Universe has been the subject of debate in several studies due to its importance as a precursor to amino acids and RNA. Current studies focus on the gas-phase and icy grain formation routes. However, formamide formation on bare silicate surfaces or hydrated Mg-bearing molecular models remains largely unexplored in warm Mg-rich dust environments, such as those potentially relevant to protoplanetary disks. This study presents a quantum-chemical investigation of formamide formation on an Mg center and a silicate surface. Mg(OH)_2·H_2O, considered here as a possible post-shock hydrated Mg-bearing species, and the bare (010) forsterite (Mg_2SiO_4) surface, used as an analog of a bare dust-grain surface , may promote formamide formation via NH_2 addition to H_2CO. This may help clarify a potential additional pathway to formamide formation in warm Mg-rich environments. I employed density functional theory (DFT), a quantum chemistry method, to study the addition of NH_2 to H_2CO with an isolated Mg(OH)_2·H_2O molecular model and on the forsterite (010) surface. First, I benchmarked various theoretical methodologies (M06-2X, PBE, PBE+U5, and VV) against the gold standard CCSD(T)-F12 and evaluated the performance of double-zeta and triple-zeta basis sets. I then examined the ability of Mg(OH)_2·H_2O and the forsterite (010) surface to promote formamide formation. Finally, for the case with an entrance barrier, I computed unimolecular rate constants and half-lives to assess its kinetic behavior. Of the DFT methods I tested, M06-2X and PBE+U5 provide the closest agreement with CCSD(T)-F12 for the addition of NH_2 to H_2CO on an Mg center. Mg(OH)_2·H_2O promotes proton transfer within the NH_2–CH_2O intermediate via the O–H group coordinated with Mg following NH_2 addition to H_2CO, whereas on the forsterite (010) surface, a barrier-less addition pathway is found when NH_2 is initially physisorbed close to the formaldehyde center, followed by proton transfer leading to formamide formation. This quantum-chemical study shows that the Mg-bearing model systems considered here can promote rapid proton transfer following NH_2 addition to H_2CO, whereas the corresponding bare gas-phase reaction proceeds through a large barrier for H abstraction to form formamide. For the Mg(OH)_2·H_2O model, the reaction step from the pre-reactive complex is predicted to occur on nanosecond timescales at temperatures approaching 300 K. The present work characterizes a potential pathway to formamide formation in Mg-bearing environments and provides molecular-level energetic and kinetic constraints for its future assessment in gas-grain astrochemical models.