Magnesium-rich dust could offer another route to formamide in space

The chemistry of a dust grain may depend as much on where molecules sit as on what they are made of. A new theoretical study suggests that magnesium-bearing environments could help formaldehyde and an NH2 radical become formamide—a molecule relevant to proposed prebiotic chemistry—provided the reactants meet in a favorable configuration.

Formamide has been observed in star-forming regions and comets, but its formation remains debated. Researchers have explored reactions in gas and on icy dust grains. In a new preprint, Dario Campisi investigates another possibility: chemistry involving exposed magnesium-rich silicate surfaces or a small hydrated magnesium species. The calculations identify promising elementary reaction pathways, not a demonstrated explanation for astronomical formamide abundances.

Formamide interests astrochemists because it contains a simple carbon–nitrogen arrangement related to peptide bonds and has been proposed as a precursor in chemistry leading toward amino acids and RNA-related molecules. This study does not simulate those later processes or the emergence of life. Its question is narrower: can magnesium-bearing environments help an NH2 radical react with formaldehyde to make formamide?

Two models, different reaction routes

Campisi examined two systems. One was an isolated Mg(OH)2·H2O molecule, used to represent a chemically defined hydrated magnesium center. The other was a repeating model of the bare (010) surface of forsterite, a magnesium-bearing silicate mineral. The isolated molecule is an exploratory model of a possible post-shock species, not a species whose presence in the proposed astronomical environments this work establishes.

The study used density functional theory, a quantum-chemical approach for estimating molecular structures and energies. Before investigating the mineral surface, Campisi compared several computational methods against a higher-level coupled-cluster reference calculation for the small magnesium model. M06-2X and a corrected version of PBE, called PBE+U5, gave the closest overall agreement among the approaches tested. Reaction-path calculations then examined bond formation and hydrogen transfer.

In the isolated magnesium model, NH2 first forms a weakly bound encounter complex with formaldehyde already coordinated to magnesium. Crossing an entrance barrier creates a carbon–nitrogen bond. The resulting intermediate then rotates toward a magnesium-coordinated hydroxyl group. After that rearrangement, hydrogen transfer proceeds without an additional resolved barrier, producing formamide and a second magnesium-coordinated water molecule.

The important assistance comes after the carbon–nitrogen bond forms. The paper finds that the hydrated magnesium model does not substantially reduce the bond-forming barrier measured from the encounter complex compared with the corresponding gas-phase reaction. Instead, it facilitates the subsequent hydrogen-transfer step, which faces a substantial barrier in the unassisted gas-phase route discussed in the paper.

On forsterite, placement matters

The most favorable mineral-surface pathway begins with formaldehyde chemically bound to forsterite and NH2 weakly attached nearby. From that particular starting arrangement, the calculations resolve no barrier for carbon–nitrogen bond formation or the subsequent transfer of hydrogen to a neighboring surface oxygen. The product remains bound to the surface.

That result does not mean every encounter on forsterite is barrier-free. Direct attack by gas-phase NH2 on surface-bound formaldehyde has a calculated barrier of 0.79 electronvolts. When NH2 instead starts chemically bound to a neighboring magnesium site, it must overcome a rotational barrier of 0.94 electronvolts and a bond-forming barrier of 0.90 electronvolts. Campisi suggests that the favorable orientation available to the weakly attached radical helps explain the difference.

The distinction is central: the low-barrier pathway starts with the reactants already close together. The study does not calculate the full sequence of arrival, long-range surface migration and encounters needed to establish how often that arrangement forms.

Fast chemistry is not an abundance prediction

For the isolated hydrated magnesium model, the calculated half-life for conversion of the encounter complex into the carbon–nitrogen-bonded intermediate is approximately 2.6 nanoseconds at 300 kelvin. That number describes one elementary step after the encounter complex exists—not the time required to assemble reactants from space or an independently calculated overall formamide-production time.

An estimated NH2 escape half-life of about 140 nanoseconds at the same temperature makes the forward step roughly 54 times faster in the adopted calculation. But the paper cautions that uncertainties in the escape-rate prefactor and binding energy could substantially change that competition.

Astronomical relevance also depends on conditions absent from these local reaction calculations. Ice can restrict access to bare silicates at low temperatures. Hydrogen atoms can compete for formaldehyde, while irradiation, desorption and other reactions influence what survives. The availability of hydrated magnesium species remains uncertain.

The paper, accepted for publication in Astronomy & Astrophysics according to the arXiv record, therefore offers molecular-level inputs for future astrochemical models rather than an observed new production mechanism. Its strongest message is conditional: magnesium-bearing environments can support favorable formamide chemistry in the modeled configurations, but their contribution in space still needs assessment.