Hydrogenation of saturated organic and inorganic molecules in metallic hydrogen

Abstract

Metallic hydrogen is the most common condensed material in the universe, however, experimental studies are extremely challenging, and understanding of this material has been led by theory. Chemistry in this environment has not been probed experimentally, so here we examine carbon, nitrogen, and oxygen in metallic hydrogen using density functional theory calculations. We find that carbon, nitrogen and oxygen react with each other and metallic hydrogen to produce molecules with covalent-type bonding based on sixfold coordinated carbon, threefold oxygen and fourfold nitrogen: CH₆, C₂H₈, C₃H₁₀, OH₃, NH₄, and CH₄OH. In view of the excess hydrogen we refer to them as hypermethane, hyperethane etc. This work suggests that molecular chemistry may take place in very different environments from those found on earth, and may be common throughout the universe. Furthermore, the solubility of C, O, and N casts doubt on whether rocky cores can exist in giant planets.

Fig. 1. Enthalpy and Gibbs free energy differences, electronic density of states, and electron localised function (ELF) of carbon in solid, metallic hydrogen.

a Enthalpy of solution (eV/atom) as a function of number of removed hydrogens from static relaxation (blue) and NPT-ensemble MD (orange). b Electronic density of states from a typical BOMD snapshot, showing the characteristic free electron form with a pseudogap at the Fermi Energy. c Phonon density of states from DFPT of a structure from a relaxed CH₁₂₄ MD snapshot (blue), compared with pure I4₁/amd hydrogen (orange). d Visualization of eigenvectors of the characteristic symmetric stretch mode of CH₆ as shown in the sharp peak above 3000 cm⁻¹ in part (c). e–g Electron Localization Function (ELF) surrounding a single carbon atom, including cross-sections of planes A and B. The high ELF values suggest the presence of covalent-type bonds, rather than the metallic bonding elsewhere. The zero value for ELF on the carbon atom is an artefact arising from the 1s electron being described by the pseudopotential.

Fig. 2. Radial function distribution, mean square displacement, and cumulative number of bonds per carbon from BOMD in NPT ensemble.

a The radial distribution function of hydrogen atom in the CH₆+H₁₁₈ BOMD simulations under the NPT ensemble at varying temperatures, and 500 GPa. The blue solid line denotes the presence of structural peaks in CH₆+H₁₁₈, while the orange and green solid lines signify the disappearance of these peaks due to system melting. b Mean square displacement (MSD) of CH₆+H₁₁₈ is examined, with the MSD depicting the crystal structure of CH₆+H₁₁₈ with a finite MSD at 300 K, represented by the blue solid line. Conversely, cases of melting at 600 K and 900K exhibit increased MSD. c The angle distribution of CH₆ in solid metallic hydrogen at 300  K between pairs of hydrogen atoms with respect to carbon for each step of MD within the radius cutoff of 1.30 Å where the average number of coordination is about 6 hydrogen atoms. Two significant peaks at 90 and 180 degrees demonstrate that the molecule is a octahedron, illustrated by four snapshots of the hypermolecule. d The radial distribution function illustrates the distribution of carbon-hydrogen pairs. The first peak of the CH-pair radial distribution function lies between 1.0 Å and 1.3 Å. e Cumulative number of bonds (CH bonds per carbon), indicating the count of hydrogens surrounding each carbon atom: the plateau at 6 confirms the hypermolecule. f As part (c) at 600 K: the angle distribution of CH₆.

Fig. 3. Radial function distribution, cumulative number of bonds in the hypermolecules, and representative snapshots from liquid simulations.

a Displays the RDF of the carbon-hydrogen pairs in metallic hydrogen with one carbon (blue line), two carbons (orange line), three carbons (green line), one oxygen and a CO pair (red line), indicating a smearing of the first peak from 1.0 Å to 1.30 Å. In the  ~10² atom simulations (main figure), structure extends throughout the supercell, however the  ~10³ atom simulations (inset) indicates a suppression of these structural oscillations at greater distances. b Shows the CDF (number of CH neighbours per carbon) at 1.30 Å, revealing values of ~6, 4, 3.3, and 4, respectively. The short-range order is nearly identical in both system sizes (solid lines:  ~10² atoms, dashed lines  ~10³ atoms) (See also in Supplementary Fig. S3 in the supplementary material) (c, d) equivalent RDF and CDF for OH and NH pairs, showing well-defined and long-lived bonding. These values suggest the formation of molecules, as depicted in (e–i): Two snapshots from each MD showcasing novel chemical species in liquid metallic hydrogen where the radial cutoff is set at minimum of RDFs of Fig. (a) and (c), including e CH₆, f C₂H₈, g C₃H₁₀, h CH₄OH and i OH₃ and j NH₄, with schematic molecular bonding shown below.