Pressure-Driven Reactivity in Dense Methane-Nitrogen Mixtures

Carbon, nitrogen, and hydrogen are among the most abundant elements in the solar system, and our understanding of their interactions is fundamental to prebiotic chemistry. CH4 and N2 are the simplest archetypical molecules formed by these elements and are both markedly stable under extremes of pressure. Through a series of diamond anvil cell experiments supported by density functional theory calculations, we observe diverse compound formation and reactivity in the CH4-N2 binary system at high pressure. Above 7 GPa two concentration-dependent molecular compounds emerge, (CH4)5N2 and (CH4)7(N2)8, held together by weak van der Waals interactions. Strikingly, further compression at room temperature irreversibly breaks the N2 triple bond, inducing the dissociation of CH4 above 140 GPa, with the near-quenched samples revealing distinct spectroscopic signatures of strong covalently bonded C-N-H networks. High temperatures vastly reduce the required pressure to promote the reactivity between CH4 and N2, with NH3 forming together with longer-chain hydrocarbons at 14 GPa and 670 K, further decomposing into powdered diamond when temperatures exceed 1200 K. These results exemplify how pressure-driven chemistry can cause unexpected complexity in the most simple molecular precursors.

Cyclic random graph models predicting giant molecules in hydrocarbon pyrolysis

Hydrocarbon pyrolysis is a complex chemical reaction system at extreme temperature and pressure conditions involving large numbers of chemical reactions and chemical species. Only two kinds of atoms are involved: carbons and hydrogens. Its effective description and predictions for new settings are challenging due to the complexity of the system and the high computational cost of generating data by molecular dynamics simulations. However, the ensemble of molecules present at any moment and the carbon skeletons of these molecules can be viewed as random graphs. Therefore, an adequate random graph model can predict molecular composition at a low computational cost. We propose a random graph model featuring disjoint loops and assortativity correction and a method for learning input distributions from molecular dynamics data. The model uses works of Karrer and Newman [Phys. Rev. E 82, 066118 (2010)10.1103/PhysRevE.82.066118] and Newman [Phys. Rev. Lett. 89, 208701 (2002)10.1103/PhysRevLett.89.208701] as building blocks. We demonstrate that the proposed model accurately predicts the size distribution for small molecules as well as the size distribution of the largest molecule in reaction systems at the pressure of 40.5 GPa, temperature range of 3200-5000 K, and H/C ratio range from 2.25 as in octane through 4 as in methane.

Synthesis and Stability of Biomolecules in C-H-O-N Fluids under Earth's Upper Mantle Conditions

How life started on Earth is an unsolved mystery. There are various hypotheses for the location ranging from outer space to the seafloor, subseafloor, or potentially deeper. Here, we applied extensive ab initio molecular dynamics simulations to study chemical reactions between NH3, H2O, H2, and CO at pressures (P) and temperatures (T) approximating the conditions of Earth's upper mantle (i.e., 10-13 GPa, 1000-1400 K). Contrary to the previous assumptions that large organic molecules might readily disintegrate in aqueous solutions at extreme P-T conditions, we found that many organic compounds formed without any catalysts and persisted in C-H-O-N fluids under these extreme conditions, including glycine, ribose, urea, and uracil-like molecules. Particularly, our free-energy calculations showed that the C-N bond is thermodynamically stable at 10 GPa and 1400 K. Moreover, while the pyranose (six-membered ring) form of ribose is more stable than the furanose (five-membered ring) form at ambient conditions, we found that the formation of the five-membered-ring form of ribose is thermodynamically more favored at extreme conditions, which is consistent with the exclusive incorporation of β-d-ribofuranose in RNA. We have uncovered a previously unexplored pathway through which the crucial biomolecules could be abiotically synthesized from geofluids in the deep interior of Earth and other planets, and these formed biomolecules could potentially contribute to the early stage of the emergence of life.

Mixture of hydrogen and methane under planetary interior conditions

We employ first-principles molecular dynamics simulations to provide equation-of-state data, pair distribution functions (PDFs), diffusion coefficients, and band gaps of a mixture of hydrogen and methane under planetary interior conditions as relevant for Uranus, Neptune, and similar icy exoplanets. We test the linear mixing approximation, which is fulfilled within a few percent for the chosen P-T conditions. Evaluation of the PDFs reveals that methane molecules dissociate into carbon clusters and free hydrogen atoms at temperatures greater than 3000 K. At high temperatures, the clusters are found to be short-lived. Furthermore, we calculate the electrical conductivity from which we derive the non-metal-to-metal transition region of the mixture. We also calculate the electrical conductivity along the P-T profile of Uranus [N. Nettelmann et al., Planet. Space Sci., 2013, 77, 143-151] and observe the transition of the mixture from a molecular to an atomic fluid as a function of the radius of the planet. The density and temperature ranges chosen in our study can be achieved using dynamic shock compression experiments and seek to aid such future experiments. Our work also provides a relevant data set for a better understanding of the interior, evolution, luminosity, and magnetic field of the ice giants in our solar system and beyond.

Massive abiotic methane production in eclogite during cold subduction

Methane (CH₄) is a critical but overlooked component in the study of the deep carbon cycle. Abiotic CH₄ produced by serpentinization of ultramafic rocks has received extensive attention, but its formation and flux in mafic rocks during subduction remain poorly understood. Here, we report massive CH₄-rich fluid inclusions in well-zoned garnet from eclogites in Western Tianshan, China. Petrological characteristics and carbon-hydrogen isotopic compositions confirm the abiotic origin of this CH₄. Reconstructed P-T-fO₂-fluid trajectories and Deep Earth Water modeling imply that massive abiotic CH₄ was generated during cold subduction at depths of 50-120 km, whereas CO₂ was produced during exhumation. The massive production of abiotic CH₄ in eclogites may result from multiple mechanisms during prograde high pressure-ultrahigh pressure metamorphism. Our flux calculation proposes that abiotic CH₄ that has been formed in HP-UHP eclogites in cold subduction zones may represent one of the largest, yet overlooked, sources of abiotic CH₄ on Earth.

Ethane and methane at high pressures: Structure and stability

We have performed a combined experimental and theoretical study of ethane and methane at high pressures of up to 120 GPa at 300 K using x-ray diffraction and Raman spectroscopies and the USPEX ab initio evolutionary structural search algorithm, respectively. For ethane, we have determined the crystallization point, for room temperature, at 2.7 GPa and also the low pressure crystal structure (phase A). This crystal structure is orientationally disordered (plastic phase) and deviates from the known crystal structures for ethane at low temperatures. Moreover, a pressure induced phase transition has been identified, for the first time, at 13.6 GPa to a monoclinic phase B, the structure of which is solved based on good agreement with the experimental results and theoretical predictions. For methane, our x-ray diffraction measurements are in agreement with the previously reported high-pressure structures and equation of state (EOS). We have determined the EOSs of ethane and methane, which provides a solid basis for the discussion of their relative stability at high pressures.

In-situ abiogenic methane synthesis from diamond and graphite under geologically relevant conditions

Diamond and graphite are fundamental sources of carbon in the upper mantle, and their reactivity with H₂-rich fluids present at these depths may represent the key to unravelling deep abiotic hydrocarbon formation. We demonstrate an unexpected high reactivity between carbons' most common allotropes, diamond and graphite, with hydrogen at conditions comparable with those in the Earth's upper mantle along subduction zone thermal gradients. Between 0.5-3 GPa and at temperatures as low as 300 °C, carbon reacts readily with H₂ yielding methane (CH₄), whilst at higher temperatures (500 °C and above), additional light hydrocarbons such as ethane (C₂H₆) emerge. These results suggest that the interaction between deep H₂-rich fluids and reduced carbon minerals may be an efficient mechanism for producing abiotic hydrocarbons at the upper mantle.

Water-Gas Shift Reaction Produces Formate at Extreme Pressures and Temperatures in Deep Earth Fluids

The water-gas shift reaction is one of the most important reactions in industrial hydrogen production and plays a key role in Fischer-Tropsch-type synthesis, which is widely believed to generate hydrocarbons in the deep carbon cycle but is little known at extreme pressure-temperature conditions found in the Earth's upper mantle. Here, we performed extensive ab initio molecular dynamics simulations and free energy calculations to study the water-gas shift reaction. We found the direct formation of formic acid from CO and supercritical water at 10-13 GPa and 1400 K without any catalyst. Contrary to the common assumption that formic acid or formate is an intermediate product, we found that HCOOH is thermodynamically more stable than the products of the water-gas shift reaction above 3 GPa and at 1000-1400 K. Our study suggests that the water-gas shift reaction may not happen in the Earth's upper mantle, and formic acid or formate may be an important carbon carrier in reducing environments, participating in many geochemical processes in deep Earth.

Carbon dioxide, bicarbonate and carbonate ions in aqueous solutions under deep Earth conditions

We investigate the effect of pressure, temperature and acidity on the composition of water-rich carbon-bearing fluids under thermodynamic conditions that correspond to the Earth's deep crust and upper mantle. Our first-principles molecular dynamics simulations provide mechanistic insight into the hydration shell of carbon dioxide, bicarbonate and carbonate ions, and into the pathways of the acid/base reactions that convert these carbon species into one another in aqueous solutions. At temperatures of 1000 K and higher, our simulations can sample the chemical equilibrium of these acid/base reactions, thus allowing us to estimate the chemical composition of diluted carbon dioxide and (bi)carbonate ions as a function of acidity and thermodynamic conditions. We find that, especially at the highest temperature, the acidity of the solution is essential to determine the stability domain of CO₂vs. HCO₃^-vs. CO₃^2-.

Decomposition and oligomerization of 2,3-naphthyridine under high-pressure and high-temperature conditions

The chemical reaction of 2,3-naphthyridine, a nitrogen-containing aromatic compound, was investigated at pressures ranging from 0.5 to 1.5 GPa and temperatures from 473 to 573 K. A distinct decrease in the amount of residual 2,3-naphthyridine was observed in the samples recovered after reaction at ˃523 K at 0.5 and 1.0 GPa, and ˃548 K at 1.5 GPa. The formation of o-xylene and o-tolunitrile accompanied a decreasing N/C ratio of the reaction products, indicating decomposition of the aromatic ring and release of nitrogen. Precise analysis of the reaction products indicated the oligomerization of decomposed products with the residual 2,3-naphthyridine to form larger molecules up to 7mers. Nitrogen in the aromatic ring accelerated reactions to decompose the molecule and to oligomerize at lower temperatures than those typically reported for aromatic hydrocarbon oligomerization. The major reaction mechanism was similar between 0.5 and 1.5 GPa, although larger products preferentially formed in the samples at higher pressure.

Phase transition in metal–organic complex trans-PtCl2(PEt3)2 under pressure: insights into the molecular and crystal structure

Molecular reorientations result in structural phase transition in trans-PtCl₂(PEt₃)₂ under pressure, leading to a hydrogen bond assisted supramolecular architecture.

Learning reduced kinetic Monte Carlo models of complex chemistry from molecular dynamics

We propose a novel statistical learning framework for automatically and efficiently building reduced kinetic Monte Carlo (KMC) models of large-scale elementary reaction networks from data generated by a single or few molecular dynamics simulations (MD). Existing approaches for identifying species and reactions from molecular dynamics typically use bond length and duration criteria, where bond duration is a fixed parameter motivated by an understanding of bond vibrational frequencies. In contrast, we show that for highly reactive systems, bond duration should be a model parameter that is chosen to maximize the predictive power of the resulting statistical model. We demonstrate our method on a high temperature, high pressure system of reacting liquid methane, and show that the learned KMC model is able to extrapolate more than an order of magnitude in time for key molecules. Additionally, our KMC model of elementary reactions enables us to isolate the most important set of reactions governing the behavior of key molecules found in the MD simulation. We develop a new data-driven algorithm to reduce the chemical reaction network which can be solved either as an integer program or efficiently using L1 regularization, and compare our results with simple count-based reduction. For our liquid methane system, we discover that rare reactions do not play a significant role in the system, and find that less than 7% of the approximately 2000 reactions observed from molecular dynamics are necessary to reproduce the molecular concentration over time of methane. The framework described in this work paves the way towards a genomic approach to studying complex chemical systems, where expensive MD simulation data can be reused to contribute to an increasingly large and accurate genome of elementary reactions and rates.

Carbon and Nitrogen Speciation in N-poor C-O-H-N Fluids at 6.3 GPa and 1100-1400 °C

Deep carbon and nitrogen cycles played a critical role in the evolution of the Earth. Here we report on successful studying of speciation in C-O-H-N systems with low nitrogen contents at 6.3 GPa and 1100 to 1400 °C. At fO2 near Fe-FeO (IW) equilibrium, the synthesised fluids contain more than thirty species. Among them, CH4, C2H6, C3H8 and C4H10 are main carbon species. All carbon species, except for C1-C4 alkanes and alcohols, occur in negligible amounts in the fluids generated in systems with low H2O, but С15-С18 alkanes are slightly higher and oxygenated hydrocarbons are more diverse at higher temperatures and H2O concentrations. At a higher oxygen fugacity of +2.5 Δlog fO2 (IW), the fluids almost lack methane and contain about 1 rel.% C2-C4 alkanes, as well as fractions of percent of C15-18 alkanes and notable contents of alcohols and carboxylic acids. Methanimine (CH3N) is inferred to be the main nitrogen species in N-poor reduced fluids. Therefore, the behaviour of CH3N may control the nitrogen cycle in N-poor peridotitic mantle. Oxidation of fluids strongly reduces the concentration of CH4 and bulk carbon. However, higher alkanes, alcohols, and carboxylic acids can resist oxidation and should remain stable in mantle hydrous magmas.

Dense Hydrocarbon Structures at Megabar Pressures

The structure, bonding, and other properties of phases in the carbon-hydrogen system over a range of conditions are of considerable importance to a broad range of scientific problems. However, the phase diagram of the C-H system at high pressures and temperatures is still not known. To search for new low-energy hydrocarbon structures, we carried out systematic structure prediction calculations for the C-H system from 100 to 300 GPa. We confirmed several previously predicted structures but found additional compositions that adopt more stable structures. In particular, a C₂H₄ structure is found that has an indirect band gap, and phonon calculations confirm that it is dynamically stable over a broad pressure range. We also identify more carbon-rich structures that are energetically favorable. The results are important for understanding carbon-hydrogen interactions in high-pressure experiments, dense astrophysical environments and the deep carbon cycle in planetary interiors.

Free energy model for solid high-pressure phases of carbon

Analytic free energy models for three solid high-pressure phases--diamond, body centered cubic phase with eight atoms in the unit cell (BC8), and simple cubic (SC)--are developed using density functional theory. We explicitly include anharmonic effects by performing molecular dynamics simulations and investigate their density and temperature dependence in detail. Anharmonicity in the nuclear motion shifts the phase transitions significantly compared to the harmonic approximation. Furthermore, we apply a thermodynamically constrained correction that brings the equation of state in accordance with diamond anvil cell experiments. The performance of our thermodynamic functions is validated against Hugoniot experiments.

Synthesis of heavy hydrocarbons at the core-mantle boundary

The synthesis of complex organic molecules with C-C bonds is possible under conditions of reduced activity of oxygen. We have found performing ab initio molecular dynamics simulations of the C-O-H-Fe system that such conditions exist at the core-mantle boundary (CMB). H₂O and CO₂ delivered to the CMB by subducting slabs provide a source for hydrogen and carbon. The mixture of H₂O and CO₂ subjected to high pressure (130 GPa) and temperature (4000 to 4500 K) does not lead to synthesis of complex hydrocarbons. However, when Fe is added to the system, C-C bonds emerge. It means that oil might be a more abundant mineral than previously thought.

Carbon precipitation from heavy hydrocarbon fluid in deep planetary interiors

The phase diagram of the carbon-hydrogen system is of great importance to planetary sciences, as hydrocarbons comprise a significant part of icy giant planets and are involved in reduced carbon-oxygen-hydrogen fluid in the deep Earth. Here we use resistively- and laser-heated diamond anvil cells to measure methane melting and chemical reactivity up to 80 GPa and 2,000 K. We show that methane melts congruently below 40 GPa. Hydrogen and elementary carbon appear at temperatures of >1,200 K, whereas heavier alkanes and unsaturated hydrocarbons (>24 GPa) form in melts of >1,500 K. The phase composition of carbon-hydrogen fluid evolves towards heavy hydrocarbons at pressures and temperatures representative of Earth's lower mantle. We argue that reduced mantle fluids precipitate diamond upon re-equilibration to lighter species in the upwelling mantle. Likewise, our findings suggest that geophysical models of Uranus and Neptune require reassessment because chemical reactivity of planetary ices is underestimated.

Quantum molecular dynamics study on the structures and dc conductivity of warm dense silane

The ionic and electronic structures of warm dense silane at the densities of 1.795, 2.260, 3.382, and 3.844 g/cm(3) have been studied with temperatures from 1000 K to 3 eV using quantum molecular dynamics simulations. At all densities, the structures are melted above 1000 K. The matter states are characterized as polymeric from 1000 to 4000 K and become dense plasma states with further increasing temperature to 1 eV. At two lower densities of 1.795 and 2.260 g/cm(3), silane first dissociates and then becomes the polymeric state via a chain state from the initial crystalline structure. At higher densities, however, no dissociation stage was found. These findings can help us understand how the warm dense matter forms. A rise is found for the direct current electric conductivity at T ∼ 1000 K, indicating the nonmetal-to-metal transition. The conductivity decreases slightly with the increase of temperature, which is due to the more disordered structures at higher temperatures.

Lithium amide (LiNH2) under pressure

Static high pressure lithium amide (LiNH(2)) crystal structures are predicted using evolutionary structure search methodologies and intuitive approaches. In the process, we explore the relationship of the structure and properties of solid LiNH(2) to its molecular monomer and dimer, as well as its valence-isoelectronic crystalline phases of methane, water, and ammonia all under pressure. A NaNH(2) (Fddd) structure type is found to be competitive for the ground state of LiNH(2) above 6 GPa with the P = 1 atm I4[overline] phase. Three novel phases emerge at 11 (P4[overline]2(1)m), 13 (P4(2)/ncm), and 46 GPa (P2(1)2(1)2(1)), still containing molecular amide anions, which begin to form N-H···N hydrogen bonds. The P2(1)2(1)2(1) phase remains stable over a wide pressure range. This phase and another Pmc2(1) structure found at 280 GPa have infinite ···(H)N···H···N(H)···H polymeric zigzag chains comprising symmetric N···H···N hydrogen bonds with one NH bond kept out of the chain, an interesting general feature found in many of our high pressure (>280 GPa) LiNH(2) structures, with analogies in high pressure H(2)O-ices. All the predicted low enthalpy LiNH(2) phases are calculated to be enthalpically stable with respect to their elements but resist metallization with increasing pressure up to several TPa. The possibility of Li sublattice melting in the intermediate pressure range structures is raised.

Stability of Positively Charged Solutes in Water: A Transition from Hydrophobic to Hydrophilic

To improve the description of solvation thermodynamics of biomolecules, we report here the dependence of solvation on the curvature and surface charge of positively charged solutes in water based on extensive molecular dynamics simulations analyzed using the two-phase thermodynamic method. At a surface charge of +0.4e, the compensating forces of favorable electrostatic stabilization and entropic destabilization cancel almost exactly, representing a molecular crossover point from hydrophobic to hydrophilic behavior, independent of curvature. These results suggest that one should include charge-dependent entropic corrections to continuum models aimed at predicting the solvation free energies of large biomolecules.