Optical studies of methane under high pressure

Activation volume and quantum tunneling in the hydrogen transfer reaction between methyl radical and methane: A first computational study

We present a theory of the effect of quantum tunneling on the basic parameter that characterizes the effect of pressure on the rate constant of chemical reactions in a dense phase, the activation volume. This theory results in combining, on the one hand, the extreme pressure polarizable continuum model, a quantum chemical method to describe the effect of pressure on the reaction energy profile in a dense medium, and, on the other hand, the semiclassical version of the transition state theory, which includes the effect of quantum tunneling through a transmission coefficient. The theory has been applied to the study of the activation volume of the model reaction of hydrogen transfer between methyl radical and methane, including the primary isotope substitution of hydrogen with deuterium (H/D). The analysis of the numerical results offers, for the first time, a clear insight into the effect of quantum tunneling on the activation volume for this hydrogen transfer reaction: this effect results from the different influences that pressure has on the competing thermal and tunneling reaction mechanisms. Furthermore, the computed kinetic isotope effect (H/D) on the activation volume for this model hydrogen transfer correlates well with the experimental data for more complex hydrogen transfer reactions.

The crystal structure of methane B at 8 GPa--an α-Mn arrangement of molecules

From a combination of powder and single-crystal synchrotron x-ray diffraction data we have determined the carbon substructure of phase B of methane at a pressure of ∼8 GPa. We find this substructure to be cubic with space group I4¯3m and 58 molecules in the unit cell. The unit cell has a lattice parameter a = 11.911(1) Å at 8.3(2) GPa, which is a factor of √2 larger than had previously been proposed by Umemoto et al. [J. Phys.: Condens. Matter 14, 10675 (2002)]. The substructure as now solved is not related to any close-packed arrangement, contrary to previous proposals. Surprisingly, the arrangement of the carbon atoms is isostructural with that of α-manganese at ambient conditions.

The refractive index and electronic gap of water and ice increase with increasing pressure

Determining the electronic and dielectric properties of water at high pressure and temperature is an essential prerequisite to understand the physical and chemical properties of aqueous environments under supercritical conditions, for example, in the Earth interior. However, optical measurements of compressed ice and water remain challenging, and it has been common practice to assume that their band gap is inversely correlated with the measured refractive index, consistent with observations reported for hundreds of materials. Here we report ab initio molecular dynamics and electronic structure calculations showing that both the refractive index and the electronic gap of water and ice increase with increasing pressure, at least up to 30 GPa. Subtle electronic effects, related to the nature of interband transitions and band edge localization under pressure, are responsible for this apparently anomalous behaviour.

Photoinduced reactivity of liquid ethanol at high pressure

The room temperature photoinduced reactivity of liquid ethanol has been studied as a function of pressure up to 1.5 GPa by means of a diamond anvil cell. Exploiting the dissociative character of the lowest electronic excited states, reached through two-photon absorption of near-UV photons (350 nm), irreversible reactive processes have been triggered in the pure system. The active species are radicals forming along two main dissociation channels involving the split of C-O and O-H bonds. The characterization of the reaction products has been performed by in situ FTIR and Raman spectroscopy. At pressures of a few megapascals, molecular hydrogen is the main reaction product, an important issue in the framework of environmentally friendly synthesis of this energetic vector. In the gigapascal range, the main products are ethane, 2-butanol, 2,3-butanediol, 1,1-diethoxyethane, and some carbonylic compounds. The relative amount of these species changes with pressure reflecting the nature of the radicals formed in the photodissociation process. As the pressure increases, the processes requiring a greater molecularity are favored, whereas those requiring internal rearrangements are inhibited. Disproportion products like CH(4), H(2)O, and CO(2) increase when the amount of ethanol decreases due to the reaction, becoming the main products only when ethanol is exhausted.

Brillouin scattering study of liquid methane under high pressures and high temperatures

Brillouin scattering measurements were performed on liquid methane using diamond anvil cell along five isotherms and at the pressures up to solidification points. Sound velocity, refractive index, and adiabatic bulk modulus of liquid methane as function of pressure were determined with the measurements from the platelet and backscattering geometries. The maximum pressure and temperature reached up to 5.12 GPa and 539 K. The sound velocity, refractive index, and adiabatic bulk modulus increased with pressure along each isotherm. The equation of state of liquid methane was determined from the present Brillouin results.

Phase transitions of methane using molecular dynamics simulations

Using a short ranged Lennard-Jones interaction and a long ranged electrostatic potential, CH4 under high pressure was modeled. Molecular dynamics simulations on small clusters (108 and 256 molecules) were used to explore the phase diagram. Regarding phase transitions at different temperatures, our numerical findings are consistent with experimental results to a great degree. In addition, the hysteresis effect is displayed in our results.

Generation of methane in the Earth's mantle: in situ high pressure-temperature measurements of carbonate reduction

We present in situ observations of hydrocarbon formation via carbonate reduction at upper mantle pressures and temperatures. Methane was formed from FeO, CaCO(3)-calcite, and water at pressures between 5 and 11 GPa and temperatures ranging from 500 degrees C to 1,500 degrees C. The results are shown to be consistent with multiphase thermodynamic calculations based on the statistical mechanics of soft particle mixtures. The study demonstrates the existence of abiogenic pathways for the formation of hydrocarbons in the Earth's interior and suggests that the hydrocarbon budget of the bulk Earth may be larger than conventionally assumed.

Hydrogen dominant metallic alloys: high temperature superconductors?

The arguments suggesting that metallic hydrogen, either as a monatomic or paired metal, should be a candidate for high temperature superconductivity are shown to apply with comparable weight to alloys of metallic hydrogen where hydrogen is a dominant constituent, for example, in the dense group IVa hydrides. The attainment of metallic states should be well within current capabilities of diamond anvil cells, but at pressures considerably lower than may be necessary for hydrogen.