A silicate dynamo in the early Earth.

The Earth's magnetic field has operated for at least 3.4 billion years, yet how the ancient field was produced is still unknown. The core in the early Earth was surrounded by a molten silicate layer, a basal magma ocean that may have survived for more than one billion years. Here we use density functional theory-based molecular dynamics simulations to predict the electrical conductivity of silicate liquid at the conditions of the basal magma ocean: 100-140 GPa, and 4000-6000 K. We find that the electrical conductivity exceeds 10,000 S/m, more than 100 times that measured in silicate liquids at low pressure and temperature. The magnetic Reynolds number computed from our results exceeds the threshold for dynamo activity and the magnetic field strength is similar to that observed in the Archean paleomagnetic record. We therefore conclude that the Archean field was produced by the basal magma ocean.

Electrical conductivity of SiO2 at extreme conditions and planetary dynamos.

Ab intio molecular dynamics simulations show that the electrical conductivity of liquid SiO2 is semimetallic at the conditions of the deep molten mantle of early Earth and super-Earths, raising the possibility of silicate dynamos in these bodies. Whereas the electrical conductivity increases uniformly with increasing temperature, it depends nonmonotonically on compression. At very high pressure, the electrical conductivity decreases on compression, opposite to the behavior of many materials. We show that this behavior is caused by a novel compression mechanism: the development of broken charge ordering, and its influence on the electronic band gap.

Preparation and Characterization of Nanocomposite Polymer Membranes Containing Functionalized SnO2 Additives.

In the research of new nanocomposite proton-conducting membranes, SnO2 ceramic powders with surface functionalization have been synthesized and adopted as additives in Nafion-based polymer systems. Different synthetic routes have been explored to obtain suitable, nanometer-sized sulphated tin oxide particles. Structural and morphological characteristics, as well as surface and bulk properties of the obtained oxide powders, have been determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared (FTIR) and Raman spectroscopies, N2 adsorption, and thermal gravimetric analysis (TGA). In addition, dynamic mechanical analysis (DMA), atomic force microscopy (AFM), thermal investigations, water uptake (WU) measurements, and ionic exchange capacity (IEC) tests have been used as characterization tools for the nanocomposite membranes. The nature of the tin oxide precursor, as well as the synthesis procedure, were found to play an important role in determining the morphology and the particle size distribution of the ceramic powder, this affecting the effective functionalization of the oxides. The incorporation of such particles, having sulphate groups on their surface, altered some peculiar properties of the resulting composite membrane, such as water content, thermo-mechanical, and morphological characteristics.

The initiation mechanisms for surface hydrosilylation with 1-alkenes.

Hydrosilylation provides a route to form substituted silanes in solution. A similar reaction has been observed in the formation of covalent organic monolayers on a hydrogen-terminated silicon surface and is called thermal hydrosilylation. In solution, the mechanism requires a catalyst to add the basal silicon and saturating hydrogen to the C=C double bond. On the silicon surface, however, the reaction proceeds efficiently at 200 °C, initiated by visible light, and more slowly at room temperature in the dark. Such low activation energy barriers for the reactions on a surface relative to that required for solution hydrosilylation are remarkable, and although many explanations have been suggested, controversy still exists. In this work using a constrained molecular dynamics approach within the density functional theory framework, we show that the free energy activation barrier for abstraction of a hydrogen from silicon by an alkene molecule can be overcome by visible light or thermal excitation. Furthermore, we show that by concerted transfer of a hydrogen from the α-carbon to the ß-carbon, a 1-alkene can insert its α-carbon into a surface Si-H bond to accomplish hydrosilylation.

Tautomers of extended reduced pyrazinacenes: a density-functional-theory based study.

We present a structural and electronic inspection of reduced pyrazinacenes within the DFT framework. Our analysis provides a clear indication that compounds in which reduced pyrazine rings are well separated from each other are rather stable. Conversely, if the reduced pyrazine rings approach each other or cluster together, the compounds become increasingly unstable. The tautomers analyzed are likely to possess properties suitable for application as proton transport materials due to protic isomerism processes. On the basis of our findings, we propose that protic transport should occur through a concerted proton transfer without involving intramolecular aggregation of the dihydropyrazine groups. Furthermore, the electronic structure analysis shows that this class of compounds can be classified as small bandgap semiconducting materials, possessing even metallic character depending on the tautomeric structure, and with potential nanotechnological applications in molecular electronics and fuel cells.

Tautomerism in Reduced Pyrazinacenes.

Monoprotic and diprotic NH tautomerism in reduced oligoazaacenes, the pyrazinacenes, was studied by using first principles simulations. Stepwise reductions in the metadynamics-sampled free energy profile were observed during consecutive monoprotic tautomerizations, with energy barriers gradually reducing with increasing proton separation during monoprotic processes. This is accompanied by an increasing contribution from the quinoidal electronic structure, as evidenced by the computed highest occupied molecular orbital (HOMO) structure. An unusual odd-even effect in the free energy profiles is also observed upon changing the length of the pyrazinacene. Calculated HOMO structures reveal an increasing tendency for delocalization of pyrazine lone pairs with an increasing number of ring annelations. The influence of tautomerism on the pyrazine lone pair delocalization was also observed. Tautomers with protons situated centrally on the pyrazinacene backbone are predicted to be more stable due to a combination of (enamine) delocalization and a loss of Clar sextet resonance stabilization in tautomers with protons at terminal pyrazine rings. Experimental evidence suggesting the structure of pyrazinacene tautomers is included and discussed as a support to the calculation.

A mechanism of adsorption of beta-nicotinamide adenine dinucleotide on graphene sheets: experiment and theory.

Beta-nicotinamide adenine dinucleotide (NAD(+)) and its reduced form (NADH) play major roles in the development of electrochemical enzyme biosensors and biofuel cells. Unfortunately, the oxidation of NADH at carbon electrodes suffers from passivation of the electrodes and a decrease in passing currents. Here, we investigate experimentally and theoretically the reasons for such passivation. High-resolution X-ray photoelectron spectroscopy (HR-XPS), voltammetry, and amperometry show that adsorption occurs on the edges and "edge-like" defects of graphene sheets. HR-XPS and ab initio molecular dynamics show that the adsorption of NAD(+) molecules on the edges of graphene happens due to interaction with oxygen-containing groups such as carboxylic groups, while graphene edges substituted only with hydrogen are prone to passivation.

Evidence for a ball-shaped cyclen cyclophane: an experimental and first principles study.

Conformational isomerism of a ball-shaped cyclophane of cyclen was studied using NMR and computational methods with the most notable conformer containing a highly symmetric cyclic benzene tetramer.

Water solvation properties: an experimental and theoretical investigation of salt solutions at finite dilution.

Our combined analysis of first-principle simulations and experiments conducted on salt solutions at finite dilution shows that the high frequency range of the infrared spectrum of an aqueous solution of NaCl displays a shift toward higher frequencies of the stretching band with respect to pure water. We ascribe this effect to a lowering of the molecular dipole moments due to a decrease in the dipole moments of molecules belonging to the first and second solvation shells with respect to bulk water. An analysis of the dipole orientation correlations proves that the screening of solutes is dominated by short-range effects. These jointly experimental and theoretical results are corroborated by the good agreement between calculated and measured dielectric constants of our target solution.

A first principles investigation of water dipole moment in a defective continuous hydrogen bond network.

First principles molecular dynamics simulations of an aqueous solution salt system at finite concentration containing both Na(+) and Cl(-) ions show that a change in the distribution of the molecular dipole moment of H(2)O monomers appears when ions are present in solution. Simulations suggest a lowering of the dipole moments of the water molecules in the solvation shells of Na(+) and Cl(-) as compared to the pure water case, while the dipoles of the rest of the molecules are hardly affected. However, finer analysis in terms of the Wannier centers distribution suggests a change in the electronic structure of the water molecules even in the bulk. Also a change of the H-bond network arrangement was found and correlation between dipole and MOH parameter evidences such subtle effects, suggesting a lowering of tetrahedral order in salty solutions. All these changes can be related to observable quantities such as the infrared spectra thus allowing for a rationalization of the experimental outcome on neutral aqueous solutions.