A Quantum Mechanical Study of the Decomposition of CF₃OCF₃ and CF₃CF₂OCF₂CF₃ in the Presence of AlF₃

The effect of AlF3 on the decomposition of CF3OCF3 and CF3CF2OCF2CF3 is investigated using ab initio theory. Previous work by Pancansky et al. [Pacansky, J.; Waltman, R. J. J Fluorine Chem. 1997, 83, 41] showed that AlF3 significantly reduces the activation energy of the decomposition of CF3OCF3 due to the strong electrostatic interaction between the aluminum trifluoride and the reactant. In this work, a new transition-state structure and reaction mechanism have been identified for the decomposition of CF3OCF3 in the presence of AlF3. This new mechanism shows that AlF3 functions by accepting a fluorine atom from one carbon and simultaneously donating a fluorine atom to the other carbon. We show that the same pathway is obtained independently of the level of theory. The reaction rate, generated via statistical mechanics and transition-state theory, is 2−3 orders of magnitude higher for the new transition state when compared to that of the old one. The study was also performed for CF3CF2OCF2CF3 in order to ascertain the effect of chain length on the reaction mechanism and rate. We find that an analogous transition state, with lower activation energy, provides the lowest-energy path for decomposition of the longer chain

Structure of oxidised silver (1 1 1) and (1 1 0) surfaces

<p>An exhaustive suite of classical molecular dynamics simulations is performed to investigate the stability of oxygen on silver (1 1 1) and (1 1 0) surfaces as a function of surface/subsurface location, binding site, fractional occupancy, and temperature. The ReaxFF potential is used to allow charge transfer between the oxygen and silver components. Comparison of the binding energies at various sites from ReaxFF and <i>ab initio</i> calculations reveals partial agreement between the two approaches. For many of the conditions sampled in the current study, we observe an initial state gives rise to a more disordered reconstruction, which is energetically more favourable. The driving force behind this reconstruction is largely an increase in the coordination of O by Ag, resulting in a more favourable binding site. The extent of reconstruction and atomic motion that initiates the reconstructive process is highly dependent on surface type, fractional occupancy, initially occupied site and temperature. For example, in the temperature range studied (77–500 K), on Ag(1 1 1) it is fractional occupancy that predominantly dictates the type and extent of reconstruction. However, on Ag(1 1 0) it is temperature rather than fractional coverage that is seen to have a more influential effect on the extent of surface reconstruction. These simulations clearly show that O atoms move from surface to subsurface sites, as has been observed experimentally.</p

Ab Initio Molecular Dynamics Simulations of an Excess Proton in a Triethylene Glycol–Water Solution: Solvation Structure, Mechanism, and Kinetics

We investigate the solvation shell structures, the distribution of protonic defects, mechanistic details, kinetics, and dynamics of proton transfer for an excess proton in bulk water and for an excess proton in an aqueous solution of triethylene glycol (TEG) via Car–Parrinello molecular dynamics simulations. The PW91, PBE, and PBE with the Tkatchenko–Scheffler (TS) density-dependent dispersion functionals were used and compared for bulk water and the TEG–water mixtures. The excess proton is found to reside predominantly on water molecules but also resides on hydroxyl groups of TEG. The lifetimes associated with structural diffusion time scales of the protonated water were found to be on the order of ∼1 ps. All three functionals studied support the presolvation requirement for structural diffusion. The highest level of theory shows a reduction in the free energy barrier for water–water proton transfer in TEG–water mixtures compared to bulk water. The effect of TEG shows no strong change in the kinetics for TEG–water mixtures compared to bulk water for this same level of theory. The excess proton displays burst-rest behavior in the presence of TEG, similar to that found in bulk water. We find that the TEG chain disrupts the hydrogen-bond network, causing the solvation shell around water to be populated by TEG chain groups instead of other waters, reducing the rigidity of the hydrogen-bond network. Methylene is a dominant hydrogen bond donor for the protonated water in hydrogen-bond networks associated with proton transfer and structural diffusion. This is consistent with previous studies that have found the hydronium ion to be amphiphilic in nature and to have higher proton mobility at oil–water interfaces

The Adsorption Properties of Amorphous, Metal-Decorated Microporous Silsesquioxanes for Mixtures of Carbon Dioxide, Methane and Hydrogen

A set of adsorbents belonging to a class of amorphous, nanoporous materials composed of spherosilicate building blocks, containing isolated titanium metal sites, was investigated for their ability to separate equimolar binary gas mixtures of CH4/H2, CO2/H2, and CO2/CH4. The cubic silicate building blocks (spherosilicate units: Si8O20) in these adsorbents are cross-linked by SiCl2O2 bridges and decorated with −OTiCl3 and −OSiMe3 groups. Adsorption isotherms, selectivity and energies at 300 K for pressures up to 100 bar for CH4/H2 and pressures up to 50 bar for CO2/H2 and CO2/CH4 were generated via molecular simulation describing physisorption using the Grand Canonical Monte Carlo method. Selectivity was also predicted using ideal adsorbed solution theory. Among the materials studied in this work, high-density material with no −OTiCl3 groups proved to be the best performing separator for the gas mixtures, with selectivities between 10 and 35 from high to low pressures for CH4/H2 mixtures, 45 and 65 for CO2/H2 mixtures, and 2 to 4 for CO2/CH4 mixtures

Molecular Dynamics Simulation of Poly(ethylene terephthalate) Oligomers

Molecular dynamics simulations of poly(ethylene terephthalate) (PET) oligomers are performed in the isobaric−isothermal (NpT) ensemble at a state point typical of a finishing reactor. The oligomer size ranges from 1 to 10 repeat units. We report thermodynamic properties (density, potential energy, enthalpy, heat capacity, isothermal compressibility, and thermal expansivity), transport properties (self-diffusivity, zero-shear-rate viscosity, thermal conductivity), and structural properties (pair correlation functions, hydrogen bonding network, chain radius of gyration, chain end-to-end distance) as a function of oligomer size. We compare the results with existing molecular-level theories and experimental data. Scaling exponents as a function of degree of polymerization are extracted. The distribution of the end-to-end distance is bimodal for the dimer and gradually shifts to a single peak as the degree of polymerization increases. The scaling exponents for the average chain radius of gyration and end-to-end distance are 0.594 and 0.571, respectively. The values of the heat capacity, isothermal compressibility, and thermal expansivity agree well with the available experimental data, which are of much longer PET chains. The scaling exponents for the self-diffusivity and zero-shear-rate viscosity are, respectively, −2.01 and 0.96, with the latter one being close to the theoretical prediction 1.0 for short-chain polymers

Coarse-Grained Molecular Dynamics Simulation of Polyethylene Terephthalate (PET)

A coarse-grained (CG) model of poly(ethylene terephthalate) (PET) was developed and implemented in CG molecular dynamics (MD) simulations of PET chains with degree of polymerization up to 50. The CG potential is parametrized to structural distribution functions obtained from atomistic simulations [J. Phys. Chem. B 2010, 114, 786] using an inversion procedure based on the Ornstein−Zernike equation with the Percus−Yevick approximation (OZPY) [ Phys. Rev. E 2010, 81, 061204]. The CGMD simulation of PET chains satisfactorily reproduces the structural and dynamic properties from atomistic MD simulation of the same systems. We report the average chain end-to-end distance and radius of gyration, relaxation time, self-diffusivity, and zero-shear-rate-viscosity’s dependence on degree of polymerization. For the longest chains, we find the scaling exponents of 0.51, 0.50, and −2.00 for average chain end-to-end distance, radius of gyration and self-diffusivity, respectively. The exponents are very close to the theoretical values of entangled polymer melt systems (0.50, 0.50, and −2.0). The study of entanglement in the longer chains shows that the tube diameter, number of monomers between entanglement points and interentanglement strand length are in close agreement with the reported values for an entangled PET melt

Structure of the Ionomer Film in Catalyst Layers of Proton Exchange Membrane Fuel Cells

The nanoscale structure of the ionomer film located in the catalyst layer of polymer exchange membrane fuel cells (PEMFCs) is of vital importance to proton transport and catalyst utilization. Classical molecular dynamic simulations are conducted to explore the molecular-level structure as well as the structure–property relationships in the ionomer film. Twenty-four systems are simulated to investigate the effect of (i) hydration, (ii) ionomer film thickness, (iii) oxidation of the carbon support surface, and (iv) the presence of catalyst nanoparticles on film adhesion and morphology. The ionomer does not form a continuous film on the carbon surface; rather, the ionomer forms irregular patches through which proton transport from the catalyst to the membrane must occur. These ionomer films are not able to retain water to the same extent as bulk ionomer membranes. However, thicker films retain proportionally more water than thinner films, allowing for a larger and better connected aqueous domain required for proton transport. Oxidation of the carbon support surface through either epoxidation or hydroxylation strongly impacts the water distribution throughout the film and thus the film adhesion. Hydroxylation enhances adhesion of the film relative to a pristine surface. Epoxidation can result in partial delamination of the film, an effect that is more pronounced for thinner films. The presence of Pt or PtO nanoparticles impacts the distribution of water and the ionomer. An aqueous layer forms around the nanoparticles and provides pathways for protons into the film. These insights provide a molecular-level basis for the experimental observations such as the inhomogeneous distribution of the Nafion film on the carbon support, the existence of an optimal content of recast ionomer in the catalyst layer, and the impact of surface oxidation on the restructuring of polymer chains and thus on PEMFC performance. This work also implies that oxidation during operation can result in ionomer film delamination, which reduces the binding energy of the catalysts, a possible precursor to catalyst detachment

Li-Ion Localization and Energetics as a Function of Anode Structure

In this work, we study the effect of carbon composite anode structure on the localization and energetics of Li-ions. A computational molecular dynamics study is combined with experimental results from neutron scattering experiments to understand the effect of composite density, crystallite size, volume fraction of crystalline carbon, and ion loading on the nature of ion storage in novel, lignin-derived composite materials. In a recent work, we demonstrated that these carbon composites display a fundamentally different mechanism for Li-ion storage than traditional graphitic anodes. The edges of the crystalline and amorphous fragments of aromatic carbon that exist in these composites are terminated by hydrogen atoms, which play a crucial role in adsorption. In this work, we demonstrate how differences in composite structure due to changes in the processing conditions alter the type and extent of the interface between the amorphous and crystalline domains, thus impacting the nature of Li-ion storage. The effects of structural properties are evaluated using a suite of pair distribution functions as well as an original technique to extract archetypal structures, in the form of three-dimensional atomic density distributions, from highly disordered systems. The energetics of Li-ion binding are understood by relating changes in the energy and charge distributions to changes in structural properties. The distribution of Li-ion energies reveals that some structures lead to greater chemisorption, while others have greater physisorption. Carbon composites with a high volume fraction of small crystallites demonstrate the highest ion storage capacity because of the high interfacial area between the crystalline and amorphous domains. At these interfaces, stable H atoms, terminating the graphitic crystallites, provide favorable sites for reversible Li adsorption

A Reactive Molecular Dynamics Algorithm for Proton Transport in Aqueous Systems

We present a model that incorporates the structural diffusion of a proton into a classical molecular dynamics simulation using a reactive molecular dynamics (RMD) algorithm. The transition state for proton transfer obtained from ab initio calculations is mapped onto a set of geometric and energetic triggers to describe the structural diffusion of the proton in the simulation. Numerical values of these triggers are parametrized to satisfy the experimental values of rate constant and activation energy in order to capture the molecular and macroscopic features of structural diffusion. The algorithm partitions the structural diffusion of a proton into three steps: (i) satisfaction of the triggers, (ii) instantaneous reaction, and (iii) local equilibration. The final step ensures that the ending point of the reaction provides the correct structure and heat of reaction. Hence, the reactivity is incorporated by the algorithm rather than through the development of a reactive potential. We have applied this scheme to study proton transport in bulk water and solutions of HCl. Total charge diffusion along with the structural and vehicular decomposition is studied as a function of temperature (280−320 K). The two components are found to be uncorrelated and the structural diffusion contributes 60−70% of the total charge diffusion in bulk water. The method is applied to HCl solutions (0.22−0.83 M) to study the effect of concentration on proton transport. The reduction in the total diffusivity of the charge with an increase in concentration is due to the reduction in structural diffusion

Rotating Phenyl Rings as a Guest-Dependent Switch in Two-Dimensional Metal–Organic Frameworks

A semirigid bis­(1,2,4-triazole) ligand binds in a syn conformation between copper­(I) chains to form a series of two-dimensional metal–organic frameworks that display a topology of fused one-dimensional metal–organic nanotubes. These anisotropic frameworks undergo two different transformations in the solid state as a function of solvation. The 2D sheet layers can expand or contract, or, more remarkably, the phenyl rings can rotate between two distinct positions. Rotation of the phenyl rings allows for the adjustment of the tube size, depending on the guest molecules present. This “gate” effect along the 1D tubes has been characterized through single-crystal X-ray diffraction. The transformations can also be followed by powder X-ray diffraction (PXRD) and solid-state ¹³C cross-polarization magic-angle-spinning (CP-MAS) NMR. Whereas PXRD cannot differentiate between transformations, solid-state ¹³C CP-MAS NMR can be employed to directly monitor phenyl rotation as a function of solvation, suggesting that this spectroscopic method is a powerful approach for monitoring breathing in this novel class of frameworks. Finally, simulations show that rotation of the phenyl ring from a parallel orientation to a perpendicular orientation occurs at the cost of framework–framework energy and that this energetic cost is offset by stronger framework–solvent interactions