Computational Modeling and Simulation of CO2 Capture by Aqueous Amines.

We review the literature on the use of computational methods to study the reactions between carbon dioxide and aqueous organic amines used to capture CO2 prior to storage, reuse, or sequestration. The focus is largely on the use of high level quantum chemical methods to study these reactions, although the review also summarizes research employing hybrid quantum mechanics/molecular mechanics methods and molecular dynamics. We critically review the effects of basis set size, quantum chemical method, solvent models, and other factors on the accuracy of calculations to provide guidance on the most appropriate methods, the expected performance, method limitations, and future needs and trends. The review also discusses experimental studies of amine-CO2 equilibria, kinetics, measurement and prediction of amine pKa values, and degradation reactions of aqueous organic amines. Computational simulations of carbon capture reaction mechanisms are also comprehensively described, and the relative merits of the zwitterion, termolecular, carbamic acid, and bicarbonate mechanisms are discussed in the context of computational and experimental studies. Computational methods will become an increasingly valuable and complementary adjunct to experiments for understanding mechanisms of amine-CO2 reactions and in the design of more efficient carbon capture agents with acceptable cost and toxicities.

How Dirhodium Catalyst Controls the Enantioselectivity of [3 + 2]-Cycloaddition between Nitrone and Vinyldiazoacetate: A Density Functional Theory Study.

The origin of enantioselectivity in the dirhodium-catalyzed [3 + 2]-cycloaddition of nitrone and vinyldiazoacetate has been investigated using dispersion-corrected density functional theory. Taking a more realistic account of bulky ligands in models of the dirhodium catalyst when investigating its catalytic behavior is crucial for describing the effects resulting from a high level of asymmetric induction. More than one active site can be located and the extra reactivity is provided by an electron-donation interaction between the substrate and an additional Rh2L4 catalyst.

Impact of distributions and mixtures on the charge transfer properties of graphene nanoflakes.

Many of the promising new applications of graphene nanoflakes are moderated by charge transfer reactions occurring between defects, such as edges, and the surrounding environment. In this context the sign and value of properties such as the ionization potential, electron affinity, electronegativity and chemical hardness can be useful indicators of the efficiency of graphene nanoflakes for different reactions, and can help identify new application areas. However, as samples of graphene nanoflakes cannot necessarily be perfectly monodispersed, it is necessary to predict these properties for polydispersed ensembles of flakes, and provide a statistical solution. In this study we use some simple statistical methods, in combination with electronic structure simulations, to predict the charge transfer properties of different types of ensembles where restrictions have been placed on the diversity of the structures. By predicting quality factors for a variety of cases, we find that there is a clear motivation for restricting the sizes and suppressing certain morphologies to increase the selectivity and efficiency of charge transfer reactions; even if samples cannot be completely purified.

Predicting properties of new ionic liquids: density functional theory and experimental studies of tetra-alkylammonium salts of (thio)carboxylate anions, RCO2⁻, RCOS⁻ and RCS2⁻.

Density functional theory calculations of alkyl-carboxylate anions and their sulfur substituted variants are presented here as an aid for the development of new ionic liquids. Electron transfer both within the anion, and between the anion and cation of an ion pair, are described using natural bond order analysis, using tetraethylammonium as a common cation. The overall stabilising effect of this electron transfer is quantified for the series of anions, and is found to correlate with clear trends in ion-pair binding energy. These and other electronic properties determine which compounds are synthesised, and experimental results validate the computational results. In combination with tetraethylammonium, a carboxylate with an unsaturated alkyl chain produces an ionic liquid at room temperature. However, computations suggest that sulfur substituted anions will produce a lower melting point and perhaps more fluid ionic liquid, but one which would be less stable against oxidation.

A molecular dynamics simulation study of LiFePO4/electrolyte interfaces: structure and Li+ transport in carbonate and ionic liquid electrolytes.

We have performed atomistic molecular dynamics (MD) simulations of the (010) surface of LiFePO(4) in contact with an organic liquid electrolyte (OLE), ethylene carbonate : dimethyl carbonate (3 : 7) with approximately 1 mol kg(-1) LiPF(6), and an ionic liquid-based electrolyte (ILE), 1-ethyl 3-methyl-imidazolium: bis(fluorosulfonyl)imide (EMIM(+) : FSI(-)) with approximately 1 mol kg(-1) LiFSI. Surface-induced structure that extends about 1 nm from the LiFePO(4) surface was observed in both electrolytes. The electrostatic potential at the LiFePO(4) surface was found to be negative relative to the bulk electrolyte reflecting an excess of negative charge from the electrolyte coordinating surface Li(+). In the ILE system negative surface charge is partially offset by a high density of EMIM(+) cations coordinating surface oxygen. The electrostatic potential exhibits a (positive) maximum about 3 A from the LiFePO(4) surface which, when combined with the reduced ability of the highly structured electrolytes to solvate Li(+) cations, results in a free energy barrier of almost 4 kcal mol(-1) for penetration of the interfacial electrolyte layer by Li(+). The resistance for bringing Li(+) from the bulk electrolyte to the LiFePO(4) surface through this interfacial barrier was found to be small for both the OLE and ILE. However, we find that the ability of EMIM(+) cations to donate positive charge to LiFePO(4)/electrolyte interface may result in a significant decrease in the concentration of Li(+) at the surface and a corresponding increase in impedance to Li(+) intercalation into LiFePO(4), particularly at lower temperatures.

Topological characterization of crystallization of gold nanoclusters.

In this study we use a new topological structure measure to analyze the local environment of 923 atom gold clusters quenched from the melt, at various quench rates, by molecular dynamics. The crystallization and geometrical rearrangements of the core atoms upon freezing can be clearly observed using our structure measure which is based on planar graphs. Our results support the hypothesis that crystallization is initiated from the surface and proceeds into the cluster core.