Ab Initio Molecular Dynamics Study of the Mechanism of Proton Recombination with a Weak Base

Despite its fundamental nature, many of the microscopic features of acid–base recombination remain poorly understood. In this work, we use ab initio molecular dynamics simulations to study the recombination of the proton with a weak base, the carbonate ion CO₃^2–. Our simulations elucidate the network structure around CO₃^2– that provides a distribution of pathways over which recombination can occur. We observe that the penultimate neutralization step involves a correlated behavior of the transferred protons that is mediated by the water wires decorating the carbonate. These concerted proton transfers are coupled to collective compressions of these water wires. We show further that these processes are dynamically coupled to the reorganization of the water molecules hydrating the CO₃^2– ion. The insights from these simulations help to bridge the structural and dynamical complexity of the microscopic mechanisms with those of phenomenological models invoked by experiments in this field

<i>Ab Initio</i> Quality NMR Parameters in Solid-State Materials Using a High-Dimensional Neural-Network Representation

Nuclear magnetic resonance (NMR) spectroscopy is one of the most powerful experimental tools to probe the local atomic order of a wide range of solid-state compounds. However, due to the complexity of the related spectra, in particular for amorphous materials, their interpretation in terms of structural information is often challenging. These difficulties can be overcome by combining molecular dynamics simulations to generate realistic structural models with an <i>ab initio</i> evaluation of the corresponding chemical shift and quadrupolar coupling tensors. However, due to computational constraints, this approach is limited to relatively small system sizes which, for amorphous materials, prevents an adequate statistical sampling of the distribution of the local environments that is required to quantitatively describe the system. In this work, we present an approach to efficiently and accurately predict the NMR parameters of very large systems. This is achieved by using a high-dimensional neural-network representation of NMR parameters that are calculated using an <i>ab initio</i> formalism. To illustrate the potential of this approach, we applied this neural-network NMR (NN-NMR) method on the ¹⁷O and ²⁹Si quadrupolar coupling and chemical shift parameters of various crystalline silica polymorphs and silica glasses. This approach is, in principal, general and has the potential to be applied to predict the NMR properties of various materials

Public Enterprise and Economic Development: The Case of Ande in Paraguay (Hydroelectric Power, Inter-American Development Bank, Itaipu Dam)

243 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1985.The presence of large state-owned enterprises in non-socialist developing economies is not uncommon. This research focuses on one public enterprise, ANDE, which generates and distributes electricity in Paraguay. Created from a nationalized Italo-Argentine firm in 1948, it has moved from thermal to hydro generation and expanded the transmission system to serve small population centers around the country. With the Brazilian state-owned power company Eletrobras, ANDE is currently constructing the world's largest hydroelectric facility, Itaipu. Investment in this project alone will exceed $15 billion.After introductory material describing the Paraguayan setting, Chapter III examines ANDE's history and traces Paraguayan-Brazilian economic cooperation which culminated in the Itaipu project. Chapters IV to VI examine ANDE's investment and pricing decisions and their impact on Paraguayan economic development between 1950 and 1982, focusing specifically on the mobilization of foreign and domestic financial resources and human capital formation.The study finds few backward linkages from the first hydroelectric project, but a significant forward linkage. Abundant power made extending service outside the capital possible. Itaipu was found to have few backward linkages because of the underdeveloped state of Paraguayan industry and the absence of adequate infrastructure in Paraguay. Important forward linkages in Paraguay appear unlikely since utilization of Itaipu energy will be left to the private sector which is unable to make the parallel investments required to make use of the power (estimated at$3 billion per turbine).ANDE's pricing policy is established in its charter; it prices to earn a rate of return sufficient to fund the local contribution required to secure international financing for expansion. The price of electric power in Paraguay is high compared to that of neighboring countries but has been declining in real terms as a result of the shift to hydro power and the concessional financing obtained.The data were collected during a one-and-a-half-year stay in Paraguay supported by a Fulbright research fellowship. In addition to the statistics collected from the firm's files, extensive use was made of interviews with past and present decision-makers.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD

Benchmarking Density Functional Based Tight-Binding for Silver and Gold Materials: From Small Clusters to Bulk

We benchmark existing and improved self-consistent-charge density functional based tight-binding (SCC-DFTB) parameters for silver and gold clusters as well as for bulk materials. In the former case, our benchmarks focus on both the structural and energetic properties of small-size Ag_<i>N</i> and Au_<i>N</i> clusters (<i>N</i> from 2 to 13), medium-size clusters with <i>N</i> = 20 and 55, and finally larger nanoparticles with <i>N</i> = 147, 309, and 561. For bulk materials, structural, energetics and elastic properties are discussed. We show that SCC-DFTB is quite satisfactory in reproducing essential differences between silver and gold aggregates, in particular their 2D–3D structural transitions, and their dependency upon cluster charge. SCC-DFTB is also in agreement with DFT and experiments in the medium-size regime regarding the energetic ordering of the different low-energy isomers and allows for an overall satisfactory treatment of bulk properties. A consistent convergence between the cohesive energies of the largest investigated nanoparticles and the bulk’s is obtained. On the basis of our results for nanoparticles of increasing size, a two-parameter analytical extrapolation of the cohesive energy is proposed. This formula takes into account the reduction of the cohesive energy for undercoordinated surface sites and converges properly to the bulk cohesive energy. Values for the surface sites cohesive energies are also proposed

Shape Modulation of Octanuclear Cu(I) or Ag(I) Dichalcogeno Template Clusters with Respect to the Nature of their Encapsulated Anions: A Combined Theoretical and Experimental Investigation

M₈L₆ clusters (M = Cu­(I), Ag­(I); L = dichalcogeno ligand) are known for their ability to encapsulate various kinds of saturated atomic anions. Calculations on the models [M₈(E₂PH₂)₆]²⁺ (M = Cu­(I), Ag­(I); E = S, Se) and the ionic or neutral [M₈(X)­(E₂PH₂)₆]^<i>q</i> (X = H, F, Cl, Br, O, S, Se, N, P, C) indicate that the cubic M₈L₆ cage adapts its shape for maximizing the host–guest bonding interaction. The interplay between size, covalent and ionic bonding favors either a cubic, tetracapped tetrahedral, or bicapped octahedral structure of the metal framework. Whereas the large third- and fourth-row main group anions maintain the cubic shape, a distortion toward a tetracapped tetrahedral arrangement of the metals occurs in the case of hydride, fluoride, and oxide. The distortion is strong in the case of hydride, weak in the case of fluoride, and intermediate in the case of oxide. Density functional theory (DFT) calculations predict a bicapped octahedral architecture in the case of nitride and carbide. These computational results are supported by X-ray structures, including those of new fluorine- and oxygen-containing compounds. It is suggested that other oxygen-containing as well as so far unknown nitride-containing clusters should be feasible. For the first time, the dynamical behavior of the encapsulated hydride has been investigated by metadynamics simulations. Our results clearly demonstrate that the interconversion mechanism between two identical tetracapped tetrahedral configurations occurs through a succession of M-H bonds breaking and forming which present very low activation energies and which involve a rather large number of intermediate structures. This mechanism is full in accordance with ¹⁰⁹Ag and ¹H state NMR measurements

Evaluation of ⁹⁵Mo Nuclear Shielding and Chemical Shift of [Mo₆X₁₄]^2– Clusters in the Liquid Phase

[Mo₆X₁₄]^2– octahedral molybdenum clusters are the main building blocks of a large range of materials. Although ⁹⁵Mo nuclear magnetic resonance was proposed to be a powerful tool to characterize their structural and dynamical properties in solution, these measurements have never been complemented by theoretical studies which can limit their interpretation for complex systems. In this Article, we use quantum chemical calculations to evaluate the ⁹⁵Mo chemical shift of three clusters: [Mo₆Cl₁₄]^2–, [Mo₆Br₁₄]^2–, and [Mo₆I₁₄]^2–. In particular, we test various computational parameters influencing the quality of the results: size of the basis set, treatment of relativistic and solvent effects. Furthermore, to provide quantum chemical calculations that are directly comparable with experimental data, we evaluate for the first time the ⁹⁵Mo nuclear magnetic shielding of the experimental reference, namely, MoO₄^2– in aqueous solution. This is achieved by combining ab initio molecular dynamics simulations with a periodic approach to evaluate the ⁹⁵Mo nuclear shieldings. The results demonstrate that, despite the difficulty to obtain accurate ⁹⁵Mo chemical shifts, relative values for a cluster series can be fairly well-reproduced by DFT calculations. We also show that performing an explicit solvent treatment for the reference compound improves by ∼50 ppm the agreement between theory and experiment. Finally, the standard deviation of ∼70 ppm that we calculate for the ⁹⁵Mo nuclear shielding of the reference provides an estimation of the accuracy we can achieve for the calculation of the ⁹⁵Mo chemical shifts using a static approach. These results demonstrate the growing ability of quantum chemical calculations to complement and interpret complex experimental measurements