A diatribe in quantum chemistry

This study presents ab initio thermochemical estimates for a series of Ti and Si compounds, for which there are no experimental data. It also offers insights in the Ti and Si chemistry, which turns out to be quite different than that of the isovalent C;Silicon compounds are intimately involved with chemical vapour deposition (CVD) processes, as well as the technology of new materials;Ti plays a very important role in catalytic processes like alkene polymerization. Its reaction with small hydrocarbons, is a phenomenon very little understood and depends on the oxidation of the metal centre and the type of hydrocarbon. There is a very limited number of studies, experimental or theoretical, on the reaction of neutral metal atoms with small hydrocarbons. The understanding of such a process would also help understanding the formation of Ti-metcars, one of new the materials;The system B/H2 is one of great theoretical and practical importance. It is subject to interstate crossing and strong diabatic effects are expected to play a crucial role in it use as a potential rocket fuel

Structure, Bonding, and Heats of Formation of Silatitanacyclobutanes

The MP2/TZVP geometries and the standard heats of formation at 0 and 298.15 K of 1,2- and 1,3-silatitanacyclobutanes and a number of smaller Ti- and/or Si-containing alkanes are calculated using the G2 model. The G2 procedure was suitably modified to allow for treatment of first-row transition elements and was directly applied to the reference compounds, which were subsequently connected to the two rings via the appropriate homodesmic reactions. The expected accuracy should be on the order of 3 kcal mol-1. Bonding and structural characteristics are discussed in terms of Boys localized orbitals and Bader density analysis

Systematic Location of Intersecting Seams of Conical Intersection in Triatomic Molecules: The 1 2A′–2 2A′ Conical Intersections in BH2

Points of conical intersection are continuously connected forming seams. Recently a quite unanticipated situation has been found in which two distinct seams of conical intersection—one symmetry-allowed and one same-symmetry—originating from the same two states intersect each other. The identification of these confluences, based on ab initio electronic wave functions has been somewhat serendipitous. A systematic approach for locating such confluences, based solely on information obtained on the symmetry-allowed portion of the seam, has been suggested. In this work that approach is applied to identify the point where a Cs seam of conical intersection intersects a symmetry-allowed C2v seam of conical intersection for the 1 2A′ and 2 2A′ states of BH2, states that correlate with B(1s22s22p,2P)+H2. It is suggested, based on this and previous work, that this unexpected situation, which has fundamental implications for our understanding of nonadiabatic processes, is not at all uncommon

Structure and Thermodynamics of Carbon and Carbon/Silicon Precursors to Nanostructures

The structures at the Hartree−Fock level, as well as the energetics, are reported for the unsaturated system C36H16, its Si-doped analogue C32Si4H16, and several smaller, unsaturated fragments. Structural effects on the electronic distribution are discussed in terms of a localized orbital energy decomposition. The standard heats of formation are calculated based on homodesmic and isodesmic reactions and the G2(MP2,SVP) method with a valence double-ζ plus polarization basis. The origin of the observed explosion of the all-carbon system (C36H16) to form carbon nanotubes was investigated by exploring a possible initial reactive channel (dimerization), which could lead to the formation of the observed onion-type nanostructures.Reprinted (adapted) with permission from Journal of the American Chemical Society 124 (2002): 6144, doi:10.1021/ja012301u. Copyright 2002 American Chemical Society.</p

Rh-Based Mixed Alcohol Synthesis Catalysts: Characterization and Computational Report

The U.S. Department of Energy is conducting a program focused on developing a process for the conversion of biomass to bio-based fuels and co-products. Biomass-derived syngas is converted thermochemically within a temperature range of 240 to 330°C and at elevated pressure (e.g., 1200 psig) over a catalyst. Ethanol is the desired reaction product, although other side compounds are produced, including C3 to C5 alcohols; higher (i.e., greater than C1) oxygenates such as methyl acetate, ethyl acetate, acetic acid and acetaldehyde; and higher hydrocarbon gases such as methane, ethane/ethene, propane/propene, etc. Saturated hydrocarbon gases (especially methane) are undesirable because they represent a diminished yield of carbon to the desired ethanol product and represent compounds that must be steam reformed at high energy cost to reproduce CO and H2. Ethanol produced by the thermochemical reaction of syngas could be separated and blended directly with gasoline to produce a liquid transportation fuel. Additionally, higher oxygenates and unsaturated hydrocarbon side products such as olefins also could be further processed to liquid fuels. The goal of the current project is the development of a Rh-based catalyst with high activity and selectivity to C2+ oxygenates. This report chronicles an effort to characterize numerous supports and catalysts to identify particular traits that could be correlated with the most active and/or selective catalysts. Carbon and silica supports and catalysts were analyzed. Generally, analyses provided guidance in the selection of acceptable catalyst supports. For example, supports with high surface areas due to a high number of micropores were generally found to be poor at producing oxygenates, possibly because of mass transfer limitations of the products formed out of the micropores. To probe fundamental aspects of the complicated reaction network of CO with H2, a computational/ theoretical investigation using quantum mechanical and ab initio molecular dynamics calculations was initiated in 2009. Computational investigations were performed first to elucidate understanding of the nature of the catalytically active site. Thermodynamic calculations revealed that Mn likely exists as a metallic alloy with Rh in Rh-rich environments under reducing conditions at the temperatures of interest. After determining that reduced Rh-Mn alloy metal clusters were in a reduced state, the activation energy barriers of numerous transition state species on the catalytically active metal particles were calculated to compute the activation barriers of several reaction pathways that are possible on the catalyst surface. Comparison of calculations with a Rh nanoparticle versus a Rh-Mn nanoparticle revealed that the presence of Mn enabled the reaction pathway of CH with CO to form an adsorbed CHCO species, which was a precursor to C2+ oxygenates. The presence of Mn did not have a significant effect on the rate of CH4 production. Ir was observed during empirical catalyst screening experiments to improve the activity and selectivity of Rh-Mn catalysts. Thus, the addition of Ir to the Rh-Mn nanoparticles also was probed computationally. Simulations of Rh-Mn-Ir nanoparticles revealed that, with sufficient Ir concentrations, the Rh, Mn and Ir presumably would be well mixed within a nanoparticle. Activation barriers were calculated for Rh-Mn-Ir nanoparticles for several C-, H-, and O-containing transitional species on the nanoparticle surface. It was found that the presence of Ir opened yet another reactive pathway whereby HCO is formed and may undergo insertion with CHx surface moieties. The reaction pathway opened by the presence of Ir is in addition to the CO + CH pathway opened by the presence of Mn. Similar to Mn, the presence of Ir was not found to not affect the rate of CH4 production

A diatribe in quantum chemistry

This study presents ab initio thermochemical estimates for a series of Ti and Si compounds, for which there are no experimental data. It also offers insights in the Ti and Si chemistry, which turns out to be quite different than that of the isovalent C;Silicon compounds are intimately involved with chemical vapour deposition (CVD) processes, as well as the technology of new materials;Ti plays a very important role in catalytic processes like alkene polymerization. Its reaction with small hydrocarbons, is a phenomenon very little understood and depends on the oxidation of the metal centre and the type of hydrocarbon. There is a very limited number of studies, experimental or theoretical, on the reaction of neutral metal atoms with small hydrocarbons. The understanding of such a process would also help understanding the formation of Ti-metcars, one of new the materials;The system B/H2 is one of great theoretical and practical importance. It is subject to interstate crossing and strong diabatic effects are expected to play a crucial role in it use as a potential rocket fuel.</p

On the electron affinity of B-2

We present the results of high-level ab initio calculations on the electron affinity of B-2. Our new best estimate of 1.93 +/- 0.03 eV is in agreement with previous calculations as well as the sole existing experimental estimate of 1.8 eV, as derived from quantities with an uncertainty of 0.4 eV. The electron affinity of atomic boron, which is much smaller, is also calculated for comparison and again found to be in good agreement with experiment

Structure, Bonding, and Heats of Formation of Silatitanacyclobutanes

The MP2/TZVP geometries and the standard heats of formation at 0 and 298.15 K of 1,2- and 1,3-silatitanacyclobutanes and a number of smaller Ti- and/or Si-containing alkanes are calculated using the G2 model. The G2 procedure was suitably modified to allow for treatment of first-row transition elements and was directly applied to the reference compounds, which were subsequently connected to the two rings via the appropriate homodesmic reactions. The expected accuracy should be on the order of 3 kcal mol-1. Bonding and structural characteristics are discussed in terms of Boys localized orbitals and Bader density analysis.Reprinted (adapted) with permission from Journal of Physical Chemistry A 101 (1997): 8714, doi:10.1021/jp971653d. Copyright 1997 American Chemical Society.</p