306 research outputs found

    Vibrational Zero-Point Energy of Organosilicon Compounds

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    In this chapter, the calculation of vibrational zero-point energies (ZPEs) of organosilicon compounds is reported. An empirical formula is used. This relationship was determined by relating vibrational zero-point energy to the nature and type of bonds forming the molecule. The calculated vibrational zero-point energies for several organosilicon derivatives belonging to different categories of compounds correlate well with the reported available values. In addition, the comparison of these results with the scaled values obtained using methods of quantum chemistry (AM1, ab initio, and by a similar empirical approach) indicates the reliability of our empirical model to reproduce vibrational zero-point energy of organosilicon compounds

    Organic reactivity. Vol XVIII. Issue 3(67) November

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    Fast evaluation of the adsorption energy of organic molecules on metals via graph neural networks

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    Modeling in heterogeneous catalysis requires the extensive evaluation of the energy of molecules adsorbed on surfaces. This is done via density functional theory but for large organic molecules it requires enormous computational time, compromising the viability of the approach. Here we present GAME-Net, a graph neural network to quickly evaluate the adsorption energy. GAME-Net is trained on a well-balanced chemically diverse dataset with C1–4 molecules with functional groups including N, O, S and C6–10 aromatic rings. The model yields a mean absolute error of 0.18 eV on the test set and is 6 orders of magnitude faster than density functional theory. Applied to biomass and plastics (up to 30 heteroatoms), adsorption energies are predicted with a mean absolute error of 0.016 eV per atom. The framework represents a tool for the fast screening of catalytic materials, particularly for systems that cannot be simulated by traditional methods

    A new approach to the energetics of clusters and related systems

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    The work described in this thesis is concerned with cluster-species and related systems, many of which are electron deficient. The term 'electron deficient' is used to describe a polynuclear species in which there are too few valence electrons to allocate a localised 2-centre 2-eleotron bond to every pair of atoms which are within normal covalent bonding distance. The bonding in these systems may be rationalised instead in terms of the relationship between the total number of skeletal electrons provided by the skeletal cluster units, and the total number of skeletal atoms. The aim of this work is to suggest new ways in which bond enthalpy contributions can be allocated to individual 2-centre links in cluster systems. In order to obtain energy terms (E) which reflect changes in bond length, (d), relationships of the form E α d(^-k)(where k=constant; 2<k≤5) are proposed. Such empirical correlations are shown to be appropriate for simple main group systems and are applied in turn to boron hydrides, borane anions, transition metal carbonyls and to complexes containing multiple metal-metal bonds. Similar relationships are used to suggest possible bond orders in some systems. Finally, the extent to which skeletal electron counting methods may be used to rationalise the bonding in boranes, carboranes, transition metal complexes and small clusters, metal π-hydrocarbon complexes and small cyclic hydrocarbons is discussed

    Thermochemical storage materials:advances in molecular modeling and applications

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    Thermochemical storage materials:advances in molecular modeling and applications

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    Thermochemistry and kinetics: hydrogen atom addition reactions with alkenes, oxidation of cyclopentadienone, trifluoroethene and transport properties

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    Thermochemical and transport properties and reaction kinetic parameters are important to understand and to model atmospheric chemistry, combustion and other thermal systems. These processes are all important to the environment. Thermochemical properties kinetic parameters and models for several atmospheric and combustion related chemical systems are determined using computational chemistry coupled with fundamentals of thermodynamics and statistical mechanics. Transport properties of hydrocarbon and oxygenated hydrocarbons which are important to the calculation of fluid dynamics of gas phase flow reactions and mixing needed for evaluation in the combustion and thermal (flow) fluid dynamic modeling. Transport properties of radicals cannot be measured so computational chemistry is method of choice. The dissertation determine dipole moment, polarizability and molecular diameters of hydrocarbon and oxygenated hydrocarbons needed for calculation of multicomponent viscosities, thermal conductivities, and thermal diffusion coefficients. Cyclopentadienone with cyclic five-member ring aromatic structure is an important intermediate in combustion systems. Thermochemical and kinetic parameters for the initial reactions of cyclopentadienone radicals with O2 and the thermochemical properties for cyclopentadienone - hydroperoxides, alcohols, alkenyl , alkoxy and peroxy radicals are determined by use of computational chemistry via Density Functional Theory (DFT) and the composite, Complete Basis Set (CBS) methods. Enthalpies of formation (ΔfH°298) with the isodesmic reaction schemes with several work reactions for each species are used for standard enthalpies. Entropy and heat capacity, S° (T) and CP° (T) (50 K ≤ T ≤ 5000 K) are also determined. Chemical activation kinetics using quantum RRK analysis for k(E) and master equation for fall-off are used for kinetic parameters and to show significance of chain branching as a function of temperature and pressure. The cyclopentadienone vinylic carbon radicals of with molecular oxygen appear to undergo chain branching via reaction with O2, to a higher extent to that of vinyl and phenyl radicals. Reaction kinetics of hydrogen atom addition to primary (P), secondary (S), tertiary (T) vinylic (olefin) carbons to form an alkyl radical is investigated using Density Functional Theory (DFT) and ab initio composite level methods. Results from calculations at different levels are compared with the experimental literature data for hydrogen atom addition to Ethylene, Propene, 1-Butene, E-2-Butene, Z-2-Butene, and Isobutene. Activation energy and rate constants for forward and reverse paths are investigated and compared with available experimental data. One goal of the study is to determine accurate calculation method for use on large molecules. Chlorofluorocarbons are widely present in the environment. Thermochemical and kinetic properties work will aid in the understanding the chlorofluorocarbons reactions in combustion and atmospheric environments. Trifluoroethene (CF2=CHF) reaction in atmospheric and combustion environment initiated via OH radical system is investigated. The HF generated channel is currently not reported in any kinetic study. It is important as the toxic gas that can cause severe respiratory damage in humans

    X-Irradiation of DNA Components in the Solid State: Experimental and Computational Studies of Stabilized Radicals in Guanine Derivatives

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    Single crystals of sodium salt of guanosine dihydrate and 9 Ethyl Guanine were X-irradiated with the objective of identifying the radical products. Study with K-band EPR, ENDOR, and ENDOR-Induced EPR techniques indicated at least four radical species to appear in both crystals in the temperature range of 6K to room temperature. Three of these radicals (Radicals R1, R2, and R3) were present immediately after irradiation at 6K. Computational chemistry and EPR spectrum simulation methods were also used to assist in radical identifications. Radical R1, the product of net hydrogen addition to N7, and Radical R2, the product of electron loss from the parent molecule, were observed in both systems. Radical R3, in Na+.Guanosine-.2H2O, is the product of net hydrogen abstraction from C1\u27 of ribose group and radical R3 in 9EtG was left unassigned due to insufficient experimental data. Radical R4, the C8-H addition radical, was also detected in both systems. For Na+.Guanosine-.2H2O, R4 was observed after warming the irradiated crystals to the room temperature. But for the 9EtG crystals the corresponding radical form was detected after irradiation at room temperature. Density functional theory (DFT) based computational studies was conducted to investigate the radical formation mechanisms and their stability. Here possibilities of proton transfers from the neighboring molecules were considered. The first approach was to consider the proton affinities of the acceptor sites and deprotonation enthalpies of the donor sites. This approach supported the formation of radicals observed in both systems. The second approach, applied only to the 9EtG system, was based on proton transfers between 9EtG base-pair anion and cation radicals. Even though the charge and spins were localized as expected, the computed thermodynamic data predicted that the proton transfer processes are unfavorable for both anionic and cationic base-pairs. This indicates the need for additional work to draw final conclusions. In addition, DFT methods were used to compute the geometries and hyperfine coupling constants of 9EtG derived radicals in both single molecule and cluster models. The calculated results agreed well with the experimental results
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