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Mechanical and Electronic Properties of the Ultra-thin Silica NanowiresLin, Kuan-Fu 29 August 2011 (has links)
In this study, we used the molecular statics, molecular dynamics, and density function theory to investigate structural, electronic, and mechanical properties of ultra-thin silica nanowires. There are two parts in this study. In the first part, we used basin-hopping method to get different diameters of silica nanowires, nemed 2MR, 2MR-2O, 3MR-3O, 4MR-4O, 5MR-5O, 4MR-3f, 4MR-4f, and 4MR-5f. The various silica nanowires were optimized by density function theory to obtain the projected density of states, Mulliken charge, and electronic density difference, and we also compared this results to £\-quartz. In the second part, the molecular dynamics simulations were performed to investigate deformation behavior of silica nanowires under axial tensile loading at 10K. The Young¡¦s modulus increases when the diameter decreases. We also
used angular correlation function to study the mechanical properties and variation of structures.
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Hydrogen storage and delivery mechanism of metal nanoclusters on a nanosheetHuang, Li-Fan 19 January 2012 (has links)
In this study, we used the Density functional theory (DFT) and Molecular dynamics (MD) to obtain the suitable hydrogen storage structure of Rh nanoclusters on the boron nitride sheet and Li atoms on the graphene. The reason of studying two type of nanoparticles is that there are two adsorption method in hydrogen storage, such as the adsorption of hydrogen molecules and hydrogen atoms. Using Rh nanoclusters on the boron nitride sheet to store hydrogen belong to the adsorption of hydrogen atoms. Using Li atoms on the graphene to store hydrogen belong to the adsorption of hydrogen molecules. We use these two models to simulate the hydrogen storage in this study. There were four parts in this study:
The first part:
The Density functional theory is utilized to obtain the configuration and corresponding energy of Rh nanoclusters, boron nitride sheet, Rh nanoclusters adsorbed on the boron nitride sheet, Li atoms adsorbed on the graphene, hydrogen adsorbed on the graphene and hydrogen adsorbed on the Li atoms. Then, we use the Force-matching method (FMM) to modify the parameters of potential function by the reference data which are obtained by Density functional theory. Finally, we use the modified parameters of potential function to perform Molecular dynamics in this study.
The second part:
In this part, the dynamical behavior of Rh nanoclusters with different sizes on the boron nitride sheet are investigated in temperature-rise period. The migration trajectory, square displacement and mean square displacement of the mass center of the Rh nanoclusters are used to analyze the dynamics behavior of Rh nanoclusters on the boron nitride sheet.
The third part:
In this part, the pristine graphene and graphen with Li atoms are investigated the efficiency of hydrogen storage at different temperature and pressure. In order to obtain the temperature (77K and 300K) and pressure effect of hydrogen storage, the densimetric distribution and gravimetric capacity (wt%) are analyzed.
The fourth part:
The Molecular dynamics is utilized to study the hydrogen storage and delivery when the distance between two graphene is different. Then, the temperature effect (77K and 300K) of hydrogen storage, the gravimetric capacity (wt%) are analyzed. In addition, the gravimetric capacity (wt%) of hydrogen delivery are also analyzed in the larger system space at 300K.
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Adsorption, dissociation and diffusion behaviors of hydrogen molecule on ultrathin Pd nanowires : the density functional theory studyHuang, Wen-Cheng 21 July 2012 (has links)
In this study, the structures of two ultrathin Pd nanowires were predicted by the simulated annealing basin-hopping method (SABH) with the tight-binding potential. The thermal stability of the Pd wires and adsorption, dissociation and diffusion behaviors were further examined by the density functional theory (DFT) calculation and DFT molecular dynamics (DFT-MD) simulation. In terms of thermal stability, these two Pd nanowires are still very stable at temperatures as high as 400 K. The configurations and adsorption energy have been calculated for H atom and H2 molecular adsorption on Pd nanowires. The minimum energy pathways and transition states of H2 molecular dissociation and H atom diffusion process on Pd nanowires were studied by the nudged elastic band (NEB) method. For the dissociation of hydrogen molecules, results show the dissociation is almost barrierless so the dissociation is easy to occur at very low temperatures, and their catalytic reactivity is very similar to the Pd bulk material. The thermal stability of the H atom within these Pd nanowires were also investigated by DFT-MD, with results showing that the H atom can only stay within Pd nanowires at temperatures much lower than room temperature (298 K). This phenomenon is very different from that of H atoms within Pd bulk material or other reported nanomaterials, leading to hydrogen embrittlement. Our results reveal that these two ultrathin Pd nanowires not only possess the same excellent catalytic activity for hydrogen molecules as the bulk Pd materials or other Pd nanomaterials do, but also avoid the hydrogen embrittlement occur.
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Understanding mechanisms for C-H bond activationVastine, Benjamin Alan 15 May 2009 (has links)
The results from density functional theory (DFT) studies into C–H bond
activation, hydrogen transfer, and alkyne–to–vinylidene isomerization are presented in
this work.
The reaction mechanism for the reductive elimination (RE) of methane from [ κ3-
TpPtIV(CH3)2H (1)] (Tp = hydridotris(pyrazolyl)borate) by oxidative addition (OA) of
benzene to form [ κ3-TpPtIV(Ph)2H] (19) was investigated through DFT calculations.
For 31 density functionals, the calculated values for the barriers to methane formation
(Ba1) and release (Ba2) from 1 were benchmarked against the experimentally reported
values of 26 (Ba1) and 35 (Ba2) kcal•mol-1, respectively. The values for Ba1 and Ba2,
calculated at the B3LYP/DZP level of theory, are 24.6 and 34.3 kcal•mol-1, respectively.
The best performing functional was BPW91 where the m.a.e. for the calculated values
of the two barriers is 0.68 kcal•mol-1.
Classic and newly proposed mechanisms for metal-mediated hydrogen transfer
(HT) were analyzed with density functional theory (DFT) and Bader's "Atoms In
Molecules" (AIM) analysis. Seven sets of bonding patterns that characterize theconnectivity in metal-mediate HT were found from the analysis of representative
models for σ-bond metathesis ( σBM), oxidative addition / reductive elimination
(OA/RE), and alternative mechanisms.
The mechanism for the formation of the alkynyl, vinylidene complex,
[(PiPr3)2Rh(CCPh)(CC(H)(Ph))] (2), by the addition of two equivalents of
phenylacetylene (PA) to [( η3-C3H5)Rh(PiPr3)2] (1) was studied through DFT
calculations. Two experimentally observed intermediates on the reaction coordinate are
the η2-PA, alkynyl complex, [(PiPr3)2Rh( η2-HCCPh)(CCPh)] (Ia) and the fivecoordinate,
pseudo square-pyramidal, RhIII–H complex, [(PiPr3)2Rh(H)(CCPh)2] (Ib),
and were found to be in equilibrium. The relative energies of Ia, Ib, and 2 (relative to 1
+ 2PA) depend on the phosphine that was used in the calculation; the predicted product
is 2 with PiPr3 and PEt3 but Ia with PMe3, PMe2Ph, PMePh2, PPh3, and PH3. The
equilibrium between Ia and Ib was calculated with PEt3 and one conformation of PiPr3.
We investigated the mechanism for the formation of 2 from Ia, and a lower energy
pathway where the π-bound PA of Ia slips to bind through the σ-C–H bond prior to the
formation of 2 through hydrogen migration was found.
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Intramolecular electronic communication between dimetal units with multiple metal??al bondsLi, Zhong 15 May 2009 (has links)
No description available.
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Theoretical Studies of Structures and Mechanisms in Organometallic and Bioinorganic Chemistry: Heck Reaction with Palladium Phosphines, Active Sites of Superoxide Reductase and Cytochrome P450 Monooxygenase, and Tetrairon Hexathiolate Hydrogenase ModelSurawatanawong, Panida 2009 May 1900 (has links)
The electronic structures and reaction mechanisms of transition-metal complexes
can be calculated accurately by density functional theory (DFT) in cooperation with the
continuum solvation model. The palladium catalyzed Heck reaction, iron-model
complexes for cytochrome P450 and superoxide reductase (SOR), and tetrairon
hexathiolate hydrogenase model were investigated.
The DFT calculations on the catalytic Heck reaction (between phenyl-bromide
and ethylene to form the styrene product), catalyzed by palladium diphosphine indicate a
four-step mechanism: oxidative addition of C6H5Br, migratory insertion of C6H5 to
C2H4, b-hydride transfer/olefin elimination of styrene product, and catalyst regeneration
by removal of HBr. For the oxidative addition, the rate-determining step, the reaction
through monophosphinopalladium complex is more favorable than that through either
the diphosphinopalladium or ethylene-bound monophosphinopalladium. In further
study, for a steric phosphine, PtBu3, the oxidative-addition barrier is lower on monopalladium monophosphine than dipalladium diphosphine whereas for a small
phosphine, PMe3, the oxidative addition proceeds more easily via dipalladium
diphosphine. Of the phosphine-free palladium complexes examined: free-Pd, PdBr-, and
Pd(h2-C2H4), the olefin-coordinated intermediate has the lowest barrier for the oxidativeaddition.
P450 and SOR have the same first-coordination-sphere, Fe[N4S], at their active
sites but proceed through different reaction paths. The different ground spin states of the
intermediate FeIII(OOH)(SCH3)(L) model {L = porphyrin for P450 and four imidazoles
for SOR} produce geometric and electronic structures that assist i) the protonation on
distal oxygen for P450, which leads to O-O bond cleavage and formation of
(FeIV=O)(SCH3)(L) H2O, and ii) the protonation on proximal oxygen for SOR, which
leads to (FeIII-HOOH)(SCH3)(L) formation before the Fe-O bond cleavage and H2O2
production. The hydrogen bonding from explicit waters also stabilizes FeIII-HOOH over
FeIV=O H2O products in SOR.
The electrochemical hydrogen production by Fe4[MeC(CH2S)3]2(CO)8 (1) with
2,6-dimethylpyridinium (LutH ) were studied by the DFT calculations of proton-transfer
free energies relative to LutH and reduction potentials (vs. Fc/Fc ) of possible
intermediates. In hydrogen production by 1, the second, more highly reductive, applied
potential (-1.58 V) has the advantage over the first applied potential (-1.22 V) in that the
more highly reduced intermediates can more easily add protons to produce H2.
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First Principles Investigation Of Hydrogen Storage In Intermetallic SystemsKinaci, Alper 01 July 2007 (has links) (PDF)
The design and production of efficient metal-hydrides for hydrogen storage is a long standing subject. Over the years, many different types of intermetallic hydride systems were studied and some of them came out to be operable. However, none of them meet all the storage criteria perfectly. In this study, total energies, hydrogen storage capacity and stability of AB (A = Al, Be, Cu, Fe, Ni, Sb, V and B = Ti) type intermetallics were investigated with the goal of spotting a potential hydrogen storage material. The relation between thermodynamic properties and the atomic and the electronic structure of hydrides are also pointed out. For this task, first principles pseudopotential method within the generalized gradient approximation (GGA) to density functional theory (DFT) was used. Calculations correctly predict experimentally determined structures except for CuTiH. Moreover, the atomic and cell parameter were found within the allowable error interval for DFT. In CuTi intermetallic, a structure having considerably lower formation energy than experimentally found mono-hydride was determined. This contradiction may be due to metastability of the experimental phase and high activation energy for the hydrogen movement in the system. It was found that AlTi and SbTi are not suitable candidates for hydrogen storage since their hydrides are too unstable. For the other intermetallic systems, the stability of the hydrides decreases in the order of VTi, CuTi, NiTi, BeTi, FeTi. For VTi, FeTi and NiTi, a change in metallic coordination around hydrogen from octahedron to tetrahedron is predicted when tetra-hydride (MTiH4) is formed. Additionally, at this composition, FeTi and NiTi have hydride structures with positive but near-zero formation energy which may be produced with appropriate alteration in chemical makeup or storage parameters. VTi is a promising intermetallic by means of storage capacity in that even VTiH6 is found to have negative formation energy but the hydrides are too stable which can be a problem during hydrogen desorption.
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Tailoring One Dimensional Novel Nano Structures For Specific Applications Using Tools Of Molecular ModelingMalcioglu, Osman Baris 01 March 2008 (has links) (PDF)
In this work, the use of theoretical tools of molecular modeling for tailoring 1D novel
nanomaterials is demonstrated. There are four selected nano-structures as examples,
each tailored for a specic demand of nano-technology that is yet to be fullled. For
the purpose of modeling/calculating the electronic and structural properties, various
methods of dening the interatomic interaction, such as empirical potential energy
functions, semi-empirical methods and density functional theory, are used. Each of
these methods have a dierent level of approximations leading to limitations in their
use. Furthermore, each method needs to be calibrated carefully in order to obtain
physically meaningful results. Examples being novel nano-structures, there does not
exist any experimental observations directly studying the material at hand. Thus, in
order to obtain a parameter set that best describes the system, a series of pre-existing
structures that are physically and/or chemically related are used. Among the methods
employed, the density functional theory (DFT) is certainly the most popular one, due to
its accuracy and more importantly the framework it provides for perturbative extensions
otherwise nearly impossible to calculate in Hartree-Fock level.
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Electronic Properties Of Dye Molecules Adsorbed On Anatase-titania Surface For Solar Cell ApplicationsTorun, Engin 01 August 2009 (has links) (PDF)
Wide band gap metal oxides have recently become one of the most investigated materials in
surface science. Among these metal oxides especially TiO2 attracts great interest, because of
its wide range applications, low cost, biocompatibility and ease of analysis by all experimental
techniques. The usage of TiO2 as a component in solar cell technology is one of the most
investigated applications of TiO2 . The wide band gap of TiO2 renders it inecient for isolated
use in solar cells. TiO2 surface are therefore coated with a dye in order to increase eciency.
This type of solar cells are called dye sensitized solar cells .
The eciency of dye sensitized solar cells is directly related with the absorbed light portion of
the entire solar spectrum by the dye molecule. Inspite of the early dyes, recent dye molcules,
which are called wider wavelength response dye molecules, can absorb a larger portion of
entire solar spectrum. Thus, the eciency of dye sensitized solar cells is increased by a
considerably amount.
In this thesis the electronic structure of organic rings, which are the fundamental components
of the dye molecules, adsorbed on anatase (001) surface is analyzed using density functionaltheory. The main goal is to obtain a trend in the electronic structure of the system as a function of increasing ring number. Electronic structure analysis is conducted through band structure
and density of states calculations. Results are presented and discussed in the framework of
dye sensitized solar cells theory.
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Ab Initio Studies Of Pentacene On Ag(111) SurfacesDemiroglu, Ilker 01 January 2010 (has links) (PDF)
In this work pentacene adsorption on both flat and stepped Ag(111) surfaces were investigated by using Density Functional Theory within Projected Augmented Wave method. On the flat Ag(111) surface favorable adsorption site for a single pentacene molecule was determined to be the bridge site with an angle of 60& / #9702 / between pentacene molecular long axis and [011] lattice direction. Potential energy surface was found to be flat, especially along lattice directions. Diffusion and rotation barriers for pentacene on this surface were found to be smaller than 40 meV indicating the possibility of a two dimensional gas phase. Calculated adsorption energies for the flat surface indicate a weak interaction between molecule and the surface indicating physisorption. On the flat surface monolayer case is found to have lower adsorption energy than the isolated case due to pentacene& / #8722 / pentacene interactions. On the stepped Ag(233) surface, close to the step edge, adsorption energy increased significantly due to the stronger interaction between pentacene molecule and low coordinated silver step atoms. On the terraces of this surface, far from step edges, however a flat potential energy surface was observed similar to the case of flat Ag(111) surface. On the stepped surface pentacene found its favorable configuration as parallel to the step with a tilt angle similar to the observed thin film phase of pentacene on Ag(111) surface. Pentacene molecule showed small distortions on stepped surface and are closer to the silver step atoms 1 Å / more than the case of flat surface, hinting a chemical interaction as well as van der Waals interactions. However on Ag(799) surface, the perpendicular orientation of the pentacene molecule to the step direction showed no strong interaction due to less matching of carbon atoms with silver step atoms.
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