The reaction of [BrRe(CO)₄]₂ with 2-(diphenylphosphino)pyridine (PN) and 6-(diphenylphosphino)-2-formylpyridine (PON) was investigated. The reactions were regiospecific and exclusively produced the phosphorus-coordinated products, BrRe(CO)₄(κᵖ-PN) and BrRe(CO)₄(κᴾ-PON). The kinetics for the chelate ring closure (κᴾ→ κᴾᴺ) in BrRe(CO)₄(κᴾ-PN) were confirmed to occur by dissociative CO loss. The reaction of [BrRe(CO)₄]₂ with 2-(diphenylphosphino)pyridine (PN) was modeled computationally by DFT calculations. The preferred reaction pathway for the substitution reaction was determined to occur by direct attack of the pnictogen donor on the dimer and formation of the κᴺ isomer as the kinetic substitution product occurs. The κᴺ kinetic product then rapidly isomerizes to the κᴾ thermodynamic product by way of a reversible ligand dissociation. Treatment of the tetrahedral cluster H₂Ru₃(CO)₃(μ₃-S) (1) with 2-(diphenylphosphino)thioanisole (PS) furnishes the cluster H₂Ru₃(CO)₇(κ²-PS)(μ₃-S) (2). Cluster 2, which exhibits a chelated thiophosphine ligand (κ²-PS), exists as a pair of diastereomers with Keq = 1.55 at 298 K that differ in their disposition of ligands at the Ru(CO)(κ²-PS) center. The PS ligand occupies the equatorial sites (Peq, Seq) in the kinetic isomer and axial and equatorial sites (Pax, Seq) in the thermodynamically favored species. The reversible first-order kinetics to equilibrium have been measured experimentally by NMR spectroscopy and HPLC over the temperature range 293-323 K. The substitution reaction involving 1 and the isomerization of the PS ligand in 2 were investigated by DFT calculations. The computational results support a phosphine-induced expansion of the cluster polyhedron that is triggered by the associative addition of the PS donor to 1. The observed isomerization of the PS ligand in 2 is best explained by a tripodal rotation of the CO and PS groups at the Ru(CO)(κ²-PS) center that is preceded by a regiospecific migration of one of the edge-bridging hydrides to the non-hydride-bridged Ru-Ru bond in 2. The chiral clusters 1,2-Ru₃(μ-H)₂(μ₃-S)(CO)₇(μ-1p1,2p2-POP) (A) and 1,2-Ru₃(μ-H)₂(μ₃-S)(CO)₇(μ-1p2,2p1-POP) (B) were formed were formed from reaction of Ru₃(μ-H)₂(μ₃-S)(CO)₉ with 1-diphenylphosphino-2-[2-(diphenylphosphino)ethoxy]benzene (POP). Chiral clusters A and B were fully characterized by IR and NMR spectroscopy. Additionally, the molecular structure of A was solved by X-ray crystallography. Chiral cluster A was resolved into its enantiomers by preparative HPLC with a chiral column. The enantiomers were characterized by electronic circular dichroism (ECD) spectroscopy and their absolute stereochemical configuration was determined by X-ray crystallography.

Density functional theory calculations have been used to investigate catalytic mechanism of polymer formation containing polar groups, from the synthesis of the monomer to the synthesis of the macromolecule. In the spirit of a sustainable and green chemistry, we initially focused attention on the coupling of CO2 as economically convenient and recyclable C1 source with C2H4 to form acrylate and/or butirro-lactone, two important polar monomers. In this process formation of a mettallolactone via oxidative coupling of CO2 and C2H4 is an important intermediate. Given this background, we explored in detail (chapter-3) several Ni based catalysts for CO2 coupling with C2H4 to form acrylate. In this thesis we report on the competitive reaction mechanisms (inner vs outer sphere) for the oxidative coupling of CO2 and ethylene for a set of 11 Ni-based complexes containing bisphosphine ligands. In another effort, considering incorporation of a C=C bond into a metal-oxygen-Functional-Group moiety is a challenging step in several polymerization reactions, we explored the details of this reaction (chapter4) using two different catalysts that are capable to perform this reaction in the synthesis of heterocycles. Specifically, the [Rh]-catalyzed intramolecular alkoxyacylation ([Rh] = [RhI(dppp)+] (dppp, 1,3-Bis-diphenylphosphino-propane), and the [Pd]/BPh3 intramolecular alkoxyfunctionalizations. Rest of the thesis we worked on understanding the details of the polymerization of polar monomers using organocatalysts based on N-heterocyclic carbenes (NHC) or N-heterocyclic olefins (NHO). In particular (chapter-5) we studied the polymerization of N-methyl N-carboxy- anhydrides, towards cyclic poly(N-substituted glycine)s, promoted by NHC catalysts. In good agreement with the experimental findings, we demonstrated that NHC promoted ring opening polymerization of N-Me N-Carboxyanhydrides may proceed via two different catalytic pathways. In a similar effort we studied polymerization of propylene oxide (PO) (chapter-6) promoted by N-heterocyclic olefins (NHO) in combination with benzylic alcohol (BnOH). Calculations support the experimental observation that there might be two different catalytic pathways namely the anionic and the zwitterionic pathways. Potential energy surfaces analysis suggested in different NHO one or other mechanism is operational which is strongly depends on steric and electronic properties of particular NHO taken in account.

This thesis covers the structural, electronic, magnetic, and vibrational properties of graphene and silicene. In Chapter I, we will start with an introduction to graphene and silicene. In Chapter II, we will briefly discuss about the methodology (i. e. density functional theory)In Chapter III, we will introduce band gap opening in graphene either by introducing defects/doping or by creating superlattices with h-BN substrate. In Chapter IV, we will focus on the structural and electronic properties of K and Ge-intercalated graphene on SiC(0001). In addition, the enhancement of the superconducting transition temperature in Li-decorated graphene supported by h-BN substrate will be discussed. In Chapter V, we will discuss the vibrational properties of free-standing silicene. In addition, superlattices of silicene with h-BN as well as the phase transition in silicene by applying an external electric field will be discussed. The electronic and magnetic properties transition metal decorated silicene will be discussed, in particular the realization of the quantum anomalous Hall effect will be addressed. Furthermore, the structural, electronic, and magnetic properties of Mn decorated silicene supported by h-BN substrate will be discussed. The conclusion is included in Chapters VI. Finally, we will end with references and a list of publications for this thesis.

Les travaux décrits dans ce manuscrit de thèse concernent l’exploitation des interactions acide-base de Lewis entre deux partenaires organophosphorés et organoborés. Quatre types d’interactions ont été particulièrement au cœur de notre étude, allant de la frustration de la liaison P-B à la formation de complexes acide base de Lewis fortement liés. L’étude de ces interactions, couplée à des outils de physico-chimie organique et de chimie théorique, nous ont permis, non seulement de comprendre les paramètres clés de la réactivité mais également de mettre en avant de nouvelles voies de synthèse. Dans un premier temps, notre étude s’est portée sur l’hydrogénation de composés insaturés par les Paires de Lewis Frustrées (FLP). Ainsi, l’origine de l’incapacité des P/B à réduire les accepteurs de Michael a été expliquée par des mesures cinétiques de la nucléophilie et de la basicité de Lewis de phosphines encombrées. Dans une seconde partie, l’hydricité de complexes de phosphine borane (PBs) variées a été mesurée, nous permettant d’établir une échelle de prédiction de la réduction de divers électrophiles en présence de ces donneurs d’hydrure ioniques. Cette étude nous a conduit à considérer les PBs comme substrats dans des réaction de borylation intramoléculaire impliquant BH3 comme source de bore. Cependant, des résultats inattendus ont mis en exergue une migration prépondérante à la borylation lors de la réaction entre les PBs et un acide de Lewis, donnant accès à une nouvelle familles de PBs possédant un atome de bore électro-déficitaire. Des études cinétiques couplées à une étude par DFT ont permis de rationaliser la réactivité observée. Enfin, l’exploitation des interactions réversibles entre une phosphine secondaire et un acide de Lewis organique ou métallique nous ont permis de contrôler la régiosélectivité lors de l’hydrophosphination d’alcènes 1,1-disubstitués par les diarylphosphines. Nos études mécanistiques soutiennent fortement l’implication d’un carbocation comme intermédiaire réactionnel. / The research in this thesis describes the exploration of the Lewis-acid Lewis-base interactions between organophopshine and organoborane compounds. We focused our attention on four types of interactions going from frustrating to stong P-B bonds. Our understanding of these interactions is based on detailed kinetics and computational calculations, allowing us to find a new reactivity of these species. In a first part, we studied the metal-free hydrogenation of unsaturated compounds using Frustrated Lewis Pairs (FLPs). Based on the measurement of the nucleophilicity and the Lewis basicity parameters of sterically hindered phosphines, we were able to explain reasons of the failure of P/B FLPs to catalyse the hydrogenation of Michael acceptors under H2. In the second chapter, we employed the Mayr’s linear-free energy relationship to measure the hydricity of various phosphine borane complexes (PBs) and compare their reactivity to commons hydride donors. Based on these kinetic parameters, we next turned our attention to investigate the effect of a strong Lewis acid, (B(C6F5)3), to prevent the expected intramolecular borylation of PBs to take place. By combining kinetic and computational investigations, we have been able to understand factors controlling this reaction. In the last chapter, we reported on the regioselective organocatalytic Markovnikov hydrophosphination of aryl alkenes. Importantly, we highlighted that the reversible formation of a phosphine-Lewis acid complex is in the core of the catalytic process. Mechanistic investigations support the formation of a carbocation in the catalytic cycle.

The current environmental situation, with respect to global warming and the ever– approaching depletion of fossil fuel sources, places significance on the development of green fuel and platform chemical production methods. In this context, processes that utilise biomass sources as feedstock, are of great interest. Cellulose, which is the most abundant biopolymer in nature, is a renewable low–cost carbon resource derived from harvest residues and sources like wood and straw. Glucose generation from cellulose requires a saccharide conversion, whereby the β-(1,4)-glycosidic bond linkages in the cellobiose polymer repeating units are cleaved. Problems arise in the hydrolysis of cellulose as experimental and theoretical studies have shown cellulose to have very low solubility in water and most other general molecular solvents. This results in the use of harsh pretreatments at high temperatures and pressures to extract cellulose from lignocellulosic material and strong acids catalysts (pKa < −3.2). Room temperature ionic liquids (RTILs) provide potentially environmentally friendly alternative. It has been shown that ILs can dissolve cellulose under relatively benign conditions and can possibly be adapted into a one-pot-like process of hydrolysis using acid-functionalised IL catalysts. This dissertation investigated the effect of various ionic liquids on the thermodynamics of cellobiose acid hydrolysis, as both a catalyst and as a solvent, using computational means. An appropriate thermodynamic cycle protocol, a DLPNO-CCSD(T)/ccpVTZ//TPSS/def2-TZVP [M05-2X/6-31+G** (SMD)] proton exchange cycle, was established through benchmarking for the prediction of Brønsted acid-functionalised ionic liquid pKa values in ionic liquids. The sulfonyl-functionalised acidic IL was shown to be the most acidic IL resulting in a lower protonation free energy. Solvation in ionic liquids resulted in higher protonation and barrier height free energies relative to solvation in water. The current environmental situation, with respect to global warming and the ever– approaching depletion of fossil fuel sources, places significance on the development of green fuel and platform chemical production methods. In this context, processes that utilise biomass sources as feedstock, are of great interest. Cellulose, which is the most abundant biopolymer in nature, is a renewable low–cost carbon resource derived from harvest residues and sources like wood and straw. Glucose generation from cellulose requires a saccharide conversion, whereby the β-(1,4)-glycosidic bond linkages in the cellobiose polymer repeating units are cleaved. Problems arise in the hydrolysis of cellulose as experimental and theoretical studies have shown cellulose to have very low solubility in water and most other general molecular solvents. This results in the use of harsh pretreatments at high temperatures and pressures to extract cellulose from lignocellulosic material and strong acids catalysts (pKa < −3.2). Room temperature ionic liquids (RTILs) provide potentially environmentally friendly alternative. It has been shown that ILs can dissolve cellulose under relatively benign conditions and can possibly be adapted into a one-pot-like process of hydrolysis using acid-functionalised IL catalysts. This dissertation investigated the effect of various ionic liquids on the thermodynamics of cellobiose acid hydrolysis, as both a catalyst and as a solvent, using computational means. An appropriate thermodynamic cycle protocol, a DLPNO-CCSD(T)/ccpVTZ//TPSS/def2-TZVP [M05-2X/6-31+G** (SMD)] proton exchange cycle, was established through benchmarking for the prediction of Brønsted acid-functionalised ionic liquid pKa values in ionic liquids. The sulfonyl-functionalised acidic IL was shown to be the most acidic IL resulting in a lower protonation free energy. Solvation in ionic liquids resulted in higher protonation and barrier height free energies relative to solvation in water.

No description available.

Since the discovery of graphene, a new era of physics called "Two Dimensional (2D)Materials" has emerged. Group IV and Group III Selenides such as SnSe and InSe arepromising members of the 2D family. Structure of Group IV selenides is unique and highlysensitive to pressure and temperature. To further tweaking their properties by structuralchanges, thorough understanding of how the structure relates to the electronic bands is veryimportant. Based on the results from DFT calculations, I carefully analyzed electronic bandstructures of layered SnSe with various interlayer stacking. The first part of this dissertationdiscussed the possible stacking-dependent indirect-direct transition of bilayer SnSe.By further analysis, these results reveal that the directionality of interlayer interactionsdetermine the critical features of their electronic band structures. Further, it demonstratedthat such changes can be achieved by substitutional chemical doping. Using a multi-scalemodeling approach by combining the result of DFT and Boltzmann Transport Theory, Idiscussed the electron transport properties of co-doped SnSe, a class of thermodynamicallyand dynamically stable structures. The second part discussed on charge transfer across InSe/Gas interface, which showsbi-polar transport properties. This finding is in a good agreement with the recent experimentalobservations. Fundamental understanding of charge transfer in few-layer InSe /gasinterfaces at the atomic level is expected to pave the path for designing gas sensing devices.

En esta tesis se ha estudiado la intercalación de Li en dos nanoestructuras de titanatos, TiO2(B) y H2Ti3O7, siendo ambos materiales candidatos para ser aplicados en baterías de ión-Li. En el caso del primer material estudiado, se realizaron estudios sobre el voltaje de intercalación del Li en las superficies ultrafinas (100) y (001) y en el sistema “bulk”, mientras que en el segundo material se estudió sólamente la intercalación en el “bulk”. En general, se obtuvieron los parámetros para las estructuras y las curvas DOS de los sistemas puros e intercalados. Se estudió la diferencia de densidad de carga y se realizó el análisis de Bader para estudiar las cargas del sistema. Se realizaron estudios vibracionales, obteniendo las phDOS, y el espectro Raman en el caso del estudio sobre TiO2(B). Se realizaron estudios de la difusión de Li en los dos materiales, y las propiedades termodinámicas de los dos sistemas “bulk” han sido estudiadas, para así poder tener una comprensión más realista para la aplicación de estos sistemas. En el desarrollo de la presente tesis, todos los cálculos fueron realizados en el marco de la Teoría del Funcional de la Densidad con la metodología de Hubbard (DFT+U). Los cálculos de optimización de geometría, estructura electrónica, estudios de carga, espectros Raman y difusión, entre otros, se obtuvieron mediante el código VASP. Los estudios de enlace se realizaron mediante el código DDEC6. Cabe destacar que el postprocesamiento para la obtención de las propiedades vibracionales y termodinámicas fueron realizadas con el código libre Phonopy. Los cálculos fueron comparados con estudios teóricos previos de DFT y con resultados experimentales, cuando había disponibles. / In this thesis, the Li intercalation of two titanates nanostructures were studied, TiO2(B) and H2Ti3O7, being both materials candidates to be applied in Li-ion batteries. In the case of the first material studied, studies about voltage intercalation of Li in the ultrathin surfaces (100) and (001) and in the “bulk” systems were performed, while in the second material, only the intercalation in the “bulk” was studied. In general, the parameters obtained for the structures and the DOS curves of the pure and intercalated systems were obtained. The charge density difference was studied and the Bader analysis was performed in order to study the charges of the system. Vibrational studies were performed, obtaining the phDOS and the Raman spectra in the case of the TiO2(B) study. Li diffusion studies in both materials were performed, and the thermodynamic properties of both “bulk” systems have been studied to have a more realistic comprehension for the application of these systems. In the development of the present thesis, all of the calculations were performed with Density Functional Theory with the Hubbard methodology (DFT+U). The calculations of optimization of geometry, electronic structure, charge studies, Raman spectra and diffusion, among others, were obtained with the VASP code. The bond studies were performed with the DDEC6 code. It is important to mention that the postprocessing to obtain the vibrational and thermodynamics properties was performed with the open source package Phonopy. The calculations were compared with previous DFT theoretical studies and experimental results, when they were available.

In this work we address two problems in computational chemistry relevant to biomolecular modeling. In the first project, we consider the conformer space of melatonin as a a representative example of “real-life” flexible biomolecules. Geometries for all 52 unique conformers are optimized using spin-component scaled MP2, and then relative energies are obtained at the CCSD (T) level near the complete basis set limit. These are then used to validate a variety of DFT methods with and without empirical dispersion corrections, as well as some lower-level ab initio methods. Basis set convergence is found to be relatively slow due to internal C-H…O and C-H…N contacts. Absent dispersion corrections, many DFT functionals will transpose the two lowest conformers. Dispersion corrections resolve the problem for most functionals. Double hybrids yield particularly good performance, as does MP2.5. In the second project, we propose a simple DFT-based diagnostic for nondynamical correlation effects. Aλ= (1-TAE [ΧλC]/TAE[XC])/λ where TAE is the total atomization energy, XC the “pure” DFT exchange-correlation functional, and ΧλC the corresponding hybrid with 100λ% HF-type exchange. The diagnostic is a good predictor for sensitivity of energetics to the level of theory, unlike most of the wavefunction-based diagnostics. For GGA functionals, Aλ values approaching unity indicate severe non-dynamical correlation. The diagnostic is only weakly sensitive to the basis set (beyond polarized double zeta) and can be applied to problems beyond practical reach of wavefunction ab-initio methods required for other diagnostics.

The price fluctuation and depletion of crude oil have led to the fervent interest in finding alternatives that can satisfy our increasing need for energy. In the past decades, two primary approaches are seen as promising ways to remedy our dependence on crude oil: first, the use of natural gas, primarily methane, to produce high-value hydrocarbons, and second, the use of ammonia as a hydrogen carrier. In this dissertation, we used density functional theory (DFT) calculation and kinetic modeling to investigate methane activation and C-C coupling on WC as well as the ammonia decomposition over the CoNi alloy surface. From our methane conversion project, we investigated the reactivity of W-terminated WC(0001) and WC(112 ̅0) surface toward methane activation and conversion to produce C2 moieties using DFT. We first calculate the intermediates binding energies and activation and reaction energies of methane dissociation. We found that WC(112 ̅0) is better at dissociating the first C–H bond than WC(0001). Our results also indicate that the surface is likely populated by (CH)ads species. The mobility of (CH)ads species on both surfaces allows the possibility of C-C coupling, resulting in a precursor for higher hydrocarbon formation. Our results also demonstrate that the WC(0001) surface favors the production of the (C2H2)ads species, whereas the WC(112 ̅0) surface dissociates CHx completely, resulting in coke formation. Thus, methane dissociates readily on the WC surfaces whereas the formation of the C2 species is sensitive to the surface structure. The DFT study on ammonia decomposition has been performed in close collaboration with the experimental study. A highly active catalyst consisting of CoNi alloy nanoparticles well-dispersed on a MgO–CeO2–SrO mixed oxide support with potassium promotion exhibited a performance matching that of the Ru-based catalysts. Extensive characterization in combination with the DFT results revealed that the CoNi alloy surface and metal/oxide interfaces are the active sites for catalytic decomposition of ammonia. Moreover, the much improved catalytic activity stems mainly from the presence of interface where the recombinative desorption of nitrogen has been greatly enhanced. These have been demonstrated by examining the detailed elementary steps of ammonia decomposition on the Co, Ni, Co2Ni, CoNi2 (111) surfaces and at the CeO2/Co2Ni interface. We calculated the binding energies of intermediates and the activation energies of each elementary step in ammonia decomposition. We found that on the Co, Ni, Co2Ni, CoNi2 surfaces, N–N bond formation is the rate-determining step, with the CoNi alloy surfaces having a lower activation energy than the pure metal surfaces. Over the CeO2/Co2Ni interface, however, N–H bond dissociation becomes rate-determining. The high catalytic activity at the CeO2/Co2Ni interface originates from the localized charge polarization due to alloying and the presence of the oxide which drastically facilitates N2* formation. We then integrated the DFT-calculated adsorption and activation energies in the microkinetic modeling of ammonia decomposition on the Co, Co2Ni, CoNi2, and Ni surfaces, focusing on the alloying effect. Two cases were investigated: ammonia decomposition in the 1) absence and 2) presence of product re-adsorption. In both cases, we determined the turnover frequencies, the apparent activation energies, the steady-state coverages, the degree of rate control, and the reaction orders. Our results show that in both cases, the alloys have higher catalytic performance than the pure metals. We also found that as the temperature increases, ammonia decomposition switches from being limited by N–N (and N–NH) bond formation to N–H bond dissociation. This change of mechanism is predicted to occur at lower temperatures on the alloy surfaces. In the case of hydrogen re-absorption, the surface H* adatom retards the last N–H bond-breaking step, resulting in the high coverage of NH* species on the surfaces, making N–NH coupling an alternative pathway for N2 formation. Furthermore, our microkinetic results show that alloying Ni with Co reduces the effect of hydrogen inhibition at high hydrogen partial pressures. In summary, this dissertation provides information for the design of efficient catalysts toward methane conversion and ammonia decomposition.