Theoretical Investigations of the Acetylene Analogues of Group 14 Elements E2X2 (E=Si-Pb, X=F-I)

In this study, the isomers of E2X2 (E=Si-Pb, X=H, F, Cl, Br and I) were investigated. In most structures, the E-E bond length and the E-X-E angle correlate with the halogen atoms. The relative energies showed that the non-planar doubly bridged structures A are global minima for all investigated E2X2 molecules except for Si2F2, although the global minimum for Si2F2 is the vinylidene structure in the triplet state. The linear isomers G are always found as energetically higher lying transition states. The investigation of the EX fragments presented that the molecules need very high excitation energies from the X2Π ground state to the a4Σ- first excited state. Due to the large excitation energies, the bond dissociation energies of the linear XEEX structures are very small, or negative, and it is found that the E-E triple bond formation is quite unfavorable. The Energy Decomposion Analyses presented that the stability of the E2X2 molecules correlate with the bond dissociation energies. The non-planar doubly bridged structures A show the largest dissociation energies. The large dissociation energies of A stem from the large dipole-dipole electrostatic interactions and the effective three-center-two-electron orbital interaction. The preparation energies of the bridged structures are larger than those of the bent structures due to the elongation of E-X bond to form the E-X-E bridged structure. The ring structures of Si2X2 show large preparation energies and those of Si2F2 are especially quite large. Due to that, the doubly bridged structures cannot compensate the energetic loss of the E-X bond elongation to form bridged structures. As a result, the dissociation energies of SiA for Si2F2 become smaller and the isomer cannot be the global minimum

Quantum Chemical Investigation of Electronic and Structural Properties of Crystalline Bismuth and Lanthanide Triborates

The origins of the optical effects and the chemical stability of BiB3O6 are studied by gradient-corrected hybrid B3PW density functional theory within the Gaussian-orbital-based CO-LCAO scheme. Including spin-orbit coupling, B3PW yields an estimate of the indirect band gap of 4.29~4.99 eV which is closer to the experimental value of 4.3 eV than the HF, LDA or GGA results. The crystal orbital overlap population is carried out to give a detailed first-principles analysis of chemical bonding. It is found that the Bi 6s couples with the O 2p in the primary interaction, which eventually forms both bonding and antibonding orbitals below the Fermi level. The Bi 6p is further involved in the secondary interaction with the filled Bi 6s-O 2p antibonding orbitals. The stereochemical activity of the Bi lone-pairs mainly originates from the primary interaction for the occupied Bi 6s-O 2p antibonding orbitals. It is found that the Bi 6p orbitals are not critically responsible for the non-spherical shape of the Bi lone-pairs. The densities of optical absorptions for the total BiB3O6 crystal, [BiO4]5- and [BO3]3- and [BO4]5- subunits are individually calculated by convoluting the total occupied density of states and the virtual densities of states of the corresponding unit. It is found that the [BiO4]5- units are mainly responsible for the optics of BiB3O6 in the long wavelength region. The reason is that the Bi-O covalent bonds lead to large spatial orbital overlappings and thus favor the electronic transfer from the occupied O 2p to the empty Bi 6p orbitals. The relativistic and correlation effects lead to fundamental differences of the band structure, chemical bonds and optical effects for BiB3O6 compared with non-relativistic and uncorrelated calculations. The harmonic frequencies of BiB3O6 are calculated by applying the numerical-difference technique. The complete 13 A and 14 B vibrational modes are assigned, graphically visualized and classified according to the Bi-O and B-O motions. Comparisons with previous experimental reports are discussed in detail. Crystal orbital adapted Gaussian (4s4p3d), (5s5p4d) and (6s6p5d) valence primitive basis sets are derived, in line with relativistic energy-consistent 4f-in-core lanthanide pseudopotentials of the Stuttgart-Köln variety, for calculating periodic bulk materials containing trivalent lanthanide ions, particularly in this thesis for the investigation of the relative stability of C2 and I2 phases of LnB3O6. Different segmented contraction schemes are calibrated on A-type Pm2O3 studying the basis set size effects. Further applications to the geometry optimization of other A-type Ln2O3 (Ln=La-Nd) show a satisfactory agreement with experimental data using the lanthanide valence basis sets (6s6p5d)/[4s4p4d]. The cohesive energies of A-Ln2O3 within both conventional Kohn-Sham DFT and the a posteriori-HF correlation DFT schemes are evaluated by using the corresponding augmented sets (8s7p6d)/[6s5p5d] with additional diffuse functions for the atomic energies of free lanthanide atoms. The I2 phases of LaB3O6 and GdB3O6 crystals are more stable than C2 phases according to both of the calculated energetic data and first-principles bond analysis. This is in agreement with the experimental results. A new method is developed to calculate the optical properties for large systems based on available wavefunction correlation approaches in the framework of the incremental scheme. The convergence behaviors of first- and second-order polarizabilities with respect to the domain distances and incremental expansion orders are examined and discussed for the model system Ga4As4H18

The interaction between BeX2 (X = H, F, Cl) molecules and ethylene/acetilene. A study on beryllium bonds

En estudios recientes se ha mostrado la existencia de un nuevo tipo de ‘interacciones no covalentes’, llamadas ‘enlaces de Berilio’[1], cuyas propiedades químicas parecen indicar un futuro prometedor en áreas como la ciencia de materiales. Estos enlaces de Berilio se producen como aductos entre moléculas de berilio que actúan como ácidos de Lewis y otras moléculas que actúan como bases de Lewis. En este marco, el presente estudio analiza el enlace entre las pequeñas moléculas BeX2 (X = H, F, Cl) actuando como ácidos y el acetileno o etileno actuando como bases. Para un primer estudio se utilizan métodos DFT, seguidos de optimizaciones a nivel MP2 y CCSD, usándose para todos los métodos la base 6-311+G(d,p), y adicionalmente se realizan cálculos a nivel MP2 y CCSD con la base aug-cc-pVTZ para proporcionar resultados más precisos para la energía. Finalmente, se usan los programas AIMAll y NBO para llevar a cabo análisis topológicos y de orbitales naturales de enlace que permitan el estudio de la natura del enlace. Efectivamente, se encuentra que existe un enlace relativamente fuerte de berilio en cada uno de los complejos y también se observan las principales participaciones de los orbitales y las deformaciones de cada molécula características de los enlaces de berilio. Dentro de cada serie, sin embargo, la tendencia seguida por la densidad del enlace de berilio y la de la energía de interacción del complejo son opuestas. Sospechamos que la deformación sufrida por la subunidad de berilio juega un papel importante en este, a priori, extraño comportamiento, y es necesario investigar más esta cuestión para poder explicarla.Máster en Química Teórica y Modelización Computaciona

Atomistic Simulations and Microscopic Experiments to Understand Nanoscale Composition Control

In this dissertation, the possibility of using the quantum mechanics calculation in combination with experimental result is explored, in order to explain experimentally observed phenomena in materials science problems. A series of published works in this theory-experiment combinatory approach will be introduced. The topics include the phase stability of Sb2O4, surface instability of MgB2, the interplay of diffusion and mechanical strengthening effect in multilayer, and latest findings of the syntheses of metallic nano-foams. The theory-experiment combinatory approach has proven to be useful in various materials science problems. The phase transformation trajectory of the Sb2O4 polymorphs, the surface reconstruction pathways of MgB2(0001), and diffusion kinetics of Cr-Cu dilute alloy system are calculated by the density functional theory coupled with the nudged elastic band method. Finally, the syntheses of Cu and Cu-Ni alloy nano-foams are reported, detailing their microstructure and morphology characterized by electron microscopies. The potential application of the theory-experiment combinatory approaches in the nano-foam synthesis is further discussed toward better understanding of the structure-property relations of the metallic nano-foams

Investigating the role of curvature on the formation and thermal transformations of soot

In this work, the role of curved polycyclic aromatic hydrocarbons (cPAH) on the initial formation mechanism and thermal transformations of soot was explored. Experimental and computational techniques were used to probe the integration, presence and impact of internal pentagonal rings on the nucleation mechanism of these particulates. A significant charge polarisation was found to occur when an internal pentagonal ring pyramidalises the aromatic network. Phase contrast transmission electron microscopy allowed for the extent of conjugation and degree of curvature to be determined in early soot nanoparticulates with 15 aromatic rings and two pentagons being the median species. The dipole moment of such a species was calculated to be 5.32 debye. The polarity was found to be persistent at flame temperatures with inversion and fluctuations being minimal. Homogeneous nucleation was considered with homodimerisation energies with one or two internal pentagonal rings within cPAH found to be comparable in energy to flat PAH (fPAH) homodimers of similar weight, with more pentagons reducing the binding energy. Ion-induced nucleation was considered with binding energies calculated between chemi-ions and cPAH suggesting small stable clusters at flame temperatures. However, physical and ion-induced nucleation of cPAH were found to be insufficient alone to explain the formation of soot. The impact of curvature on the reactivity of PAH were then studied. Strong crosslinks between σ-radicals and cPAH were found to form at their rim due to decreased aromaticity. Partially saturated rim-based pentagonal rings were also found to form localised π-radicals that allow stacked and bonded complexes to form, suggesting a covalently stabilised soot nucleation. Finally, the curved geometry of highly annealed soot, otherwise known as non-graphitising carbon, was explored using annealed molecular dynamics simulations and a discrete mesh analysis method. Analysis of the angular defect of the meshes revealed an excess of negative curvature. The coexistence of curved and layered ribbon-like structures was found to be possible due to the presence of a small number of non-sp² defects such as screw dislocations and free edges, which will impact the synthesis of novel carbon materials and the oxidation of thermally annealed soot. The incorporation of curvature and pentagonal rings is therefore considered critical for understanding the properties, formation and destruction of combustion generated carbonaceous particles and other carbon materials.This project is supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme

Study of the Geometry and Electronic Structure of Self-Assembled Monolayers on the Au(111) Surface

123 p.[EN]A study of the geometries and electronic properties of organic molecular thin films adsorbed on Au(111) has been presented in this thesis. It contains DFT-based first principles calculations, that have been compared to experimental results, mainly STM/STS measurements. Three different systems have been analysed in detail: chemisorption of small thiolates, donor–acceptor charge-transfer complex adsorption, and co-adsorption of complementary polyarenes. These systems cover a wide range of the different strategies used to grow SAMs. Indeed, the relative strength of the interactions that steer the selfassembly of such films, ranging from strong to weak adsorbate–substrate interactions with distinct adsorbate–adsorbate interactions, results in a perfect playground to understand the underlaying physics in the formation of SAMs. With that aim we have used different theoretical tools throughout this work, that include, besides relaxed geometries and energetic analysis, induced electron densities, PDOS on atomic sites and molecular orbitals, band structure calculations, charge transfer analysis, etc.[ES]Esta tesis presenta un estudio teórico, mediante el formalismo de teoría funcional de densidad (dft), de las geometrías y propiedades electrónicas de films delgados de moléculas orgánicas adsorbidad en Au(111). En función de la intensidad relativa de las interacciones adsorvato-adsorbato y adsorbato-substrato, que gobiernan el autoensamblado de dichos films, tres sistemas diferentes son analizados en detalle: quimisorcion de pequeños thiolatos, adsorción de complejos donor-acceptor, y co-adsorción de poliarenos complemetarios. Con el objetivo de entender la física subyacente en la formacion de monocapas autoensambladas, resultados experimentales disponibles (principalmente medidas STM y STS) son contrastados gracias al uso de distintas herramientas teóricas, que incluyen, proyección de densidad de estados en orbitales atómicos y moleculares, cálculo de la estructura de bandas, transferencia de carga, etc

Exploring the chemical properties of elementary actinide species in the gas phase

Tese de doutoramento, Química (Química-Física), Universidade de Lisboa, Faculdade de Ciências, 2015Gas-phase ion chemistry studies were conducted to search for new elementary actinide (An) species and reactions and to enhance the fundamental understanding of these elements. In the gas phase, with the absence of external perturbations (solvents), physical and chemical properties of elementary actinide species were better studied and relationships between electronic structure, reactivity and energetics established. The role of the 5f electrons and the covalency in these heavy elements were important aspects examined. Most of the studies were performed in parallel with theoretical calculations (through international collaborations), which were essential for complementing the understanding of actinide chemistry. The studies were performed with the use of mass spectrometry techniques, namely Laser Ionization Fourier Transform Ion Cyclotron Resonance (LI-FT-ICR-MS) and Electrospray Ionization Quadrupole Ion Trap (ESI-QIT-MS). The use of these ion traps were very effective for the ion/molecule reaction studies, enabling to perform complex sequences of ion manipulation and obtaining kinetic and mechanistic information. New actinide species and processes were identified, with the actinyl ions, AnO2+/2+, being the highlight of the work: oxo-exchange with water and methanol; addition reactions of water and oxygen to actinyl ions, with a focus in the formation of superoxide uranyl(VI) ions from uranyl(V); synthesis of sulfur “analogues” of actinyls from reactions of An+/2+ ions with COS; determination of the effective charge density of U in uranyl(VI), comparable to that of the late trivalent lanthanides, from the dissociation of bimetallic clusters; evaluation of III/IV redox stabilities of lanthanides and actinides by measurement of hydrolysis rates for MO(NO3)3- species; and coordination studies of biologically relevant ligands, amino acids in particular, that allowed establishing trends of relative stabilities/affinities towards the actinyl ions.Foram efetuados estudos de química de iões em fase gasosa com o objetivo de obter novas espécies e reações de actinídeos (An) e ampliar o conhecimento destes elementos a nível fundamental. Devido à ausência de fatores externos na fase gasosa, nomeadamente solventes, é possível realizar estudos mais precisos das propriedades físicas e químicas de espécies com actinídeos e estabelecer relações entre reatividade, energética e estrutura eletrónica. O papel dos eletrões 5f e a covalência neste tipo de elementos foram dois aspetos importantes abordados. A parte experimental foi realizada em simultâneo com cálculos teóricos, através de colaborações internacionais, que foram essenciais uma vez que permitiram complementar o conhecimento da química dos actinídeos. Recorreu-se ao uso de técnicas de espectrometria de massa, Ionização Laser/Ressonância Ciclotrónica de Iões com Transformada de Fourier (LI-FT-ICR-MS) e Ionização por Eletronebulização/Trapa de Iões Quadrupolar (ESI-QIT-MS). As trapas de iões utilizadas foram bastante vantajosas no estudo de reações ião/molécula, permitindo realizar sequências complexas de manipulação de iões e estudos cinéticos e mecanísticos. Foram identificadas novas espécies e processos envolvendo actinídeos, em que os iões actinilos foram o alvo principal do estudo: oxo-permuta com água e metanol; adição de água e oxigénio molecular a iões actinilos, em que a formação de iões superóxidos de uranilo(VI) partindo de uranilo(V) foi relevante; síntese de espécies análogas de actinilos com enxofre, partido de reações de iões An+/2+ com COS; determinação da carga efetiva do U no uranilo(VI), a qual se verificou ser comparável à dos iões lantanídeos trivalentes, através da dissociação de agregados bimetálicos; avaliação de estabilidades redox dos estados III/IV de lantanídeos e actinídeos estimadas através de reações de hidrólise de espécies MO(NO3)3-; e estudos de coordenação de ligandos aminoácidos com iões actinilos, onde ordens de estabilidade/afinidade foram estimadas.Fundação para a Ciência e a Tecnologia (FCT), SFRH/BD/70475/201

Defect Chemistry of Novel Transparent Conductors

The combination of optical transparency and metallic-like conductivity in a single material is unusual, such that only a handful of transparent conductors have been discovered in the last century. However, they play a crucial rôle as transparent electrodes in the operation of modern devices such as display screens and solar cells. The increasing ubiquity of opto-electronic technology therefore motivates the continued development and discovery of transparent conductors. This work uses computational methods to investigate the defect chemistry and rationalise the performance of a wide variety of transparent conducting materials. The insights from this project will guide the optimisation of existing technologies through alternative doping strategies, and act as a stepping stone towards the development of new materials for transparent conducting applications. An overview of the electronic structure requirements and some applications of transparent conducting materials are offered in Chapter 1. Next, the computational theory that enables the calculation of electronic structure is presented in Chapter 2, followed by a section on methodology and implementation in Chapter 3. Chapter 4 introduces a selection of post-transition metal oxides (ZnO, In₂O₃, Ga₂O₃ and ZnSb₂O₆) as n-type transparent conductors and investigates doping strategies for achieving metallic-like conductivity. This is followed by Chapter 5, which examines CaCuP and CuI as potential p-type transparent conductors, with a focus on their defect chemistry (specifically the copper vacancy) and charge transport behaviour. Finally, p-type doping strategies are considered in the transparent perovskite BaSnO₃ in Chapter 6, followed by a summary of the results and suggestions of future work in Chapter 7