42 research outputs found
Resolving a puzzling anomaly in the spin-coupled generalized valence bond description of benzene
In an earlier study of benzene, Small and Head-Gordon found that the spin-coupled generalized valence bond (SCGVB) wave function for the π system predicted a distorted (non-D6h) geometry, one with alternating CC bond lengths. However, the variations in the energy were very small and the predictions were made using a very small basis set (STO-3G). We re-examined this prediction using a much larger basis set (aug-cc-pVTZ) to determine the dependence of the energy of benzene on the distortion angle, ΔθCXC (ΔθCXC = 0° corresponds to the D6h structure). We also found a distorted geometry with the optimum ΔθCXC being 0.31° with an energy 0.040 kcal mol⁻¹ lower than that for the D6h structure. In the optimum geometry, adjacent CC bond lengths are 1.3861 Å and 1.4004 Å. Analysis of the SCGVB wave function led us to conclude that the cause of the unusual non-D6h geometry predicted by the SCGVB calculations seems to be a result of the interaction between the Kekulé and Dewar components of the full SCGVB wave function. The addition of doubly ionic configurations to the SCGVB wave function leads to the prediction of a D6h geometry for benzene and a dependence on ΔθCXC essentially the same as that predicted by the complete active space self-consistent field wave function
Quantum frustration in organic Mott insulators: from spin liquids to unconventional superconductors
We review the interplay of frustration and strong electronic correlations in
quasi-two-dimensional organic charge transfer salts, such as k-(BEDT-TTF)_2X
and Et_nMe_{4-n}Pn[Pd(dmit)2]2. These two forces drive a range of exotic phases
including spin liquids, valence bond crystals, pseudogapped metals, and
unconventional superconductivity. Of particular interest is that in several
materials there is a direct transition as a function of pressure from a spin
liquid Mott insulating state to a superconducting state. Experiments on these
materials raise a number of profound questions about the quantum behaviour of
frustrated systems, particularly the intimate connection between spin liquids
and superconductivity. Insights into these questions have come from a wide
range of theoretical techniques including first principles electronic
structure, quantum many-body theory and quantum field theory. In this review we
introduce the basic ideas of the field by discussing a simple frustrated
Heisenberg model with four spins. We then describe the key experimental
results, emphasizing that for two materials, k-(BEDT-TTF)_2Cu_2(CN)_3 and
EtMe_3Sb[Pd(dmit)_2]_2, there is strong evidence for a spin liquid ground
state, and for EtMe_3P[Pd(dmit)_2]_2, a valence bond solid ground state. We
review theoretical attempts to explain these phenomena, arguing that this can
be captured by a Hubbard model on the anisotropic triangular lattice at half
filling, and that resonating valence bond wavefunctions can capture most of the
essential physics. We review evidence that this model can have a spin liquid
ground state for a range of parameters that are realistic for the relevant
materials. We conclude by summarising the progress made thus far and
identifying some of the key questions still to be answered.Comment: Major rewrite. New material added and many typos corrected. 67 pages,
41 figures. Thanks to those who commented on the previous versio
Light scattering spectroscopy: studies of electronic excitations and atomic vibrations in matter
This thesis comprises a compendium of forty-two publications on
various research topics unified by a common theme s light scattering
spectroscopy. The research work presented ranges from experimental
studies of the dynamics of atoms in various phases of matter through
to theoretical investigations of the light scattering process. The
topics discussed are the dynamics of structural phase transitions in
proper and improper ferroelectrics, and in antiferroelectrics, for
both ordered and disordered systems; the magnetic phase transitions
in ordered and disordered compounds; the electronic properties of
divalent transition metal ions in pure and dilute systems; the
lattice vibrations of ionic, covalent and molecular crystals; the
selection rules governing higher-order light scattering processes;
the weak interactions occurring between chemical species in both
aqueous and non-aqueous solvents; and the use of automation
techniques in light scattering spectroscopy
More is Different: Modern Computational Modeling for Heterogeneous Catalysis
La combinació d'observacions experimentals i estudis de la Density Functional Theory (DFT) és un dels pilars de la
investigació química moderna. Atès que permeten recopilar informació física addicional d'un sistema químic,
difícilment accessible a través de l'entorn experimental, aquests estudis es fan servir àmpliament per modelar i predir
el comportament d'una gran varietat de compostos químics en entorns únics. A la catàlisi heterogènia, els models
DFT s'utilitzen habitualment per avaluar la interacció entre els compostos moleculars i els catalitzadors, vinculant
aquestes interpretacions amb els resultats experimentals. Tanmateix, l'alta complexitat trobada tant als escenaris
catalítics com a la reactivitat, implica la necessitat de metodologies sofisticades que requereixen automatització,
emmagatzematge i anàlisi per estudiar correctament aquests sistemes. Aquest treball presenta el desenvolupament i
la combinació de múltiples metodologies per avaluar correctament la complexitat d'aquests sistemes químics. A més,
aquest treball mostra com s'han utilitzat les tècniques proporcionades per estudiar noves configuracions catalítiques
d'interès acadèmic i industrial.La combinación de observaciones experimentales y estudios de la Density Functional Theory (DFT) es uno de los
pilares de la investigación química moderna. Dado que permiten recopilar información física adicional de un sistema
químico, difícilmente accesible a través del entorno experimental, estos estudios se emplean ampliamente para
modelar y predecir el comportamiento de una gran variedad de compuestos químicos en entornos únicos. En la
catálisis heterogénea, los modelos DFT se emplean habitualmente para evaluar la interacción entre los compuestos
moleculares y los catalizadores, vinculando estas interpretaciones con los resultados experimentales. Sin embargo, la
alta complejidad encontrada tanto en los escenarios catalíticos como en la reactividad, implica la necesidad de
metodologías sofisticadas que requieren de automatización, almacenamiento y análisis para estudiar correctamente
estos sistemas. Este trabajo presenta el desarrollo y la combinación de múltiples metodologías con el objetivo de
evaluar correctamente la complejidad de estos sistemas químicos. Además, este trabajo muestra cómo las técnicas
proporcionadas se han utilizado para estudiar nuevas configuraciones catalíticas de interés académico e industrial.The combination of Experimental observations and Density Functional Theory studies is one of the pillars of modern
chemical research. As they enable the collection of additional physical information of a chemical system, hardly
accessible via the experimental setting, Density Functional Theory studies are widely employed to model and predict
the behavior of a diverse variety of chemical compounds under unique environments. Particularly, in heterogeneous
catalysis, Density Functional Theory models are commonly employed to evaluate the interaction between molecular
compounds and catalysts, lately linking these interpretations with experimental results. However, high complexity
found in both, catalytic settings and reactivity, implies the need of sophisticated methodologies involving automation,
storage and analysis to correctly study these systems. Here, I present the development and combination of multiple
methodologies, aiming at correctly asses complexity. Also, this work shows how the provided techniques have been
actively used to study novel catalytic settings of academic and industrial interest
Carbon in the Galaxy: Studies from Earth and Space
Presented here is the text of the invited papers presented during a meeting entitled, Carbon in the Galaxy: Studies from Earth and Space, that was held at NASA Ames Research Center on November 5 and 6, 1987. For completeness, abstracts from all of the poster papers and the text of a paper summarizing what was learned during the course of the meeting are also included. The underlying premise for the meeting was that there is much to be gained by bringing together scientists from very different disciplines, all of whom study carbon in different ways for different reasons. The interchanges took place during the meeting and the contents of the enclosed papers validate that premise
Hierarchical Intermolecular Interaction Models of N-Heteroaromatic STM Adlayer Structures
The molecular scale electronic device concept was initiated in 1974 with the semi-quantitative analysis of a hemiquinone molecule. Because of the molecule's electron donor and acceptor properties, and ability to transfer electrons along the -network, it was proposed that the molecule could perform as a circuit rectifier. Many investigations of molecular scale systems have occurred since then, in particular, of organic molecules with large, fused ring systems that spontaneously self-organize after deposition onto a substrate. The directionality and molecular specificity of hydrogen bonding differentiates it from the other weak interactions, driving molecules into specific arrangements and enabling spontaneous rearrangement after addition of only a small amount of enthalpic energy. A direct application of molecular recognition through self-assembly has been the design of patterned self-assembled monolayers (SAMs) for the construction of microelectrodes and supramolecular templates. However, the intermolecular interactions that drive ordered structures to form, including molecular chains and large aggregates, has not been well understood.
To elucidate a quantitative description of the intermolecular forces of network systems of aromatics that control such features as packing density and porosity, two individual model heteroaromatic systems of 9-acridinecarboxylic acid and isonicotinic acid are investigated using both experimental and computational resources. Supported by scanning tunneling microscopy (STM) topographies, x-ray diffraction (XRD) data and x-ray photoelectron (XPS) spectra, this class of N-heteroaromatics adsorbed on Ag (111) serves as a model system to systematically investigate 2-dimensional intermolecular (2-D) interactions and their impact on forming different structural phases of molecular chain domains. To approach an understanding of the dynamics of N-heteroaromatic film growth, an intermolecular interaction model of 1-D single phase chains and clusters is performed. The model considers the anisotropy of the electrostatic force interactions to determine what charge arrangements (dipole, quadrupole, etc.) better characterize the molecular interactions. Furthermore, the competition between phase chain types is shown to be length dependent and in qualitative agreement with the coverage dependent STM structural phase composition