Nuevos índices de reactividad en sistemas discretos y extendidos
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Archivos
Fecha
2007
Autores
Profesor/a Guía
Facultad/escuela
Idioma
es
Título de la revista
ISSN de la revista
Título del volumen
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Universidad Andrés Bello
Nombre de Curso
Licencia CC
Licencia CC
Resumen
Tal como su título lo dice esta Tesis tenía como objetivo principal el estudio,
con métodos químico-cuánticos, de grandes estructuras compuestas por carbono,
nitrógeno y boro. Su objeto era brindar una buena descripción de las propiedades
electrónicas de dichas estructuras a fin de poder enténder el origen de algunas de
sus propiedades.
Sin embargo pronto se pudo apreciar que, para lograr ese objetivo, había
que analizar alguno~ puntos dados como supuestos. Eso condujo a la observación
de que el lenguaje que emplea la Teoría de los Orbitales Moleculares adolecía de
una cierta falta de claridad para lograr algunas descripciones correctas de estas
estructuras.
Además fue posible notar una ausencia relativa de índices de reactividad
locales, lo cual indujo a proponer algunos nuevos y analizar algo más
detalladamente algunos ya propuestos. Esto, a su vez, exigió hallar una solución
al llamado "problema de las poblaciones electrónicas negativas".
Finalmente hubo que estudiar un par de ejemplos de la posible aplicación
de los nuevos índices locales de reactividad propuestos.
Esta Tesis presenta el resultado de todas esas exploraciones científicas.
Cada capítulo es una unidad completa en sí mismo pero que está
lógicamente ligado a los que lo preceden. Por ello se adoptó la determinación de
estructurar cada capítulo con sus propias secciones. Habría sido imposible, desde
el punto de vista lógico, haberlo hecho de otra forma.
La necesidad de explorar obligatoriamente nuevos rumbos en esta Tesis ha
dejado una enorme cantidad de material que no pudo ser trabajado
adecuadamente. Por ello las proyecciones de lo aquí presentado dan al menos
para un par de años o más de trabajo extra.
The study of carbon, nitrogen and boron macrostructures required the clarification of several aspects of the theory that were taken for granted. First, we proceeded to find the most appropriate mathematical function (and the optimal value of its parameters) for the widening of the whole eigenvalue spectrum to obtain the Total Density of States (DOS) spectrum. Next, a comparison between the Extended Hückel Method with DFT and ab initio results to analyze the capacity of the former method to reproduce DOS spectra in medium-sized molecules was carried out. The basic conditions under which the results of the three above mentioned methods converge to similar spectra were established. The next step was to examine the concept of "minimal molecular model" (the smallest model representing a macrostructure). lt was shown that the choice of only the invariance of the electronic chemical potential with the growth of the molecular structure is not enough in several cases. lt is necessary to add the analysis of other features, such as the variation of the frontier molecular orbital energies. Moreover, in sorne cases we need to analyze the nature and localization of the molecular orbitals to distinguish between triplet states and metallic behavior. All the above results were applied to propase an hypothesis of why sorne metallic nanotubos are more metallic than others. The most desirable quantum-chemical description of a macrostructure is the one providing us with the possible maximum of correct information. One of these descriptions if the Mulliken Population Analysis which has two problems: it gives occupation numbers greater than 2.0 and it produces negative electronic populations. Given that there is not an agreement about the exact origin we developed an empirical method to correct negative populations. The results are excellent and they open, for the first time, the way to obtain correct numerical values for the Fukui's index and the superdelocalizabilities. The next step was the analysis of sorne global reactivity indices (electronic chemical potential, hardness, softness, global electrophilic inidex). We explored the possibility to build up a set of values for atoms to have a comparison scale allowing us to see their changes when a molecule is formed. This set is basis-dependent. Later, and on the basis of the application of sorne conditions to the Fukui index, we defined two sets of local reactivity indices. The definitions are based on modifications of the original definitions of the global indices. The only way to meas u re the performance of these kinds of indices is to test them against experimental results. The first test was to try to correlate them in a qualitative way with several electrophilic aromatic substitution reactions of sorne monosubstituted benzene derivatives. A positive result is that, in contradiction with other theoretical results, one local reactivity index is not able to explain all the above mentioned reactions. The last step of this Thesis was to include the new local reactivity índices into the KPG model for Quantitative Structure-Affinity Relationship (QSAR) studies. This model relates the variation of the affinity constant in a family of molecular with the variation of several local reactivity índices belonging to a common molecular skeleton. We studied the binding of a of a series of N,N-dialkyl-2-phenylindol-3- ylglyoxylamide derivatives interacting with the peripheral benzodiazepine receptor. We employed DFT and ab initio methods to obtain the electronic structure and the local reactivity indices for these molecules. We obtained for the first time a good QSAR equation at this level of the theory but, unhappily, none of the new local reactivity índices appeared in it. lt is suggested that a full revision of the theory underlying the KPG model is necessary first. The next stage should be the use of these new local índices in other fields of chemistry to find them a different use than to be only a group of molecular descriptors.
The study of carbon, nitrogen and boron macrostructures required the clarification of several aspects of the theory that were taken for granted. First, we proceeded to find the most appropriate mathematical function (and the optimal value of its parameters) for the widening of the whole eigenvalue spectrum to obtain the Total Density of States (DOS) spectrum. Next, a comparison between the Extended Hückel Method with DFT and ab initio results to analyze the capacity of the former method to reproduce DOS spectra in medium-sized molecules was carried out. The basic conditions under which the results of the three above mentioned methods converge to similar spectra were established. The next step was to examine the concept of "minimal molecular model" (the smallest model representing a macrostructure). lt was shown that the choice of only the invariance of the electronic chemical potential with the growth of the molecular structure is not enough in several cases. lt is necessary to add the analysis of other features, such as the variation of the frontier molecular orbital energies. Moreover, in sorne cases we need to analyze the nature and localization of the molecular orbitals to distinguish between triplet states and metallic behavior. All the above results were applied to propase an hypothesis of why sorne metallic nanotubos are more metallic than others. The most desirable quantum-chemical description of a macrostructure is the one providing us with the possible maximum of correct information. One of these descriptions if the Mulliken Population Analysis which has two problems: it gives occupation numbers greater than 2.0 and it produces negative electronic populations. Given that there is not an agreement about the exact origin we developed an empirical method to correct negative populations. The results are excellent and they open, for the first time, the way to obtain correct numerical values for the Fukui's index and the superdelocalizabilities. The next step was the analysis of sorne global reactivity indices (electronic chemical potential, hardness, softness, global electrophilic inidex). We explored the possibility to build up a set of values for atoms to have a comparison scale allowing us to see their changes when a molecule is formed. This set is basis-dependent. Later, and on the basis of the application of sorne conditions to the Fukui index, we defined two sets of local reactivity indices. The definitions are based on modifications of the original definitions of the global indices. The only way to meas u re the performance of these kinds of indices is to test them against experimental results. The first test was to try to correlate them in a qualitative way with several electrophilic aromatic substitution reactions of sorne monosubstituted benzene derivatives. A positive result is that, in contradiction with other theoretical results, one local reactivity index is not able to explain all the above mentioned reactions. The last step of this Thesis was to include the new local reactivity índices into the KPG model for Quantitative Structure-Affinity Relationship (QSAR) studies. This model relates the variation of the affinity constant in a family of molecular with the variation of several local reactivity índices belonging to a common molecular skeleton. We studied the binding of a of a series of N,N-dialkyl-2-phenylindol-3- ylglyoxylamide derivatives interacting with the peripheral benzodiazepine receptor. We employed DFT and ab initio methods to obtain the electronic structure and the local reactivity indices for these molecules. We obtained for the first time a good QSAR equation at this level of the theory but, unhappily, none of the new local reactivity índices appeared in it. lt is suggested that a full revision of the theory underlying the KPG model is necessary first. The next stage should be the use of these new local índices in other fields of chemistry to find them a different use than to be only a group of molecular descriptors.
Notas
Tesis (Doctor en Fisicoquímica Molecular)
Palabras clave
Compuestos de Carbono, Boro, Nitrógeno, Fisicoquímica Molecular