Catalytic tuning of phosphinoethane ligand. An improved catalyst for hydrocarbon

Report for the scientific sojourn carried out at the Department of Chemistry University of North Texas (USA) from September until November 2006. It includes the performance of two computational chemistry studies: an experimental and computational study toward the intra- and intermolecular hydroarylation of isonitriles and the development of an improved catalyst for hydrocarbon functionalization.

Mecanismes de reacció de sistemes catalítics amb èmfasi en carbens de Fischer i quantificació de l'aromaticitat en petites molècules inorgàniques cícliques i planes.

El tema principal del treball és sobre l'estudi teòric de l'estructura i reactivitat en carbens de Fischer de la forma (CO)5Cr=C(X)R (X= OH, NH2, OMe, NMe2 i R= CHCH2 i Ph). Particularment, el nostre interès va sorgir del tipus de reaccions de cicloadicció que donen lloc a la síntesi de productes naturals i fàrmacs de gran valor afegit. Hem estudiat els mecanismes de reacció dels casos més comuns de cicloanul•lació: la reacció de benzanul•lació de Dötz i ciclopentanulacions que es troben en competència amb el primer cas, derivats de l'inserció de acetilè i fenilacetilè. En l'últim pas de les reaccions que comporten la formació d'un sistema de més d'un anell, hi tenim una migració del complex metàl•lic de crom d'un extrem a un altre anomenat com rearranjament haptotròpic. Aleshores, hem investigat sobre els mecanismes de migració haptotròpica de Cr(CO)3 sobre hidrocarburs aromàtics policíclics analitzant l'efecte de la mida i la curvatura del sistema així com la complexació d'un segon fragment metàl•lic a la manera de coordinació als anells. D'una altre banda, vam estudiar l'aromaticitat en 54 cúmuls cíclics de molecules inorgàniques mitjançant metodologia desenvolupada al nostre grup de recerca i altres. Vam proposar la tècnica del scan-NICS com nova mesura quantitativa d'aromaticitat i reportar l'escassa correlació entre els distints índexs d'aromaticitat a la literatura. Finalment, com resultat de col•laboracions en estades de recerca, he desenvolupat propostes de mecanismes de reacció en sistemes catalítics de isonitrils i fosfinetà-amides i en dímers de gassos nobles.

The nature of the Cr-C bond: direct estimation of aromaticity in Fischer carbenes

Report for the scientific sojourn at the the Philipps-Universität Marburg, Germany, from september to december 2007. For the first, we employed the Energy-Decomposition Analysis (EDA) to investigate aromaticity on Fischer carbenes as it is related through all the reaction mechanisms studied in my PhD thesis. This powerful tool, compared with other well-known aromaticity indices in the literature like NICS, is useful not only for quantitative results but also to measure the degree of conjugation or hyperconjugation in molecules. Our results showed for the annelated benzenoid systems studied here, that electron density is more concentrated on the outer rings than in the central one. The strain-induced bond localization plays a major role as a driven force to keep the more substituted ring as the less aromatic. The discussion presented in this work was contrasted at different levels of theory to calibrate the method and ensure the consistency of our results. We think these conclusions can also be extended to arene chemistry for explaining aromaticity and regioselectivity reactions found in those systems.In the second work, we have employed the Turbomole program package and density-functionals of the best performance in the state of art, to explore reaction mechanisms in the noble gas chemistry. Particularly, we were interested in compounds of the form H--Ng--Ng--F (where Ng (Noble Gas) = Ar, Kr and Xe) and we investigated the relative stability of these species. Our quantum chemical calculations predict that the dixenon compound HXeXeF has an activation barrier for decomposition of 11 kcal/mol which should be large enough to identify the molecule in a low-temperature matrix. The other noble gases present lower activation barriers and therefore are more labile and difficult to be observable systems experimentally.

Neutral atoms in ionic lattices: Excited states of KCl:Ag(0)

The optical-absorption spectrum of a cationic Ag0 atom in a KCl crystal has been studied theoretically by means of a series of cluster models of increasing size. Excitation energies have been determined by means of a multiconfigurational self-consistent field procedure followed by a second-order perturbation correlation treatment. Moreover results obtained within the density-functional framework are also reported. The calculations confirm the assignment of bands I and IV to transitions of the Ag-5s electron into delocalized states with mainly K-4s,4p character. Bands II and III have been assigned to internal transitions on the Ag atom, which correspond to the atomic Ag-4d to Ag-5s transition. We also determine the lowest charge transfer (CT) excitation energy and confirm the assignment of band VI to such a transition. The study of the variation of the CT excitation energy with the Ag-Cl distance R gives additional support to a large displacement of the Cl ions due to the presence of the Ag0 impurity. Moreover, from the present results, it is predicted that on passing to NaCl:Ag0 the CT onset would be out of the optical range while the 5s-5p transition would undergo a redshift of 0.3 eV. These conclusions, which underline the different character of involved orbitals, are consistent with experimental findings. The existence of a CT transition in the optical range for an atom inside an ionic host is explained by a simple model, which also accounts for the differences with the more common 3d systems. The present study sheds also some light on the R dependence of the s2-sp transitions due to s2 ions like Tl+.

Ground- and excited-state properties of M-center oxygen vacancy aggregates in the bulk and the surface of MgO

Aggregates of oxygen vacancies (F centers) represent a particular form of point defects in ionic crystals. In this study we have considered the combination of two oxygen vacancies, the M center, in the bulk and on the surface of MgO by means of cluster model calculations. Both neutral and charged forms of the defect M and M+ have been taken into account. The ground state of the M center is characterized by the presence of two doubly occupied impurity levels in the gap of the material; in M+ centers the highest level is singly occupied. For the ground-state properties we used a gradient corrected density functional theory approach. The dipole-allowed singlet-to-singlet and doublet-to-doublet electronic transitions have been determined by means of explicitly correlated multireference second-order perturbation theory calculations. These have been compared with optical transitions determined with the time-dependent density functional theory formalism. The results show that bulk M and M+ centers give rise to intense absorptions at about 4.4 and 4.0 eV, respectively. Another less intense transition at 1.3 eV has also been found for the M+ center. On the surface the transitions occur at 1.6 eV (M+) and 2 eV (M). The results are compared with recently reported electron energy loss spectroscopy spectra on MgO thin films.

Aggregation under a forced convective flow

Morphological transitions are analyzed for a radial multiparticle diffusion-limited aggregation process grown under a convective drift. The introduction of a tangential flow changes the morphology of the diffusion-limited structure, into multiarm structures, inclined opposite to the flow, whose limit consists of single arms, when decreasing density. The case of shear flow is also considered. The anisotropy of the patterns is characterized in terms of a tangential correlation function based analysis. Comparison between the simulation results and preliminary experimental results has been done.

Theoretical modeling of photon and electron-stimulated Na and K desorption from SiO2

The mechanism of generation of atomic Na and K from SiO2 samples has been studied using explicitly correlated wave function and density functional theory cluster calculations. Possible pathways for the photon and electron stimulated desorption of Na and K atoms from silicates are proposed, thus providing new insight on the generation of the tenuous Na and K atmosphere of the Moon.

Regioselectividad en fullerenos, una visión computacional

Fullerenes are wonderful molecules that exhibit a wealth of interesting properties, which promises a major role for this family in the future in the (bio)medicine and nanotechnology fields. However, there are still many unknowns such as for instance the reactivity of these molecules and how this is affected by metals. These aspects are difficult to apprehend by experiments alone, for which reason there is a widespread interest in the description and simulation of them by theoretical chemistry. Here we briefly describe our results from the past five years and their future impact

Metodologia a seguir per la reconstrucció d’imatges mèdiques i la seva transformació en geometries mallades per a l’ús computacional

Estudi elaborat a partir d’una estada a l'Imperial College of London, Gran Bretanya, entre setembre i desembre 2006. Disposar d'una geometria bona i ben definida és essencial per a poder resoldre eficientment molts dels models computacionals i poder obtenir uns resultats comparables a la realitat del problema. La reconstrucció d'imatges mèdiques permet transformar les imatges obtingudes amb tècniques de captació a geometries en formats de dades numèriques . En aquest text s'explica de forma qualitativa les diverses etapes que formen el procés de reconstrucció d'imatges mèdiques fins a finalment obtenir una malla triangular per a poder‐la processar en els algoritmes de càlcul. Aquest procés s'inicia a l'escàner MRI de The Royal Brompton Hospital de Londres del que s'obtenen imatges per a després poder‐les processar amb les eines CONGEN10 i SURFGEN per a un entorn MATLAB. Aquestes eines les han desenvolupat investigadors del Bioflow group del departament d'enginyeria aeronàutica del Imperial College of London i en l'ultim apartat del text es comenta un exemple d'una artèria que entra com a imatge mèdica i surt com a malla triangular processable amb qualsevol programari o algoritme que treballi amb malles.

Caracterización química y métodos de prevención de los precipitados que se forman en el exterior del chorizo

El objetivo general de este proyecto consiste en la mejora del proceso de maduración, envasado y comercialización de chorizo para evitar la formación de cristales en la superficie, que son rechazados por el consumidor. Para lograr este objetivo se planteó conocer la composición de los cristales más frecuentes en chorizos y la evaluación de medidas preventivas para frenar su aparición.

Variabilitat temporal i geogràfica i caracterització química de la contaminació atmosfèrica particulada a Sabadell

Estudi de 3 punts amb diferent intensitat de trànsit a Sabadell: punt de trànsit, fons urbà i fons suburbà. Nivells de PM i altres gasos contaminants van ser mesurats durant 1 mes. Els objectius principals són la correlació dels nivells de partícules i NOx als 3 punts d’estudi (ja que als estudis epidemiològics s’utilitza NO2 com a indicador del nivell de partícules); la caracterització química de les partícules per determinar quina porció té origen en les emissions dels tubs d’escapament i identificar similituds i diferències entre els nivells i composició de partícules entre les diferents estacions seleccionades.