Dinámica de la reacción F + D2(JI) (flecha) DF(vf,jf) + D en un experimento de dispersión de haces moleculares cruzados de alta resolución

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Física Atómica, Molecular y Nuclear, leída el 19-06-1996La dinámica de la reacción f+d2(ji) df(vf.jf)+d ha sido investigada en un experimento de dispersión de haces moleculares cruzados de alta resolución. Se han medido espectros de tiempo de vuelo de los productos df(vf.jf) dispersados en una amplia región angular a cinco energías de colisión en el intervalo ecm=90-240 mev. La resolución en energía de los espectros, de hasta 20 mev, permite la extracción secciones eficaces reactivas diferenciales e integrales en valor absoluto para los distintos estados vibracionales y rotacionales finales de los productos. La sección eficaz integral total de reaccion f+d2 crece monótonamente con la energía de colisión en todo el intervalo investigado, donde tiene un valor absoluto entre 1.5 a2 y 3.0 a2. La sección eficaz diferencial reactiva toma, en general, su maximo valor en los ángulos de dispersión en el sistema de referencia de centro de masas, 0cm, grandes. La sección eficaz diferencial de todos los productos se desplaza rápidamente con la energía de colisión hacia ángulos 0cm menores. La sección eficaz para los productos df (vf=4) se extiende en todo el intervalo de ángulos de dispersión con un valor apreciable y presenta un maximo en 0cm=0 cuya magnitud aumenta con la energía de colisión llegando a constituir el maximo absoluto de la distribucion angular de este estado vibracional. La energía de colisión favorece la excitación rotacional de todos los productos vibracionales, siendo aquellos dispersados en los ángulos 0cm intermedios los mas altamente excitados. En particular, la distribucion rotacional de los productos df(vf=2) y df(vf=3) presentan una pronunciada estructura bimodal a las energías de colisión mas bajas investigadas. La influencia de la rotación de los reactivos en la dinámica de la reaccion f+d2(ji) se ha investigado en experimentos de dispersión adicionales en los que se han variado las poblaciones rotacionales iniciales del d2. La presencia de rotación inicial resulta tener un efecto en general beneficioso en la reactividad, selectivo en estados finales y ángulos de dispersión. Se han realizado cálculos dinámicos cuanticos y cuasiclasicos en las superficies de energía potencial mas recientes para el sistema fd2, los cuales reproducen en general satisfactoriamente las principales características de las secciones eficaces diferenciales e integrales reactivas experimentales. Sin embargo, se observan también discrepancias significativas tanto a nivel cuantitativo como cualitativo.Depto. de Estructura de la Materia, Física Térmica y ElectrónicaFac. de Ciencias FísicasTRUEpu

Liquid crystal behavior of the Kihara fluid

The liquid crystal phases of the Kihara fluid have been studied in computer simulations. The work focuses on the isotropic–nematic–smectic-A triple point region, especially relevant for the understanding of the properties and the design of real mesogens with specific phase diagrams. The Kihara interaction resembles more appropriately than other related models, the shape of elongated polymers and biomolecules, and a closer assertion is provided for the role of the configurational entropy and the dispersive interactions in the behavior of such molecules in dense phases or under macromolecular crowding conditions.Dirección Genaral de Investigación Científico y Técnica BQU2001-3615-C02Instituto de Salud Carlos III 01/1664Plan Andaluz de Investigación FQM-205, FQM-31

A novel orientation-dependent potential model for prolate mesogens

An intermolecular potential is introduced for the study of molecular mesogenic fluids. The model combines distinct features of the well-known Gay-Berne and Kihara potentials by incorporating dispersive interactions dependent on the relative pair orientation to a spherocylinder molecular core. Results of a Monte Carlo simulation study focused on the liquid crystal phases exhibited by the model fluid are presented. For the chosen potential parameters, molecular aspect ratio L*55 and temperatures T*52, 3, and 5, isotropic, nematic, smectic-A, and hexatic phases are found. The location of the phase boundaries as well as the equation of state of the fluid and further thermodynamical and structural parameters are discussed and contrasted to the Kihara fluid. In comparison to this latter fluid, the model induces the formation of ordered liquid crystalline phases at lower packing fractions and it favors, in particular, the appearance of layered hexatic ordering as a consequence of the greater attractive interaction assigned to the parallel side-to-side molecular pair configurations. The results contribute to the evaluation of the role of specific interaction energies in the mesogenic behavior of prolate molecular liquids in dense environments

Use of Parsons-Lee and Onsager theories to predict nematic and demixing behavior in binary mixtures of hard rods and hard spheres

Parsons-Lee and Onsager theories are formulated for the isotropic-nematic transition in a binary mixture of hard rods and hard spheres. Results for the phase coexistence and for the equation of state in both phases for mixtures with different relative sizes and composition are presented. The two theories explain correctly the general behavior observed in experiments and computer simulations for these fluids. In particular, the theory accounts for the destabilization of the nematic phase when spherical or globular macromolecules are added to a system of rodlike colloids, and the entrance of the system into a demixed regime at high volume fractions of the spherical particles. Upon demixing a nematic state rich in rods coexists in equilibrium with an isotropic state much more diluted in the rodlike component. Onsager theory fails on quantitative grounds for aspect ratios of the rodlike molecules smaller than 100, and in the cases where the molar fractions of spheres becomes close to unity. On the contrary, the Parsons-Lee approximation remains accurate down to aspect ratios as small as 5. The spinodal analysis indicates that the isotropic-isotropic and nematic-nematic coexistences become feasible for sufficiently large spheres and long rods, respectively. The latter type of coexistence interferes partially with the isotropic-nematic coexistence regime of interest to the present work. Overall, the study serves to rationalize and control key aspects of the behavior of these binary nematogenic colloidal systems, which can be tuned with an appropriate choice of the relative size and molar fractions of the particles.Ministerio de Educación y Ciencia CTQ2004- 07730-C02 VEM2003-20574-C03Junta de Andalucía PAI FQM-205 FQM-31

Monte Carlo study of liquid crystal phases of hard and soft spherocylinders

We report on a Monte Carlo study of the liquid crystal phases of two model fluids of linear elongated molecules: ~a! hard spherocylinders with an attractive square-well ~SWSC! and ~b! purely repulsive soft spherocylinders ~SRS!, in both cases for a length-to-breadth ratio L*55. Monte Carlo simulations in the isothermal–isobaric ensemble have been performed at a reduced temperature T*55 probing thermodynamic states within the isotropic ~I!, nematic ~N!, and smectic A ~Sm A! regions exhibited by each of the models. In addition, the performance of an entropy criterion to allocate liquid crystalline phase boundaries, recently proposed for the isotropic–nematic transition of the hard spherocylinder ~HSC! fluid, is successfully tested for the SWSC and the SRS fluids and furthermore extended to the study of the nematic–smectic transition. With respect to the more extensively studied HSC fluid, the introduction of the attractive square well in the SWSC model brings the I–N and N–Sm A transitions to higher pressures and densities. Moreover, the soft repulsive core of the SRS fluid induces a similar but quite more significant shift of both of these phase boundaries toward higher densities. This latter effect is apparently in contrast with very recent studies of the SRS fluid at lower temperatures, but this discrepancy can be traced back to the different effective size of the molecular repulsive core at different temperatures

A Cl− hinge for cyclen macrocycles: Ionic Interactions and tweezer-like complexes

The supramolecular networks derived from the complexation of polyazamacrocycles with halide anions constitute fundamental building blocks of a broad range of modern materials. This study provides insights into the conformational framework that supports the binding of protonated cyclen macrocyles (1,4,7,10-Tetraazacyclododecane) by chloride anions through NH δ+ ···Cl - interactions. The isolated complex comprised of two cyclen hosts linked by one Cl - anion is characterized by means of infrared action spectroscopy and ion mobility mass spectrometry, in combination with quantum chemical computations. The Cl - anion is found to act as a hinge that bridges the protonated NH2+ moieties of the two macrocycles leading to a molecular tweezer configuration. Different types of conformations emerge, depending on whether the trimer adopts an open arrangement, with significant freedom for internal rotation of the cyclen moieties, or it locks in a folded conformation with intermolecular H-bonds between the two cyclen backbones. The ion mobility collision cross section supports that folded configurations of the complex are dominant under isolated conditions in the gas phase. The IRMPD spectroscopy experiments suggest that two qualitatively different families of folded conformations coexist at room temperature, featuring either peripheral or inner positions of the anion with respect to the macrocycle cavities, These findings should have implications in the growth of extended networks in the nanoscale and in sensing applicationsThe research leading to this result has been supported by the project CALIPSOplus under the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020. This study is part of project P12-FQM-4938 of the research programme of Junta de Andalucia and FEDER

The dynamics of the hydrogen exchange reaction at 2.20 eV collision energy: Comparison of experimental and theoretical differential cross sections

The H+D2(v = 0, j = 0)→HD(v′, j′)+D isotopic variant of the hydrogen atom exchange reaction has been studied in a crossed molecular beam experiment at a collision energy of 2.20 eV. Kinetic energy spectra of the nascent D atoms were obtained by using the Rydberg atom time-of-flight technique. The extensive set of spectra collected has permitted the derivation of rovibrationally state-resolved differential cross sections in the center-of-mass frame for most of the internal states of the HD product molecules, allowing a direct comparison with theoretical predictions. Accurate 3D quantum mechanical calculations have been carried out on the refined version of the latest Boothroyd-Keogh-Martin-Peterson potential energy surface, yielding an excellent agreement with the experimentally determined differential cross sections. The comparison of the results from quasi-classical trajectory calculations on the same potential surface reveals some discrepancies with the measured data, but shows a good global accordance. The theoretical calculations demonstrate that, at this energy, reactive encounters are predominantly noncollinear and that collinear collisions lead mostly to nonreactive recrossing. The experimental results are satisfactorily accounted for by theoretical calculations without consideration of Geometric Phase effects. © 1999 American Institute of Physics.Financial support through the program ‘‘Acciones para la Incorporación de Doctores y Tecnólogos’’ of the Ministry of Education and Culture of Spain. We also gratefully acknowledge the computer resources ~Cray T-90! provided by the Leibniz Rechenzentrum in Munich ~Germany. Funded by the German Science Foundation ~Grant No. SCH 435/3! and the Deutscher Akademischer Austauchdienst‘‘Acciones Integradas’’. The Spanish part was financed by the DGICYT ~PB95-0918-C03! and by the ‘‘Acciones Integradas’’ Program of the Ministry of Educationand Culture.Peer Reviewe

High-resolution mass spectrometry identification of dye compounds and their degradation products in American cochineal from a historic shipping cargo

Cochineal dyes constitute paradigmatic organic compounds that guide in the elucidation of historic cultural and economical exchanges. This study combines liquid chromatography and high-resolution tandem mass spectrometry to assess the degradation products of the organic dyes of American cochineal collected from the cargo of a 16th century sunken wreck. The identification of biological materials of historical and archeological origin is often challenging due to molecular degradation. In this scenario, the mapping of the chemical routes underlying the fate of the organic and biochemical compounds employed as taxonomic biomarkers becomes of crucial importance. This work shows that, under harsh environmental conditions, the original carminic and (flavo) kermesic acid structures of cochineal dyes are prone to chemical transformation, typically losing the carboxylic acid and glucoside groups. The anthraquinone core is preserved in the majority of degradation products identified in this study, with eventual side chains reminiscent of a partial degradation of the glucoside moiety. The comparison of these observations with the analysis of modern cochineal samples allows us to lay out an updated chart of dye compounds and degradation products which should constitute a seminal benchmark for future biomolecular analysis of historic and archeological dye materials.Instituto Andaluz del Patrimonio Históric

The H+D2 reaction in the vicinity of the conical intersection

Scattering measurements performed at an energy slightly higher than that of the lowest crossing between the ground and the first electronically excited state of the H3 system are reported. The essentials of the reactive dynamics are describable in terms of a classical motion of the nuclei on the lowest adiabatic electronic potential. © 1997 American Institute of Physics.The German part of this work was supported by the German Science Foundation. The Spanish part was funded by the DGICYT of Spain under Grant PB95-0918-C02. Both parts are grateful to the German–Spanish scientific exchange program ‘‘Acciones Integradas.’’ Partial funding by the Alexander von Humboldt StiftungPeer Reviewe