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Publicación A comprehensive study of the molecular vibrations in solid-state benzylic amide [2]catenane(Royal Society of Chemistry, 2019) Romero Muñiz, Carlos; Paredes Roibás, Denís; Hernanz, Antonio; Gavira Vallejo, José MaríaThe interpretation of vibrational spectra is often complex but a detailed knowledge of the normal modes responsible for the experimental bands provides valuable information about the molecular structure of the sample. In this work we record and assign in detail the infrared (IR) spectrum of the benzylic amide [2]catenane, a complex molecular solid displaying crimped mechanical bonds like the links of a chain. In spite of the large size of the unit cell, we calculate all the vibrational modes of the catenane crystal using quantum first-principles calculations. The activity of each mode is also evaluated using the Born effective charges approach and a theoretical spectrum is constructed for comparison purposes. We find a remarkable agreement between the calculations and the experimental results without the need to apply any further empirical correction or fitting to the eigenfrequencies. A detailed description in terms of the usual internal coordinates is provided for over 1000 normal modes. This thorough analysis allows us to perform the complete assignment of the spectrum, revealing the nature of the most active modes responsible for the IR features. Finally, we compare the obtained results with those of Raman spectroscopy, studying the effects of the rule of mutual exclusion in vibrational spectroscopy according to the different levels of molecular symmetry embedded in this mechanically interlocked molecular compound.Publicación Assignment of the Raman Spectrum of Benzylic Amide [2]Catenane: Raman Microscopy Experiments and First-Principles Calculations(American Chemical Society, 2018) Romero Muñiz, Carlos; Paredes Roibás, Denís; Hernanz, Antonio; López García, Concepción; Gavira Vallejo, José MaríaIn this work, we use Raman spectroscopy and quantum first-principles calculations to unveil the experimental spectrum of a complex molecular solid-like benzylic amide [2]catenane, a representative example of a mechanically interlocked molecular architecture. We use large-scale density functional theory calculations to obtain the complete set of vibrational normal modes of the catenane crystal, whose unit cell contains 544 atoms. Subsequently, we demonstrate that these calculations are able to accurately reproduce the experimental Raman spectrum of this molecular compound, without introducing any empirical corrections or fittings in the calculated eigenfrequencies. Thanks to the good agreement between the experimental and theoretical spectra, it is possible to carry out the complete assignment of the main vibrational modes responsible for the whole spectrum. A detailed description in terms of the usual internal coordinates is given for all of these representative modes. This description, rather difficult from the experimental point of view, provides valuable information about the molecular structure of this compound, compatible with experimental evidences reported in the literature.Publicación Can gas hydrate structures be described using classical simulations?(American Institute of Physics (AIP), 2010-02-13) Conde, M. M.; Vega, Carlos; Noya, Eva G.; Ramírez, RafaelQuantum path-integral simulations of the hydrate solid structures have been performed using the recently proposed TIP4PQ/2005 model. By also performing classical simulations using this model, the impact of the nuclear quantum effects on the hydrates is highlighted; nuclear quantum effects significantly modify the structure, densities, and energies of the hydrates, leading to the conclusion that nuclear quantum effects are important not only when studying the solid phases of water but also when studying the hydrates. To analyze the validity of a classical description of hydrates, a comparison of the results of the TIP4P/2005 model (optimized for classical simulations) with those of TIP4PQ/2005 (optimized for path-integral simulations) was undertaken. A classical description of hydrates is able to correctly predict the densities at temperatures above 150 K and the relative stabilities between the hydrates and iceIh. The inclusion of nuclear quantum effects does not significantly modify the sequence of phases found in the phase diagram of water at negative pressures, namely, Ih→sII→sH. In fact the transition pressures are little affected by the inclusion of nuclear quantum effects; the phase diagram predictions for hydrates can be performed with reasonable accuracy using classical simulations. However, for a reliable calculation of the densities below 150 K, the sublimation energies, the constant pressure heat capacity, and the radial distribution functions, the incorporation of nuclear quantum effects is indeed required.Publicación Cavity Ring-Down Absorption Spectroscopy: Optical Characterization of ICl Product in Photodissociation of CH2ICl at 248 nm(American Chemical Society, 2018-10-02) Paredes Roibás, Denís; Balaganesh, Muthiah; Kasai, Toshio; Lin, King Chuen; Gavira Vallejo, José MaríaPublicación Dielectric and Molecular Dynamics Study of the Secondary Relaxations of Poly(styrene-co-methylmethacrylate) Copolymers: Influence of the Molecular Architecture(EDP Sciences ; Springer Verlag (Germany) ; Società Italiana di Fisica, 2011-11-07) Encinar, M.; Prolongo, M. G.The effect of the structure of copolymers (random, alternate or diblock) on their dynamics has been studied by dielectric spectroscopy. Six copolymers of styrene and methyl methacrylate (three diblocks, one alternate and two random) have been studied. The results show that the sub-Tg transitions of the diblock samples can be described by one asymmetric Havriliak-Negami (HN) function, while two are necessary for the rest of the copolymers (β and γ relaxations). The characteristic times of the sub-Tg relaxations show an Arrhenius temperature dependence and there is a strong coupling of the α and β relaxations at high temperatures. The deconvolution of the merging relaxations has been made in the framework of the Williams Ansatz set out in terms of Havriliak-Negami distributions. The γ relaxation may be assigned to the rotation of the methyl methacrylate group in a styrene-rich environment. The Molecular Dynamics simulations of a poly(methyl methacrylate) homopolymer and of the alternate copolymer are in qualitative agreement with the experimental results, although they predict smaller values for the activation energy of the sub-Tg relaxations.Publicación Fibrillar morphologies of hydrogels obtained from a lamellar medium(Elsevier, 2011-10-17) Renamayor, Carmen S.; Esteban Pacios, María IsabelHydrogels are suitable for multiple applications and their properties are strongly dependent on their morphology. Sponge-like morphologies are obtained in conventional hydrogels when the polymerization is performed in isotropic media. Fibrillar morphologies imparting new properties to hydrogels are expected when the reacting medium is anisotropic. Here, we synthesize such fibrillar hydrogels by polymerization in a lamellar medium formed by 1,4- bis(2-ethylhexyl)sodium sulfosuccinate (AOT) and water. At high surfactant content this objective is achieved, and the thickness of the obtained fibrils (0.1-1 μm) can be correlated with the physical properties of the lamellar system. At intermediate AOT concentrations, the morphology is hierarchical, with primary closed pores containing a secondary fibrillar structure that fills the pores. For low AOT concentrations, the crosslinking process is unhindered, and the hydrogels are mechanically consistent with a sponge-like morphology having large (10-102 μm) void pores. The time evolution of the mesophase as polymerization advances is followed by small angle X-ray scattering.Publicación Fractionation of polyacrylamide in lamellar mesopashes(2014-05-16) Agzenai Ben Salem, Yahya; Renamayor, Carmen S.; Esteban Pacios, María IsabelPublicación Heat capacity of water : a signature of nuclear quantum effects(American Institute of Physics (AIP), 2010-01-03) Vega, Carlos; Conde, M. M.In this note we present results for the heat capacity at constant pressure for the TIP4PQ/2005 model, as obtained from path-integral simulations. The model does a rather good job of describing both the heat capacity of iceIh and of liquid water. Classical simulations using the TIP4P/2005, TIP3P, TIP4P, TIP4P-Ew, simple point charge/extended, and TIP5P models are unable to reproduce the heat capacity of water. Given that classical simulations do not satisfy the third law of thermodynamics, one would expect such a failure at low temperatures. However, it seems that for water, nuclear quantum effects influence the heat capacities all the way up to room temperature. The failure of classical simulations to reproduce Cp points to the necessity of incorporating nuclear quantum effects to describe this property accurately.Publicación Impact of Small Adsorbates in the Vibrational Spectra of Mg- and Zn-MOF-74 Revealed by First-Principles Calculations(American Chemical Society, 2020) Romero Muñiz, Carlos; Merkling, Patrick; Calero, Sofía; Gavira Vallejo, José MaríaIn this work, we analyze the influence of small adsorbates on the vibrational spectra of Mg- and Zn-metal–organic framework MOF-74 by means of first-principles calculations. In particular, we consider the adsorption of four representative species of different interaction strengths: Ar, CO2, H2O, and NH3. Apart from a comprehensive characterization of the structural and energetic aspects of empty and loaded MOFs, we use a fully quantum ab initio approach to evaluate the Raman and IR activities of the normal modes, leading to the construction of the whole vibrational spectra. Under this approach, not only are we able to proceed with the complete assignment of the spectra in terms of the usual internal coordinates but also we can discern the most relevant vibrational fingerprints of the adsorbates and their impact on the whole MOF spectra. On the one hand, some of the typical vibrational modes of the small molecules are slightly shifted but still visible when adsorbed on the MOFs, especially those appearing at high wavenumbers where the empty MOFs lack IR/Raman signals. On the other hand, some bands arising from the organic ligands are affected by the presence of the absorbates, displaying non-negligible frequency shifts, in agreement with recent experiments. We find a strong correlation between all of these frequency shifts and the interaction strength of the adsorbate with the hosting framework. The findings presented in this work expand the capabilities of vibrational spectroscopy techniques to analyze porous materials and can be useful for the design of sensors and new devices based on MOF technology.Publicación Micro Raman Spectroscopy of Epipalaeolithic Decorated Pebbles from Arroyo Moreras 2 (Parque Darwin, Madrid)(The Royal Society of Chemistry, 2019-11) Hernanz, Antonio; Bueno Ramírez, Primitiva; Balbín Behrmann, Rodrigo de; Morín de Pablos, Jorge; Juana Ortín, Cristina de; Gavira Vallejo, José MaríaPigment vestiges on the three decorated pebbles from the excavation of Parque Darwin (Madrid), level 8470 ± 70 BP have been identified by Micro Raman Spectroscopy. Haematite and amorphous carbon were used as pigments. Microscopic contamination spots with ε-copper-phthalocyanine blue, have been detected. The analysis of the pigments used, together with their previously unknown chronology and open-air location, constitute a new point of reference for the technical and cultural uses of symbology among hunter-gatherers in southern Europe.Publicación Micro-Raman spectroscopy of rock paintings from the Galb Budarga and Tuama Budarga rock shelters, Western Sahara(Elsevier, 2018-03) Iriarte, Mercedes; Hernanz, Antonio; Sáenz de Buruaga, Andoni; Gavira Vallejo, José María; Martín Fernández, SantiagoRock paintings of two recent discovered rock shelters, Galb Budarga and Tuama Budarga, from the southeastern area of the Western Sahara, Sahrawi Arab Democratic Republic, have been studied by micro-Raman spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectrometry in order to characterize the composition of the materials present in the painting panels. An unusual white pigment has been used in the zoomorphic pictographs of the Galb Budarga shelter which main components are the anhydrite (CaSO4) polymorphs I and II. Red and orange zoomorphic figures and ancient Berber scripts have been painted in the Tuama Budarga rock shelter. Haematite (α-Fe2O3) is the main component of the paints used; amorphous carbon and different manganese oxides have also been detected. Accretions of gypsum (CaSO4·2H2O) and anhydrite have been observed on the shelter wall used to paint. α-Quartz (α-SiO2), albite (NaAlSi3O8), dolomite, CaMg(CO3)2, calcite (CaCO3) and traces of hydroxylapatite (Ca10(PO4)6(OH)2) have been identified in the rocks supporting the paintings of both sites. Layers of calcium oxalates, whewellite (CaC2O4·H2O) and weddellite (CaC2O4·(2 + x)H2O, x ≤ 0.5) cover the pictorial panels of these rock shelters. A microestratrigraphic study of the paint used in the Tuama Budarga shelter revealed that the pigment layer is bracketed between oxalate layers.Publicación Non-stoichiometric polymer-surfactant complexes obtained in a lamellar lyotropic medium(Springer Verlag (Germany), 2012-03-08) Agzenai Ben Salem, Yahya; Renamayor, Carmen S.; Esteban Pacios, María IsabelThe addition of a polyelectrolyte to lamellar media formed by an oppositely charged surfactant often leads to the coexistence of several phases without macroscopic phase separation, which makes their characterization difficult. Here, the effect of the polydiallyldimethylammonium chloride (PD) on the lamellar liquid crystal formed by the anionic surfactant Aerosol OT (AOT) and water is investigated. SAXS results are discussed regarding the changes in the lamellar spacing as a function on the PD or AOT concentrations. In most of the samples two lamellar phases, without macroscopic phase separation, are detected. One of them is a typical swollen phase, while the other is a collapsed phase, which corresponds to the polymer-surfactant complex. At concentrations of polymer up to 3 %wt the two lamellar phases coexist, however, at a critical concentration higher than 3 %wt, the swollen phase becomes isotropic, and a macroscopic phase separation takes place. A simple model is proposed to calculate the composition of the phases when macroscopic phase separation does not occur. The results thus calculated show that generally the polymer-surfactant complexes are non-stoichiometric containing a lesser amount of polymer than ideally expected.Publicación On the accurate direct computation of the isothermal compressibility for normal quantum simple fluids : application to quantum hard spheres(American Institute of Physics (AIP), 2012-15-30) Sesé, Luis M.A systematic study of the direct computation of the isothermal compressibility of normal quantum fluids is presented by analyzing the solving of the Ornstein-Zernike integral equation (OZ2) for the pair correlations between the path-integral necklace centroids. A number of issues related to the accuracy that can be achieved via this sort of procedure have been addressed, paying particular attention to the finite-N effects and to the definition of significant error bars for the estimates of isothermal compressibilities. Extensive path-integral Monte Carlo computations for the quantum hard-sphere fluid (QHS) have been performed in the (N,V,T) ensemble under temperature and density conditions for which dispersion effects dominate the quantum behavior. These computations have served to obtain the centroid correlations, which have been processed further via the numerical solving of the OZ2 equation. To do so, Baxter-Dixon-Hutchinson’s variational procedure complemented with Baumketner-Hiwatari’s grand-canonical corrections have been used. The virial equation of state has also been obtained and several comparisons between different versions of the QHS equation of state have been made. The results show the reliability of the procedure based on isothermal compressibilities discussed herein, which can then be regarded as a useful and quick means of obtaining the equation of state for fluids under quantum conditions involving strong repulsive interactions.Publicación Path integral Monte Carlo simulations for rigid rotors and their application to water(Taylor and Francis, 2010-09-23) Noya, Eva G.; Sesé, Luis M.; Ramírez, Rafael; McBride, Carl; Conde, M. M.; Vega, CarlosIn this work the path integral formulation for rigid rotors, proposed by M¨user and Berne [Phys. Rev. Lett. 77, 2638 (1996)], is described in detail. It is shown how this formulation can be used to perform Monte Carlo simulations of water. Our numerical results show that whereas some properties of water can be accurately reproduced using classical simulations with an empirical potential which, implicitly, includes quantum effects, other properties can only be described quantitatively when quantum effects are explicitly incorporated. In particular, quantum effects are extremely relevant when it comes to describing the equation of state of the ice phases at low temperatures, the structure of the ices at low temperatures, and the heat capacity of both liquid water and the ice phases. They also play a minor role in the relative stability of the ice phases.Publicación Path integral Monte Carlo study of quantum-hard sphere solids(American Institute of Physics (AIP), 2013-06-28) Sesé, Luis M.A path integral study of the fcc, hcp, and bcc quantum hard-sphere solids is presented. Ranges of densities within the interval of reduced de Broglie wavelengths 0.2 * 0.8 B ≤ λ ≤ have been analyzed using Monte Carlo simulations with Cao-Berne propagator. Energies, pressures, and structural quantities (pair radial correlation functions, centroid structure factors, and Steinhardt order parameters) have been computed. Also, applications of the Einstein crystal technique (J. Chem. Phys. 126, 164508 (2007)) have been made to compute the free energies of the fcc and hcp solids. Some technical points related to the latter technique are discussed, and it is shown that these calculations produce consistent results with increasing sample sizes. The fluid-solid (fcc and hcp) equilibria have been studied, thus completing prior work by this author on the fluid-fcc equilibrium. Within the accuracy attained no significant differences between the relative stabilities of the fcc and hcp lattices have been detected. The bcc case stands apart from the other two lattices, as the simulations lead either to irregular lattices (two types) that keep some traces of bcc-memory, or to spontaneous transitions to hcp-like lattices. The latter transitions make manifestly clear the potential repercussions that the quantum hard-sphere behavior can have on solid-solid equilibria at low temperatures in real systems (e.g. helium).Publicación Path-integral Monte Carlo and static structures in condensed matter(2012-06-08) Sesé, Luis M.Conferencia presentada en el Congreso: PIMD School (CECAM), Toulouse (France), 8 junio 2012Publicación Rock Art(The Royal Society of Chemistry, 2021-06-04) Hernanz, Antonio; Gavira Vallejo, José MaríaAn overview of representative studies on rock art paintings applying physicochemical techniques to investigate their composition is presented. Advantages and limitations of the different techniques, protocols and recommendations for sampling and proposals to deal with difficulties are discussed. Portable instrumentation and non-destructive multi-technique methodology are highly advisable. Contamination and the presence of non-pictorial materials must always be taken into account because rock art is an open-air scenario. Accretions, alterations, deterioration and possible remedies are considered. The results obtained are very useful for archaeologists and conservators. The use of new techniques and instrumentation augurs a promising future in this field.