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The nature of the electronic ground state of the Ti2C MXene is unambiguously determined by making use of density functional theory-based calculations including hybrid functionals together with a stringent computational setup providing numerically converged results up to 1 meV. All the explored density functionals (i.e., PBE, PBE0, and HSE06) consistently predict that the Ti2C MXene has a magnetic ground state corresponding to antiferromagnetic (AFM)-coupled ferromagnetic (FM) layers. A spin model, with one unpaired electron per Ti center, consistent with the nature of the chemical bond emerging from the calculations, is presented in which the relevant magnetic coupling constants are extracted from total energy differences of the involved magnetic solutions using an appropriate mapping approach. The use of different density functionals enables us to define a realistic range for the magnitude of each of the magnetic coupling constants. The intralayer FM interaction is the dominant term, but the other two AFM interlayer couplings are noticeable and cannot be neglected. Thus, the spin model cannot be reduced to include nearest-neighbor interactions only. The Néel temperature is roughly estimated to be in the 220 ± 30 K, suggesting that this material can be used in practical applications in spintronics and related fields.

In this work, the effect of CuPtB ordering on the optoelectronic properties of Ga0.5In0.5P is studied by combining in situ transmission electron microscopy measurements and density functional theory (DFT) calculations. GaInP layers were grown by metal organic vapor phase epitaxy with a CuPtB single-variant-induced ordering due to the intentional misorientation of the Ge(001) substrate. Moreover, the degree of order was controlled using Sb as the surfactant without changing other growth parameters. The presence of antiphase ordered domain boundaries (APDBs) between the ordered domains is studied as a function of the order parameter. The in situ electrical measurements on a set of samples with controlled degree of order evidence a clear anisotropic electrical conductivity at the nanoscale between the [110] and [1-10] orientations, which is discussed in terms of the presence of APDBs as a function of the degree of order. Additionally, DFT calculations allow to determine the differences in the optoelectronic properties of the compound with and without ordering through the determination of the dielectric function. Finally, the anisotropy of the electrical conductivity for the ordered case is also discussed in terms of the effective mass calculated from the band structure on specific k-paths. By comparing the experimental measurements and the theoretical calculations, two factors have been presented as the main contributors of the electric conductivity anisotropy of CuPtB-type ordered GaInP thin films: antiphase boundaries that separate domains with uniform order (APDBs) and the anisotropy of the effective mass due to the alternating of In/Ga rich planes.

The search for environmentally sustainable and economically affordable catalysts is incessant in the world of chemistry, especially when it comes to reduce polluting gases such as CO2. Capture and sequestration of CO2 are very interesting research fields, although it seems clear that the efforts must be routed towards the potential use of CO2 as an economical feedstock. In this sense, the CO2 cycloaddition to epoxides to give rise to cyclic carbonates has attracted much attention. Here the results for the formation of cyclic organic carbonates catalyzed by metal-salen complexes, previously achieved with yttrium and scandium, are compared with those of analogous complexes containing metals from the first transition series, such as cobalt or chromium. DFT calculations allow to determine whether this switch of metals will be feasible and provide the basis for instigating future experimental efforts in this regard. The calculations analyzing the structure and electronics of the catalysts allow us to give not only a clear picture of whether these catalysts will be efficient, but also allow to assess which metal center is the most convenient and/or whether the catalytic reaction will occur under mild conditions. Advanced buried volume calculations with the SambVca packages shed light on the different catalytic pockets of monometallic first row transition metals versus group III salen complexes. Our results show that the bite O-M-O angle plays an essential role in the catalysis S.P.P. thanks the Spanish Ministerio de Ciencia e Innovacion for Juan de la Cierva Formacion fellowship (FJC2019-039623-I). A.P. is a Serra Húnter Fellow and ICREA Academia Prize 2019. M.S. and A.P. thank the Spanish MINECO for projects PID2020-113711GB-I00, PID2021- 127423NB-I00, and PGC2018-097722-B-I00 and the Generalitat de Catalunya for project 2017SGR39

The synthesis of spiro compounds featuring seven- and six-membered rings in the spirobicyclic motif is successfully achieved through a cascade process encompassing a rhodium(I)-catalyzed cycloisomerization followed by a highly selective Diels-Alder homodimerization. The scope of the reaction is analyzed based on a series of synthetic substrates, and control experiments and DFT calculations led us to justify the exquisite degree of selectivity observed We are grateful for financial support from the Ministerio de Ciencia e Innovación (PID2020-113711GB-I00 MCIN/AEI/ 10.13039/501100011033), the Generalitat de Catalunya (Project 2017-SGR-39), and the UdG for an IF predoctoral grant to J.V. Open Access funding provided thanks to the CRUE-CSIC agreement with ACS

In this study, the effect of nitrogen substitution in wolframite-type ScTaO4 was investigated using density- functional theory calculations. First, structural and mechanical properties, as well as the dynamical stability of ScTaO4 were examined deeply for the ambient-pressure structure. Subsequently, we studied how lattice vibrations are affected by hydrostatic pressure and determined the elastic moduli of ScTaO4. The results of our study show that the monoclinic structure of ScTaO4 is rigid and non-compressible. In addition, band- structure calculations show that ScTaO4 has a wide direct band-gap of 4.04 eV, which in turn leads to a possible tuning of electronic properties. We have found that this task can be conducted by partially substi- tuting oxygen atoms in the unit cell with nitrogen atoms. Both band-structure calculations and charge- density analyses revealed a narrowing in the band gap caused by the presence of nitrogen atoms, which act as a shallow acceptor state, resulting in weak repulsive interactions and structural distortions in both Sc and Ta coordination polyhedra; reducing the crystal symmetry from monoclinic to triclinic.

A density functional theory-based study was performed to investigate the stability of single-atom catalysts (SACs) over a series of O-terminated MXenes with the stoichiometry of M2CO2. Accordingly, we selected the first-row transition metals as adatoms and nine MXene surfaces as supports. From a thermodynamic viewpoint, the stability of the resulting SACs is favorable for all combinations between adatoms and MXenes. However, the adatoms tend to cluster together to a rather low energy barrier, promoting diffusion, which significantly affects their stability. The present systematic study showed that Sc@ and Ti@M2CO2 are the most feasible SACs because of their high metal-support interaction, low tendency to form metal clusters and high diffusion barriers. The present results can be useful to experimentalists aimed at synthesizing SACs based on MXenes.

All electron density functional theory based calculations are carried out to investigate the properties of metal/oxide nanostructures taking the case of a Ru13 cluster supported on an octahedral anatase (TiO2)84 nanoparticle, as a representative systems. The interaction between both system is exothermic showing binding energies below -4 eV. In spite of the large interaction, the structure of the (TiO2)84 nanoparticle remains unaltered. However, the metal-support interaction promotes the deformation of the Ru13 cluster atomic structure. This deformation is more accentuated when the Ru13 nanocluster is situated in the facet region of the (TiO2)84 nanoparticle than when the interaction involves the edge regions. The formation of the Ru13/(TiO2)84 heterostructure leads to a decrease of the energy gap inherent to the bare (TiO2)84 nanoparticle, becoming almost negligible. This is due to the contribution of the partially filled Ru 4d orbitals with Kohn-Sham energies spanning in the energy range of those of the O 2p occupied and Ti 3d empty manifolds. This feature systematically observed no matter the interaction involves the oxide nanoparticle facet or edge regions. This study constitutes a first step in designing a strategy to investigate metal-semiconductor nanostructures using realistic models that go beyond the use of extended surfaces

Metal-support interactions can dramatically affect the properties of nanocomposite materials. Nevertheless, comprehensive studies of the interfaces between metal nanoparticles and oxide supports remain scarce due to challenges in experimental characterization. A significant understanding of the interactions at such interfaces can be obtained by combining state-of-the-art experiments with density functional calculations. In particular, this Perspective illustrates how theory and experiment can be combined to study interfacial charge transfer, the short- or long-range natures of nanoparticle-support interactions and the effects of oxide nanostructuring on the properties of supported metal particles. These studies aid our understanding of the role of metal-oxide interactions in industrially employed nanocomposites and the design of interfaces with unique properties for future applications.

International audience; The clustering, collision, and relaxation dynamics of pristine and doped helium nanodroplets is theoretically investigated in cases of pickup and clustering of heliophilic argon, collision of heliophobic cesium atoms, and coalescence of two droplets brought into contact by their mutual long-range van der Waals interaction. Three approaches are used and compared with each other. The He time-dependent density functional theory method considers the droplet as a continuous medium and accounts for its superfluid character. The ring-polymer molecular dynamics method uses a path-integral description of nuclear motion and incorporates zero-point delocalization while bosonic exchange effects are ignored. Finally, the zero-point averaged dynamics approach is a mixed quantum–classical method in which quantum delocalization is described by attaching a frozen wavefunction to each He atom, equivalent to classical dynamics with effective interaction potentials. All three methods predict that the growth of argon clusters is significantly hindered by the helium host droplet due to the impeding shell structure around the dopants and kinematic effects freezing the growing cluster in metastable configurations. The effects of superfluidity are qualitatively manifested by different collision dynamics of the heliophilic atom at high velocities, as well as quadrupole oscillations that are not seen with particle-based methods, for droplets experiencing a collision with cesium atoms or merging with each other.

MXenes, a rather new family of 2D carbides and nitrides, have shown to be promising materials in many technological applications, particularly in electrocatalysis. The as-synthesized MXenes exhibit a variety of surface terminations involving mixtures of O, OH, H, or F surface groups. These terminations play a crucial role in the electrocatalytic performance of these materials as these may change depending on the reaction conditions. The Pourbaix diagrams have long being used to provide the thermodynamically stable surface under certain conditions of pH and potential, U. However, experimental determination of Pourbaix diagrams may be quite challenging while first-principles studies, considering the most likely terminations, allow deriving reliable insights. Here, Pourbaix diagrams for a series of representative MXenes are provided; the Ti2C, Ti3C2, V2C, and Mo2C MXenes, with the novelty of considering single and several double mixed terminations. The possible implications of the obtained results are discussed, especially for a proper choice of models in theoretical electrocatalysis studies, including the water splitting related hydrogen evolution reaction (HER), or the oxygen reduction reaction (ORR), but also serving as a guide to any further computational studies and also to electrocatalytic experiments.

Treballs Finals de Grau d'Enginyeria Química, Facultat de Química, Universitat de Barcelona, Curs: 2021-2022, Tutor: Federico Calle Vallejo, Nowadays, the use of fossil fuels is exaggerated and the development and optimization of technologies capable of fulfilling human society’s energetic needs is utterly important. Not only fossil fuels are a non-renewable source of energy, but they are also harmful for the environment and the population. A possible alternative for replacing fossil fuels and internal combustion energies for automotive applications are low-temperature fuel cells. The fuel used in those cells is H2 and the electrochemical process only produces water as by-product. Sustainable supply of H2 must be guaranteed if those technologies are to be globally adopted. However, currently more than 90 % of the H2 produced comes from steam reforming of natural gas and other light fossil fuels. Alternatively, hydrogen can be produced electrochemically by means of the hydrogen evolution reaction (HER) in water electrolyzers. The best catalyst for that purpose is platinum, an expensive and scarce metal. Therefore, alloying is an appealing option to reduce the loading of Pt during hydrogen evolution. Furthermore, positioning a subsurface layer of transition metals increases the activity of the electrolyzer. In this research work, using density functional theory calculations, the computational hydrogen electrode and a simple descriptor-based analysis, we determined the best near-surface alloys (NSAs) of Pt and transitions metals for the HER. We determined that for (111) facets of Pt NSAs the best alloying candidates are Fe, Co and Re. Instead, for (100) facets of Pt NSAs the best alloying candidates are Y and Mo

The shift towards renewable energy is one of the main challenges of this generation. Dye-sensitized solar cells (DSSC), based on donor-acceptor architectures, can help on this transition as they present excellent photovoltaic efficiencies yet cheap and simple manufacturing. For molecular heterojunctions DSSCs, donor-acceptor pairs are linked in a covalent manner, which facilitates their tailoring and rational design. Nevertheless, reliable computational characterization of charge transfer rate constants (kCT) is needed to speed this development process up. In this context, the performance of time-dependent density functional theory for the calculation of kCT's in donor-acceptor fullerene-based dyads has not been benchmarked yet. Herein, we present a detailed analysis on the performance of seven well-known density functional approximations (DFAs) for this type of systems, focusing on several parameters as the reorganization energies (λ), electronic couplings (VDA), and Gibbs energies (ΔG^0_CT), as well as in the final rate constants. The amount of exact exchange at short range (SR) and long range (LR) electron-electron distances (and the transition from SR to LR) turned out to be key for the success of the prediction. The tuning of these parameters improves significantly the performance of current DFAs This work was supported with funds from the Ministerio de Economía y Competitividad (MINECO) of Spain (project CTQ2017- 85341-P to M.S. and A.A.V.), the Spanish government MICINN (project PGC2018-098212-B-C22 to J.M.L), and the Generalitat de Catalunya (project 2017SGR39 to M.S. and J.M.L.). We thank the Spanish government for the predoctoral grant to P.B.-S. (FPU17/02058)

Density functional theory method used to investigate the interaction of the simplest sulfonamide with the B36 nanocluster. The obtained results indicate that although sulfonamide weakly interacts with convex and concave sides of the nanocluster, the B atom at the edge of B36 nanocluster is favorable position for adsorbing. The interactions have been also studied in terms of natural bond order charges analysis. The electronic properties of the nanocluster are significantly altered once the sulfonamide molecule is adsorbed. Thus, the energy gap between HOMO and LUMO orbitals is reduced which could be applied as a chemical signal. Moreover, the relative high dipole moments obtained for B36/sulfonamide configurations suggest that these structures could be solubilized or dispersed in polar mediums like water. According to the results obtained, the B36 nanostructure could be a potential carrier for delivering sulfonamides in nanomedicine applications.

Fluorinated carboxylic acids may soon become a relevant class of compounds for new materials development and as synthons in medicinal chemistry. In this report, a potential method based on a hydroboration/copper-catalyzed carboxylation reaction sequence has been computationally explored to check whether these moieties could be synthetically accessible. DFT calculations fully support the possibility of obtaining fluorinated carboxylic acids from simple fluorinated alkenes and gaseous carbon dioxide, paving the way to future preparation of this class of chemicals.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2022, Tutors: Francesc Viñes Solana, Carme Sousa Romero, Finding adequate photocatalysts for the generation of hydrogen fuel through photocatalytic water splitting via sunlight is a difficult challenge using the traditional trial-and-error method. Thanks to vast advancements in computational methodologies, such as with Density Functional Theory (DFT), the process of identifying suitable photocatalysts may now be accelerated. The present study focuses on MXene compounds, a relatively recently discovered family of two-dimensional early transition metal carbides and nitrides with chemical formula Mn+1Xn, which have been proposed as potentially photoactive materials. An essential step to evaluate their possible use in sunlight-driven photochemistry is the analysis of their bandstructure. Here, DFT simulations are employed in order to inspect the bandstructure of pristine and O-terminated MXene compounds —with M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W and X = C, N— as a function of structure, composition, and oxygen surface termination, with the goal of finding suitable compounds for photocatalytic water splitting. From the studied MXenes and their possible configurations, pristine MXenes have shown metallic character for all cases, while adding the O-termination has proven to increase the chance of becoming a semiconductor, especially for Group III and IV MXenes, which posed as the most promising bandgap cases for photocatalysis. Concerning the X element, C-MXenes exhibit more cases with bandgap than N-MXenes, and generally with larger values. Factors such O-termination hollow position and stacking may affect the bandgap under certain circumstances, but such influences are found to be subtle and irregular. Finally, for all the cases presenting a bandgap larger than 1.23 eV, the minimum thermodynamically demanded to carry out the water splitting process, the band alignment with respect the water splitting half-reactions potentials was studied. Results highlighted Zr2CO2 and Y2NO2 as, a priori, adequate photocatalysts for this process.

Here we show, combining CO stripping voltammograms on different PtCu nanoparticle (NP) low-temperature fuel cell electrocatalysts and density functional calculations, that surface chemical ordering and the presence of certain defects explain the CO tolerance vs. poisoning of such systems. The CO withdrawal for these duelling CO-slingers depends on whether they are well-shaped core@shell Cu@Pt NPs, more CO-tolerant, or having Cu-surrounded surface Pt atoms or adatoms/vacancies surface defects, less CO-tolerant. The latter sites are critical on nm-sized PtCu NPs, displaying stronger CO adsorption compared to pure Pt NPs. Avoiding such sites is key when designing less expensive and CO-poisoned Cu@Pt NP-based electrocatalysts.

electrochemical technologies for water treatment have flourished over the last decades. However, it is still challenging to treat the actual complex water effluents by a single electrochemical process, often requiring coupling of technologies. In this study, an upgraded peroxi-coagulation (PC) process with a magnetically assembled mZVI/DSA anode has been devised for the first time. COD, NH3-N and total phosphorous were simultaneously and effectively removed from livestock wastewater. The advantages, influence of key parameters and evolution of electrogenerated species were systematically investigated to fully understand this novel PC process. The fluorescent substances in livestock wastewater could also be almost removed under optimal conditions (300 mA, 0.2 g ZVI particles and pH 6.8). The interaction between ¿OH and active chlorine yielded ClO¿ with a high steady-state concentration of 6.85 × 10􀀀 13 M, which did not cause COD removal but accelerated the oxidation of NH3-N. The Mulliken population suggested that ¿OH and NH3-N had similar electron-donor behavior, whereas ClO¿ acted as an electron-withdrawing species. Besides, although the energy barrier for the reaction between ¿OH and NH3-N (17.0 kcal/mol) was lower than that with ClO¿ (18.8 kcal/mol), considering the tunneling in the H abstraction reaction, the Skodje-Truhlar method adopted for calculations evidenced a 17-fold faster NH3-N oxidation rate with ClO¿. In summary, this work describes an advantageous single electrochemical process for the effective treatment of a complex water matrix.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2022, Tutors: Ángel Morales García, Francesc Viñes Solana Single-atom catalysts (SACs) are the maximal expression of nanostructuring, of vital importance to maximize the efficiency of late, scarce, and expensive transition metals (TMs), regularly used as catalysts for a large variety of industrial chemical applications. SACs are fundamentally determined by their stability, where proper substrates that disperse them are key. Focusing on these, two-dimensional (2D) transition metal carbides, nitrides, and carbonitrides, known as MXenes, have gained much interest due to their inherent 2D nature, which suggests them as a suitable platform for disperse TM adatoms. Here we study, by density functional theory (DFT), the TM single-atom adsorption energy and stability for 4d and 5d TMs on nine O-functionalized MXenes with stoichiometry M2CO2 (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W), reliable models according to experimental evidences. The TM adsorption sites and heights are identified, analyzing adsorption energy trends along the d series, as well the MXene groups and series, identifying that the adsorption strengths decay along the d series, and are highly influenced by the TM stability, where especially stable d5 and d10 configurations show the smallest adsorption energies. Aside, the adsorption strength increases along the MXene series, but decays down the MXene groups. SACs are found to be energetically favorable for early and late TMs, and specially enhanced on V2CO2, Cr2CO2, Mo2CO2, and W2CO2 MXenes, although dispersion and/or aggregation can be affected by the TM adatom diffusion energy barriers

The reductive amination process under hydrogen at high pressure catalyzed by iron complexes is of great synthetic interest. In this work, we report Density Functional Theory (DFT) studies on the reductive amination catalyzed by a Knölker-type iron complex. Different modifications of the catalyst are explored to improve the efficiency and guide experiments towards milder conditions. DFT calculations in conjunction with analysis of the chemical structure in terms of geometry, fragment partial charges, Effective Oxidation States (EOS) and aromaticity, allows to conclude that the presence of electron-withdrawing substituents on the cyclopentadienone ring induces a decrease of the activation barriers of most relevant steps, leading to a more efficient catalysis. The present work is a clear example that predictive catalysis can have a fundamental role in sustainable catalytic transformations S.G., N.J., and J.-L.R. gratefully acknowledge financial support from the “Ministere de la Recherche et des Nouvelles ̀ Technologies”, Normandie Université, CNRS, and the LABEX SynOrg (ANR-11-LABX-0029). N.J. acknowledges the Graduate School of Research XL-Chem (ANR-18-EURE0020 XL-Chem) for the PhD fellowship. A.P., P.S., and M.S. thank the Ministerio de Economía y Competitividad (MINECO) of Spain for projects PGC2018-097722-B-I00, PGC2018-098212-B-C22, and PID2020-13711GB-I00 and Generalitat de Catalunya for projects 2017SGR39 and ICREA Academia prize 2019. A.P. is a Serra Hunter fellow. ́ M.G. thanks the Generalitat de Catalunya and Fons Social Europeu for the predoctoral fellowship (2018 FI_B01120). S.E. thanks Universitat de Girona and Donostia International Physics Center (DIPC) for an IFUdG2019 predoctoral fellowship Open Access funding provided thanks to the CRUE-CSIC agreement with ACS

Understanding the water splitting mechanism in photocatalysis is a rewarding goal as it will allow producing clean fuel for a sustainable life in the future. However, identifying the photocatalytic mechanisms by modeling photoactive nanoparticles requires sophisticated computational techniques based on multiscale modeling. In this review, we will survey the strengths and drawbacks of currently available theoretical methods at different length and accuracy scales. Understanding the surface-active site through Density Functional Theory (DFT) using new, more accurate exchange-correlation functionals plays a key role for surface engineering. Larger scale dynamics of the catalyst/electrolyte interface can be treated with Molecular Dynamics albeit there is a need for more generalizations of force fields. Monte Carlo and Continuum Modeling techniques are so far not the prominent path for modeling water splitting but interest is growing due to the lower computational cost and the feasibility to compare the modeling outcome directly to experimental data. The future challenges in modeling complex nano- photocatalysts involve combining different methods in a hierarchical way so that resources are spent wisely at each length scale, as well as accounting for excited states chemistry that is important for photocatalysis, a path that will bring devices closer to the theoretical limit of photocatalytic efficiency.

[eng] Nanoparticles are highly relevant for catalysis due to their nanostructure, which increases the fraction of surface atoms, and the size dependent morphology and properties. Catalytic properties of nanoalloys also depend on the composition and chemical ordering. Therefore, it is important to understand how geometrical and topological features of mono- and bimetallic nanoparticles affect their stability and reactivity of the exposed surface sites. This Thesis deals with computational study of monometallic and bimetallic nanoparticles using methods based on Density Functional Theory (DFT). First, the exchange-correlation functional PBE was identified to reproduce experimental bulk and surface properties of Transition Metals overall more accurately than other examined functionals VWN, PBEsol, RPBE and TPSS. The performance was further improved by choosing exchange and correlation parameters between those of PBE and PBEsol functionals or restoring the local spin approximation of PBEsol. The d-band centre was found the most robust and transferrable among the studied electronic descriptors for different functionals. For different monometallic Pd nanoparticles it was shown that their energies can be quantitatively described using linear dependence on the quantity of atoms with different coordination numbers. Whereas employing such morphological features as number of corner sites, length of the edges, particle area and volume allowed only a qualitative energy prediction. Equilibrium chemical ordering for bare PdRh, PtNi, PtAu, PtAg, and PtCu bimetallic nanoparticles at low temperature was determined employing the so-called Topological Method parameterized on DFT data. Effects of temperature and reactive environment on the chemical ordering were also evaluated. A remarkable experimental finding of our collaborators is that CO molecules can bind stronger on some core@shell Cu@Pt particles than on pure Pt particles. It was rationalised by DFT calculations of specific surface sites, such as single Pt atom surrounded by surface Cu atoms, under-coordinated Pt atom, Pt adatom and vacancies with missing Pt atoms., [spa] Las nanopartículas son muy relevantes para la catálisis debido a su nanoestructura, implicando un aumento de la fracción de átomos superficiales, y la morfología y propiedades dependientes del tamaño. Las propiedades catalíticas de las nanoaleaciones también dependen de la composición y el patrón de posiciones atómicas de los elementos que las conforman. Por lo tanto, es importante comprender cómo las características geométricas y topológicas de las nanopartículas mono y bimetálicas afectan en su estabilidad y en la reactividad de los átomos superficiales. Esta Tesis aborda el estudio computacional de nanopartículas monometálicas y bimetálicas bajo el marco de la Teoría del Funcional de la Densidad (o DFT, por sus siglas en inglés). Primeramente, se identificó el funcional PBE para reproducir, en general, con mayor precisión las propiedades experimentales de bulk y superficie de los metales de transición respecto a otros funcionales examinados (VWN, PBEsol, RPBE y TPSS). Además, esta reproducción se mejoró variando los parámetros de intercambio y correlación entre los de los funcionales PBE y PBEsol o restaurando la aproximación de espín local en PBEsol. El centro de la banda d fue el descriptor electrónico más robusto y transferible entre los estudiados para diferentes funcionales. Para nanopartículas monometálicas de Pd diferentes, se demostró que sus energías se pueden describir cuantitativamente utilizando una dependencia lineal con la cantidad de átomos con diferente número de coordinación. Mientras que el empleo de características morfológicas como el número de vértices, la longitud de los bordes, el área y el volumen de las partículas permitieron sólo una predicción energética cualitativa. El patrón de posiciones atómicas de equilibrio a baja temperatura se determinó en nanopartículas bimetálicas de PdRh, PtNi, PtAu, PtAg y PtCu empleando el Método Topológico. También se evaluaron los efectos de la temperatura y del medio reactivo sobre él. Un hallazgo experimental de nuestros colaboradores muestra que las moléculas de CO pueden unirse más fuerte a algunas nanopartículas de Cu@Pt que a nanopartículas de Pt puro. Esto se racionalizó mediante cálculos DFT de sitios específicos de la superficie, como un átomo de Pt rodeado por átomos superficiales de Cu o un átomo de Pt subcoordinado.

Fe(III) complexes are receiving ever-increasing attention as spin crossover (SCO) systems because they are usually air stable, as opposed to Fe(II) complexes, which are prone to oxidation. Here, we present the first systematic study exclusively devoted to assess the accuracy of several exchange-correlation functionals when it comes to predicting the energy gap between the high-spin (S = 5/2) and the low-spin (S = 1/2) states of Fe(III) complexes. Using a dataset of 24 different Fe(III) hexacoordinated complexes, it is demonstrated that the B3LYP* functional is an excellent choice not only for predicting spin-state energy gaps for Fe(III) complexes undergoing spin-transitions but also for discriminating Fe(III) complexes that are either low- or high-spin in the whole range of temperatures. Our benchmark study has led to the identification of a very versatile Fe(III) compound whose SCO properties can be engineered upon changing a single axial ligand. Overall, this work demonstrates that B3LYP* is a reliable functional for screening new spin-crossover systems with tailored properties.

Here we present room-temperature spin-dependent charge transport measurements in single-molecule junctions made of metalloporphyrin-based supramolecular assemblies. They display large conductance switching for magnetoresistance in a single-molecule junction. The magnetoresistance depends acutely on the probed electron pathway through the supramolecular wire: those involving the metal center showed marked magnetoresistance effects as opposed to those exclusively involving the porphyrin ring which present nearly complete absence of spin-dependent charge transport. The molecular junction magnetoresistance is highly anisotropic, being observable when the magnetization of the ferromagnetic junction electrode is oriented along the main molecular junction axis, and almost suppressed when it is perpendicular. The key ingredients for the above effect to manifest are the electronic structure of the paramagnetic metalloporphyrin, and the spinterface created at the molecule-electrode contact., The research reported here was supported by the Spanish Ministerio de Ciencia, e Innovación (grants PGC2018-093863-B-C21, PID2019-108794GB-100 and MDM-2017-0767) and the ERC Grants Fields4CAT (grant 772391) and Tmol4TRANS (grant 724981). I.D.-P. thanks King's College London for start-up funds support. A.C.A. thanks the European Union for a H2020-MSCA-IF-2018 Fellowship (TECh-MoDE, 844668). A.M.R. thanks the Ministerio de Ciencia, e Innovación for a predoctoral FPI grant. E.R. thanks Generalitat de Catalunya for an ICREA Academia award and for the SGR2017-1289 grant. N.A.-A. and A.G.-C. thank the Generalitat de Catalunya for the grant 2017SGR1277 and the Severo Ochoa Program for Centers of Excellence in R&D (FUNFUTURE, CEX2019-000917-S). A.G.C. thanks Ministerio de Ciencia e Innovación for Ramon y Cajal grant (RYC-2017-22910). The authors acknowledge the general facilities of the Universitat de Barcelona (CCiT-UB) and the computer resources, technical expertise and assistance provided by the Barcelona Supercomputing Centre., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

This theoretical work presents the electronic and magnetic properties of Cr doped SrFe12O19 using first-principles calculations based on density functional theory. An improvement in the magnetic properties of SrFe12O19 is predicted when doped with the Cr3+ ion. It was found that the magnetic Cr3+ ion preferentially replaces Fe3+ ion at 2a, 4f1, and 12k sites. The calculation demonstrates that if Cr ion occupies the 4f1 site, a total magnetization of 52.920 emu/g is obtained. On the other hand, SrFe11.5Cr0.5O19 behaves as a semiconductor when Cr occupies the 2a and 12k sites while behaves like half-metal when Cr occupies the 4f1 site. The higher magnetization, identified for the SrFe11.5Cr0.5O19 compound, provides unique qualities in the applications of permanent magnets and/or related devices like spintronics and magnetic semiconductors.

DFT modelling of the oxygen reduction and evolution reactions (ORR and OER) habitually makes use of semiempirical corrections to oxygen in the gas phase. Although such corrections are tacit in the model, they should not be overlooked. In this article, we calculate the errors in the total energy of oxygen for commonly used exchange-correlation functionals, PW91, RPBE, PBE, and BEEF-vdW, to show that, for all functionals tested, the error is at least 0.3 eV. We discuss the impact this sizeable error in oxygen has on the modelling of the ORR and the OER. The error due to oxygen affects not only the overall equilibrium potential of the reaction, but also the energies of individual mechanistic steps. This illustrates that understanding the reasoning behind the semiempirical corrections for oxygen is important for researching new catalysts which may have different potential limiting steps.

Nanostructured materials based on CeO2 and Pt play a fundamental role in catalyst design. However, their characterization is often challenging due to their structural complexity and the tendency of these materials to change under reaction conditions. In this work, we combine calculations based on the density functional theory, a machine-learning assisted global optimization method, and ab initio thermodynamics to characterize stable oxidation states of ceria-supported PtyOx particles under different environmental conditions. The collection of global minima structures for different stoichiometries resulting from the global optimization effort is used to assess the effect of temperature, oxygen pressure, and support interactions on the phase diagrams, oxidation states, and geometries of the PtyOx particles. We, thus, identify favored structural motifs and O:Pt ratios, revealing that oxidized states of freestanding and ceria-supported platinum particles are more stable than reduced ones under a wide range of conditions. These results indicate that studies rationalizing activity of ceria-supported Pt clusters must consider oxidized states and that previous understanding of such materials obtained only with fully reduced Pt clusters may be incomplete.

Low-valent titanium species were prepared by reaction of [TiCp*X3] (Cp* = eta5-C5Me5; X = Cl, Br, Me) with LiEH4 (E = Al, B) or BH3(thf), and their structures elucidated by experimental and theoretical methods. The treatment of trihalides [TiCp*X3] with LiAlH4 in ethereal solvents (L) leads to the hydride-bridged heterometallic complexes [{TiCp*(mu-H)}2{(mu-H)2AlX(L)}2] (L = thf, X = Cl, Br; L = OEt2, X = Cl). Density functional theory (DFT) calculations for those compounds reveal an open-shell singlet ground state with a Ti-Ti bond and can be described as titanium(II) species. The theoretical analyses also show strong interactions between the Ti-Ti bond and the empty s orbitals of the Al atom of the AlH2XL fragments, which behave as sigma-accepting (Z-type) ligands. Analogous reactions of [TiCp*X3] with LiBH4 (2 and 3 equiv) in tetrahydrofuran at room temperature and at 85 ºC lead to the titanium(III) compounds [{TiCp*(BH4)(mu-X)}2] (X = Cl, Br) and [{TiCp*(BH4)(mu-BH4)}2], respectively. The treatment of [TiCp*Me3] with 4 and 5 equiv of BH3(thf) produces the diamagnetic [{TiCp*(BH3Me)}2(mu-B2H6)] and paramagnetic [{TiCp*(mu-B2H6)}2] complexes, respectively., Ministerio de Ciencia, Innovación y Universidades, Universidad de Alcalá

Partition coefficients define how a solute is distributed between two immiscible phases at equilibrium. The experimental estimation of partition coefficients in a complex system can be an expensive, difficult, and time-consuming process. Here a computational strategy to predict the distributions of a set of solutes in two relevant phase equilibria is presented. The octanol/water and octanol/air partition coefficients are predicted for a group of polar solvents using density functional theory (DFT) calculations in combination with a solvation model based on density (SMD) and are in excellent agreement with experimental data. Thus, the use of quantum-chemical calculations to predict partition coefficients from free energies should be a valuable alternative for unknown solvents. The obtained results indicate that the SMD continuum model in conjunction with any of the three DFT functionals (B3LYP, M06-2X, and M11) agrees with the observed experimental values. The highest correlation to experimental data for the octanol/water partition coefficients was reached by the M11 functional; for the octanol/air partition coefficient, the M06-2X functional yielded the best performance. To the best of our knowledge, this is the first computational approach for the prediction of octanol/air partition coefficients by DFT calculations, which has remarkable accuracy and precision.

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble‐metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low‐coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low‐coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity.

Bis(phenoxy-imine) complexes of zirconium or titanium, a type of Fenokishi-Imin (FI) catalysts, allow the production of polyethylenes with well-defined bimodal molecular weight distributions. Interestingly, by substitution of phenyl rings in the bis(phenoxy-imine) ligands by perfluorinated phenyl rings, the polymerization reaches a desired unimodal behaviour. These catalysts have 3 isomers of similar energy that can be easily interconverted. It is likely that the bimodal behaviour is due to the coexistence of more than one isomer in the reaction vessel. Here, we perform static and dynamic DFT calculations to understand the isomerization of the catalytic active species. We analyze the relative Gibbs energies of the different isomers and the barriers for the isomerization processes. Further characterization of the isomers is obtained through stereo maps, aromaticity measures, and NCI plots. Our results show that by changing the phenyl group by a perfluorinated phenyl ring, one of the isomers is particularly stabilized, thus explaining the unimodal behaviour of the polyethylene production process S.E. acknowledges financial support through IFUG2019 Ph.D. fellowship from Universitat de Girona (UdG) and Donostia International Physics Center (DIPC). A.P. is a Serra Húnter Fellow. A.P. and M.S. thank the Ministerio de Economía y Competitividad (MINECO) of Spain for projects CTQ2014-59832-JIN, PGC2018-097722-B-I00 and CTQ2017-85341-P; Generalitat de Catalunya for project 2017SGR39, Xarxa de Referència en Química Teòrica i Computacional, and ICREA Academia prize 2019 to A.P. N.B.-L. appreciates Iran Polymer and Petrochemical Institute (IPPI) for all of the supports provided under the grant number of 43794110. We thank support by Mitsui Chemicals, Inc

Bimetallic alloys are actively investigated as promising new materials for catalytic and other energy-related applications. However, the stable arrangements of the two metals in prevailing nanostructured systems, which define their structure and surface reactivity, are seldom addressed. The equilibrium chemical orderings of bimetallic nanoparticles are usually different from those in the corresponding bulk phases and hard to control experimentally, which hampers assessment of the relations between composition, structure, and reactivity. Herewith, we study mixtures of platinum an essential metal in catalysis alloyed with coinage metals gold, silver, and copper. These systems are interesting, for instance, for reducing the costly Pt content and designing improved multifunctional catalysts, but the chemical orderings in such mixtures at the nanoscale are still debated. We therefore explore chemical orderings and properties of Pt-containing nanoalloys by means of a topological method based on density functional calculations. We determine the lowest-energy chemical orderings in 1.4 to 4.4 nm large Pt-Au, Pt-Ag and Pt-Cu particles with different contents of metals. Chemical ordering, bonding, and charge distribution in the nanoparticles are analyzed, identifying how peculiar structural motifs relevant for catalysis and sensing applications, such as monometallic skins and surface single-atom sites, emerge. We compare these results with previous data for the corresponding Pd-based particles, identifying trends in chemical ordering, deepening understanding of the behaviour of catalytically relevant bimetallic compositions, and establishing appropriate models for studying the bimetallic nanoalloys.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2021, Tutors: Ángel Morales García, Francesc Viñes Solana, Heterogeneous catalysis is crucial on the development of chemical products in industrialized societies. A great deal of major industrial chemical transformations involves heterogeneous catalysts, which are typically comprised of transition metals and metal oxides as substrates. Two-dimensional metastable transition-metal carbides and nitrides, known as MXenes, have garnered increasing attention for nearly a decade because of their versatile composition and structure, stability under certain conditions of interest in catalysis, and numerous appealing properties. The synthesis of Singe-atom Catalyst (SACs) is one of the main active areas in heterogeneous catalysis with the ultimate goal of promoting small-size metal particles as Sisngle Atoms (SAs). Thus, the search for appropriate substrate for SACs is highly demanded for the next generation of heterogeneous catalysis. The present study aims at introducing low-dimensional transition metal carbides as feasible substrates for anchoring single metal atoms by using first-principles based on density functional theory (DFT), calculations. In particular, we focussed on the 4d transition metal atoms (TMs) and nine bare MXene surfaces with M2C stoichiometry (M= Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, and W). The adsorption energies of four different active sites were studied to find the most stable site, HM, and the situation where the SA isolation would be thermodynamically favourable. This energetic parameter is used to find a correlation with respect to structural features such as the atomic height over the MXene surface, the MAX exfoliation energies, and by comparing 4d with 3d TMs. In addition, few TMs from Pt-group plus Au are also investigated in this project. The results show, shortly, that Pt and Au are better dispersed than Pd and Ag and all the 5d TMs are more prone to be isolated with the V2C MXene

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2021, Tutores: Carme Rovira Virgili, Alba Nin-Hill, A computational study of the possible β-D-glucopyranose conformations and energies has been performed using Density Functional Theory (DFT). Three different density functionals have been used; PBE, BLYP and HCTH. The calculations have been performed using the metadynamics method combined with Car-Parrinello Molecular Dynamics (CPMD). The results obtained show that the most stable conformation of β-D-glucopyranose corresponds to a chair conformation, 4C1, with B3,O as the second most stable conformation. The most reliable results are obtained using the PBE functional

Rhenium disulfide (ReS2), known in nature as the mineral rheniite, is a very interesting compound owing to its remarkable fundamental properties and great potential to develop novel device applications. Here we perform density functional theory (DFT) calculations to investigate the structural properties and compression behavior of this compound and also of the (Re,Mo)S-2 solid solution as a function of Re/Mo content. Our theoretical analysis is complemented with high-pressure X-ray diffraction (XRD) measurements, which have allowed us to reevaluate the phase transition pressure and equation of state of 1T-ReS2. We have observed the 1T-to-1T' phase transition at pressures as low as similar to 2 GPa, and we have obtained an experimental bulk modulus, B-0, equal to 46(2) GPa. This value is in good agreement with PBE+D3 calculations, thus confirming the ability of this functional to model the compression behavior of layered transition metal dichalcogenides, provided that van der Waals corrections are taken into account. Our experimental data and analysis confirm the important role played by van der Waals effects in the high-pressure properties of 1T-ReS2., J.I. and C.P. acknowledge the Spanish Ministry of Science, Innovation and Universities via Grant FIS2017-2017-83295-P. T.W. acknowledges financial support from Polish Ministry of Science and Higher Education via Diamond Grant no. D\2015 002645.

Reducing the operating temperature of oxidation catalysts is important for designing energy efficient processes, extending catalyst lifetime, and abating pollutants, especially in cold climates. Herein, high CO oxidation activity at sub-ambient temperatures is reported for Pt/CeO2 catalysts with high content of Pt in the form of dispersed Pt2+ and Pt4+ centers. Whereas the reference 1 wt%Pt catalyst was active for CO oxidation only above 100ᵒC, the 8 and 20 wt%Pt catalysts converted 60 and 90 % of CO, respectively, below 0ᵒC. Ionic platinum was shown to facilitate oxygen release from ceria and lower the light-off temperature of the reaction occurring through the Mars-van-Krevelen mechanism. However, the remarkable activity observed at sub-ambient temperatures for the ≥8 wt%Pt catalysts is proposed to involve O2 and CO reactants weakly adsorbed on PtOx clusters. The synergies between ionic platinum and nanostructured ceria reported in this work advance the knowledge-driven design of catalysts for low-temperature oxidation reactions.

Interatomic processes play a crucial role in weakly bound complexes exposed to ionizing radiation; therefore, gaining a thorough understanding of their efficiency is of fundamental importance. Here, we directly measure the timescale of interatomic Coulombic decay (ICD) in resonantly excited helium nanodroplets using a high-resolution, tunable, extreme ultraviolet free-electron laser. Over an extensive range of droplet sizes and laser intensities, we discover the decay to be surprisingly fast, with decay times as short as 400 fs, nearly independent of the density of the excited states. Using a combination of time-dependent density functional theory and ab initio quantum chemistry calculations, we elucidate the mechanisms of this ultrafast decay process, where pairs of excited helium atoms in one droplet strongly attract each other and form merging void bubbles, which drastically accelerates ICD., Physical Review X, 11 (2), ISSN:2160-3308

Reducing the operating temperature of oxidation catalysts is important for designing energy efficient processes, extending catalyst lifetime, and abating pollutants, especially in cold climates. Herein, high CO oxidation activity at sub-ambient temperatures is reported for Pt/CeO2 catalysts with high content of Pt in the form of dispersed Pt2+ and Pt4+ centers. Whereas the reference 1 wt%Pt catalyst was active for CO oxidation only above 100ᵒC, the 8 and 20 wt%Pt catalysts converted 60 and 90 % of CO, respectively, below 0ᵒC. Ionic platinum was shown to facilitate oxygen release from ceria and lower the light-off temperature of the reaction occurring through the Mars-van-Krevelen mechanism. However, the remarkable activity observed at sub-ambient temperatures for the ≥8 wt%Pt catalysts is proposed to involve O2 and CO reactants weakly adsorbed on PtOx clusters. The synergies between ionic platinum and nanostructured ceria reported in this work advance the knowledge-driven design of catalysts for low-temperature oxidation reactions.

Various COx species formed upon the adsorption and oxidation of CO on palladium and silver single atoms supported on a model ceria nanoparticle (NP) have been studied using density functional calculations. For both metals M, the ceria-supported MCOx moieties are found to be stabilised in the order MCO &lt, MCO2 &lt, MCO3, similar to the trend for COx species adsorbed on M-free ceria NP. Nevertheless, the characteristics of the palladium and silver intermediates are different. Very weak CO adsorption and the small exothermicity of the CO to CO2 transformation are found for O4Pd site of the Pd/Ce21O42 model featuring a square-planar coordination of the Pd2+ cation. The removal of one O atom and formation of the O3Pd site resulted in a notable strengthening of CO adsorption and increased the exothermicity of the CO to CO2 reaction. For the analogous ceria models with atomic Ag instead of atomic Pd, these two energies became twice as small in magnitude and basically independent of the presence of an O vacancy near the Ag atom. CO2-species are strongly bound in palladium carboxylate complexes, whereas the CO2 molecule easily desorbs from oxide-supported AgCO2 moieties. Opposite to metal-free ceria particle, the formation of neither PdCO3 nor AgCO3 carbonate intermediates before CO2 desorption is predicted. Overall, CO oxidation is concluded to be more favourable at Ag centres atomically dispersed on ceria nanostructures than at the corresponding Pd centres. Calculated vibrational fingerprints of surface COx moieties allow us to distinguish between CO adsorption on bare ceria NP (blue frequency shifts) and ceria-supported metal atoms (red frequency shifts). However, discrimination between the CO2 and CO32− species anchored to M-containing and bare ceria particles based solely on vibrational spectroscopy seems problematic. This computational modelling study provides guidance for the knowledge-driven design of more efficient ceria-based single-atom catalysts for the environmentally important CO oxidation reaction.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2021, Tutors: Francesc Viñes Solana, Pablo Gamallo Belmonte, The present study focuses on the possible use of grazynes as membranes to separate biogas methane (CH4) from carbon dioxide (CO2), an impurity found when biogas is gained from the anaerobic digestion of organic matter. This is assessed by theoretical simulations using Density Functional Theory (DFT) in conjunction with the Perdew-Burke-Ernzerhof (PBE) exchangecorrelation functional, and including a description of dispersive forces by means of Grimme D3 method (PBE-D3). The analysis focused on the CO2 and CH4 diffusions through the pores of [1],[1]-, [2],[1]-, and [3],[1]-grazynes. To this end, adsorption energies for both molecules on two possible conformations were acquired as a matter of coverage and pore size, revealing that low-coverages are needed for a molecular diffusion, otherwise high-steric repulsions and diffusion energy barriers are found. As far as pore size is concerned, the larger the pore, the smaller the energy barriers. Aside, the interaction of the molecules with the grazynes is weak, mostly due to dispersive forces, which lower the adsorption energies and diffusion energy barriers. The study found that such grazynes are impermeable to both molecules, and only when achieving acetylenic defects on c(2×2) supercells, one could effectively expect a biogas upgrading. There, the CO2 molecule is found to trespass the membrane when overcoming energy barriers of ca. 0.5-0.6 eV when approaching with its molecular axis perpendicular to the grazyne pore. Diffusion is found to be impossible for other conformations, e.g. parallel CO2, or any conformation of CH4 molecule, revealing mechanisms of grazyne bulging out the impinging molecules, or, when forcing it, the observation of large deformations of the structure and/or the molecule. Thus, CO2 could be 100% selectively separated when using defective [1],[1,2]{0,1}-, [2],[1,2]{0,1}-, and [3],[1,2]{0,1}-grazynes, enabling them to be used to get a purified methane stream in biogas upgrading

The issue of formation of dust grains in the interstellar medium is still a matter of debate. One of the most developed proposals suggests that atomic and heteromolecular seeds bind together to initiate a nucleation process leading to the formation of nanostructures resembling very small grain components. In the case of silicates, nucleated systems can result in molecular nanoclusters with diameters ≤ 2 nm. A reasonable path to further increase the size of these proto-silicate nanoclusters is by mutual aggregation. The present work deals with modeling this proto-silicate nanocluster aggregation process by means of quantum chemical density functional theory calculations. We simulate nanocluster aggregation by progressively reducing the size of a periodic array of initially well-separated nanoclusters. The resulting aggregation leads to a set of silicate bulk structures with gradually increasing density which we analyze with respect to structure, energetics and spectroscopic properties. Our results indicate that aggregation is a highly energetically favorable process, in which the infrared spectra of the finally formed amorphous silicates match well with astronomical observations.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2021, Tutor: Francesc Viñes Solana, Density Functional Theory would be exact in estimating a polyelectronic chemical system energy, when the exchange-correlation (xc) functional would be known. Unfortunately, it is not, and has to be approximated, with dozens of xc functionals developed in the last decades, belonging to different rungs of Jacob’s ladder of xc improvement. In the case of Transition Metals (TMs), mostly describing few late TMs and their structural properties. Recent studies expanded the analysis covering different bulk properties and surface features. Here we carry out such performance evaluation on so far ignored xc functionals, either within the most basic local density approximations, including the, Hendin-Lundqvist (HL) and Perdew-Zunger (PZ) xc functionals, or within the Generalized Gradient Approximation (GGA), exploring the revised Perdew-Burke-Ernzerhof (revPBE) and the Armiento-Mattson (AM05) xc functionals. Aside from these, the recent meta-GGA Strongly Constrained Appropriately Normed (SCAN) functional is analysed, characterized by fulfilling all 17 mathematical conditions an xc must comply, plus the Bayesian Error Estimation Functional (BEEF) is explored, a functional where artificial intelligence, in the form of a machine learning algorithm, was used to adjust the mathematical expression to a large and diverse set of experimental results. The present results, acquired for 27 TM bulks in their crystallographic structures —body-centred cubic, face-centred cubic, and hexagonal close-packed—, reveal that none of the explored functionals is best in describing TM bulks, were Viñes-Vega (VV) excels, and highlighting that, while SCAN performance is acceptable, BEEF is not. When accounting for TM surface properties, acquired on 81 low-index Miller surfaces, the same situation applies, not improving the VV xc adapted for solids (VVsol).

Background: Time-dependent techniques in nuclear theory often rely on mean-field or Hartree-Fock descriptions. Beyond mean-field dynamical calculations within the time-dependent density matrix (TDDM) theory have often invoked symmetry restrictions and ignored the connection between the mean-field and the induced interaction. Purpose: We study the ground states obtained in a TDDM approach for nuclei from $A=12$ to $A=24$, including examples of even and odd-even nuclei with and without intrinsic deformation. We overcome previous limitations using three-dimensional simulations and employ density-independent Skyrme interactions self-consistently. Methods: The correlated ground states are found starting from the Hartree-Fock solution, by adiabatically including the beyond-mean-field terms in real time. Results: We find that, within this approach, correlations are responsible for $\approx 4-5 \%$ of the total energy. Radii are generally unaffected by the introduction of beyond mean-field correlations. Large nuclear correlation entropies are associated to large correlation energies. By all measures, $^{12}$C is the most correlated isotope in the mass region considered. Conclusions: Our work is the starting point of a consistent implementation of the TDDM technique for applications into nuclear reactions. Our results indicate that correlation effects in structure are small, but beyond-mean-field dynamical simulations could provide new insight into several issues of interest., Comment: 14 pages, 7 figures, 8 tables

MXenes are two-dimensional nanomaterials isolated from MAX phases by selective extraction of the A component&mdash, a p-block element. The MAX exfoliation energy, Eexf, is considered a chemical descriptor of the MXene synthesizability. Here, we show, by density functional theory (DFT) estimations of Eexf values for 486 different MAX phases, that Eexf decreases (i) when MAX is a nitride, (ii) when going along a metal M component d series, (iii) when going down a p-block A element group, and (iv) when having thicker MXenes. Furthermore, Eexf is found to bias, even to govern, the surface chemical activity, evaluated here on the CO2 adsorption strength, so that more unstable MXenes, displaying larger Eexf values, display a stronger attachment of species upon.

Single-atom catalysts (SACs) have emerged as a novel class of catalysts that show unique catalytic properties thanks to their easy tuneability. In this context, zeolites are widely used as support materials for SACs, since the steric restrictions from their channels can effectively suppress the aggregation of the metal particles and provide encapsulated metal atoms with exceptional stability. Although significant efforts have been made to study zeolite-supported SACs, only a few transition metal atoms have been tested, and the determination of precise locations of single metal atoms within the zeolite matrix by experiments remains a nontrivial task. Herein we provide a thorough dispersion corrected density functional theory study of the energetic, structural, magnetic and electronic properties for the full sets of 3d, 4d and 5d transition metal atoms encapsulated on MFI-type Silicalite-1 zeolite, namely TM@S-1. Most TM atoms prefer to adsorb in the center of the S-1 channels, weakly interacting with the zeolite, with the exception of Group 3 − Sc, Y, La − , Group 10 − Ni, Pd, Pt − , Ru and Rh atoms, making them good candidates as stable SACs with superior catalytic performance. The optimized geometry files for the full set of TM@S-1 have been uploaded to a public online repository, paving the road for further catalytic studies of these systems. The behavioral trends gained from this study provide a solid theoretical background that can be used as the basis for interpreting, understanding, and discussing already observed experimental features, and future findings as well.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2020, Tutors: Francesc Viñes Solana, Ángel Morales García, One of the major challenges of our nowadays society is the fight against climate change derived from the gradual global warming of the Earth surface due to the high concentration of detrimental greenhouse gases in the atmosphere, such as CO2 or CH4. These gases absorb the radiated heat emitted by the Earth toward space consequently promoting the so-called greenhouse effect. In principle, this effect is beneficial, because it leads to the thermal stability that is essential for life. However, the anthropogenic activity has derived into a great dependency of fossil fuels, especially since the industrial revolution, generating an excessive concentration of the above-mentioned gases, and so increasing the Earth temperature leading as well to the acidification of oceans. Nowadays, the CO2 concentration in the atmosphere has reached 360 ppm, and this amount is expected to be doubled by the end of this century. The direct consequence of such concentration is the increase of the Earth’s temperature, expected to rise 2-5 ºC, leading to the melting of the poles, augmenting the level of the sea, and causing floods and other devastating phenomena across the world. To minimize, even avoid such consequences, new technologies are sought to reduce these greenhouse gases concentrations. The usage of CO2 sweeper derived materials emerges as a suitable technology for Carbon Capture and Storage (CCS). In this context, Transition Metal Carbides (TMCs) materials have been suggested as potential substrates for CO2 capture and activation due to the exothermic CO2 adsorption even at low partial pressures. Interestingly, the two-dimensional counterparts of TMCs, called MXenes, have shown an even better performance for CCS. These MXenes are synthetized by MAX phase precursors exfoliation techniques, thus depending on the MAX phases composition and demanding a certain energy to foster the layers separation. Indeed, such an exfoliation energy could be a descriptor to seize the resulting MXene chemical activity, here analyzed in depth through computational simulations, correlating the CO2 adsorption energies on MXenes with the estimates of exfoliation energies

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2020, Tutor: Francesc Viñes Solana, Carbon monoxide (CO) has been used as a probe molecule to study, typically, the chemical activity of Transition Metals (TMs) surfaces, but its study has been normally focused on those TMs with a face-centered cubic (fcc) crystalline structure, which are close in the Periodic Table, and so, perhaps, does not allow capturing reliable trends along the d series. Here we analyze such trends on hexagonal closed-pack (hcp) TMs by means of theoretical simulations, here relying on the accuracy of Density Functional Theory (DFT) using the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional using suitable slab models. The detailed analysis of the obtained results allows disentangling different trends, alongside with particular surface reconstructions and CO adsorption modes no previously reported in the literature. Those TMs with low number of valence d electrons adsorb the CO stronger, specially connecting its two atoms. While the number of d electrons increase, the CO changes to be adsorbed only through the C atom, up to being no adsorbed. This trend goes along quantitatively with the d-band centre descriptor model, but qualitatively with the surface energy or the C-metal distance ones. The bonding analysis reveals a donation/back-donation mechanism, which eventually weakens the CO bond order, and so, makes its bonds longer. This has a direct effect, e.g. on the simulated infrared (IR) spectra, implying lower frequencies for the CO bond stretching vibration, which can become invisible when the CO molecule adsorbs parallel to surface due to the surface dipole selection rule. Furthermore, the results on the CO dissociation energy reveals that such a metal surface catalysed process is thermodynamically favourable for earlier TMs, but grows endothermic when going along the d series

Two-dimensional porous MXenes with the formula M2C (M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W) are proposed as very promising sorbent materials for carbon dioxide (CO2) separation from methane (CH4) in the critical step of biogas upgrading. Density functional theory calculations including dispersion show that MXenes present very high CO2 uptakes and selectivities even under the most adverse working conditions.

Treballs Finals de Grau de Química, Facultat de Química, Universitat de Barcelona, Any: 2020, Tutor: Jordi Cirera, Since its discovery, spin-crossover compounds have attracted attention due to their intrinsic applications as molecular switches. These compounds have two electronic states which can be accessed with similar energies, causing that an external stimulus can determine which one becomes the predominant. Present work explores the spin-crossover properties of indenyl chromium (II) compound, as some of them exhibit thermally induced spin-crossover transition despite having a rather than unusual metallic centre in the field of spin-crossover properties. To do so, density functional theory and ab initio ligand field theory calculations are performed, with the aim to identify the trends caused by the influence of functionalization of indenyl ligand. The final goal of the present work is the design of new indenyl chromium (II) compounds with spin-crossover properties and tailored transition temperatures.

International audience; Light absorption or fluorescence excitation spectroscopy of alkali atoms attached to 4He droplets is investigated as a possible way for detecting the presence of vortices. To this end, we have calculated the equilibrium configuration and energetics of alkali atoms attached to a 4He1000 droplet hosting a vortex line using 4He density functional theory. We use them to study how the dipole absorption spectrum of the alkali atom is modified when the impurity is attached to a vortex line. Spectra are found to be blue-shifted (higher frequencies) and broadened compared to vortex-free droplets because the dimple in which the alkali atom sits at the intersection of the vortex line and the droplet surface is deeper. This effect is smaller for lighter alkali atoms and all the more so when using a quantum description since, in this case, they sit further away from the droplet surface on average due to their zero-point motion. Spectral modifications due to the presence of a vortex line are minor for np ← ns excitation and therefore insufficient for vortex detection. In the case of higher n′ p ← ns or n′ s ← ns (n′ > n) excitations, the shifts are larger as the excited state orbital is more extended and therefore more sensitive to changes in the surrounding helium density.

Nature has developed supramolecular constructs to deliver outstanding charge-transport capabilities using metalloporphyrin-based supramolecular arrays. Herein we incorporate simple, naturally inspired supramolecular interactions via the axial complexation of metalloporphyrins into the formation of a single-molecule wire in a nanoscale gap. Small structural changes in the axial coordinating linkers result in dramatic changes in the transport properties of the metalloporphyrin-based wire. The increased flexibility of a pyridine-4-yl-methanethiol ligand due to an extra methyl group, as compared to a more rigid 4-pyridinethiol linker, allows the pyridine-4-yl-methanethiol ligand to adopt an unexpected highly conductive stacked structure between the two junction electrodes and the metalloporphyrin ring. DFT calculations reveal a molecular junction structure composed of a shifted stack of the two pyridinic linkers and the metalloporphyrin ring. In contrast, the more rigid 4-mercaptopyridine ligand presents a more classical lifted octahedral coordination of the metalloporphyrin metal center, leading to a longer electron pathway of lower conductance. This works opens to supramolecular electronics, a concept already exploited in natural organisms., Angewandte Chemie. International Edition, 59 (43), ISSN:1433-7851, ISSN:1521-3773, ISSN:0570-0833