Butene Isomerization on Palladium Surfaces: Time-Dependent Monte Carlo Studies

A new time-dependent Monte Carlo approach, tdMC, is presented. This allows one to manage the quantum-chemical information relating to surface catalytic processes and rationalize, with atomistic dynamical perspectives, the corresponding reaction mechanism by providing descriptors that can be compared with experimentally obtained data. The approach, which falls into the more general microkinetic paradigm, is strictly self-consistent as it exploits information framed in just one computational method based on the density functional theory. The results simulated by the tdMC algorithm concern the isomerization of but-1-ene to cis- and trans-but-2-ene on Pd surfaces. This reaction was chosen mainly to focus on the development and implementation of the model as well as to point out the characteristics of the code and the soundness of the approach. In order to reach these goals, the simulated findings were compared to related experimental and computational literature data. From the study, it clearly emerges that the tdMC approach, although conceptually very straightforward and simple, is flexible enough to pinpoint the main characteristics of the reaction, which is just seemingly elementary and conversely governed by a complex mechanism involving, besides isomerization, even hydrogenation and dehydrogenation processes. Noticeably, new insights into the title reaction were also provided by the proposed approach

DFT insights into competing mechanisms of guaiacol hydrodeoxygenation on a platinum cluster

In a scenario of declining fossil resources and increasing demand for renewable and sustainable alternatives, biomass is the only source able to offer an easy and gradual transition in the use of current energy technologies based on the exploitation of carbon derivatives. Its conversion to liquid fuels has oriented our study towards the computational mechanistic analysis of the guaiacol catalytic hydrodeoxygenation, which is currently considered one of the most challenging routes for upgrading biomass-derived bio-oils. For this purpose, a subnanometric Pt10 platinum cluster was chosen as the catalyst model, with Pt as a computational reference element for catalytic hydrogenation, and guaiacol as a model compound of bio-oils. DFT calculations revealed that the energy barriers related to the cleavage of C(sp2)-O bonds in the direct deoxygenation mechanism are significantly lower (by an average of 60 kJ mol−1) than those in the deoxygenation-through-hydrogenation mechanism in which C(sp3)-O bond breaking from a saturated ring occurs. Even if the ring hydrogenation is easier in the oxygenated compound, the analysis reveals that the direct deoxygenation mechanism is favoured at all temperatures. Furthermore, the results obtained highlight that, from a thermodynamic perspective, the removal of oxygen groups preferentially occurs by the elimination of the -OCH3 fragment as methanol and then of the -OH fragment as a water molecule

DFT study on zeolites’ intrinsic Brønsted acidity: The case of BEA

Since Brønsted acidity is a crucial aspect for the applications of zeolitic materials in heterogeneous catalysis, great effort was devolved to characterize the number, strength and location of the potentially active acidic sites. Quantum chemical calculations can turn out essential in estimating the intrinsic acidity by computing deprotonation energy (DPE) values, although each method comes with its own difficulties. In this context, three approaches within density functional theory were employed to study the intrinsic acidity of 30 topologically distinct Brønsted sites in the β-zeolite framework. Advantages and disadvantages of the three methods were outlined and the acidity order between the sites was assessed, being the DPE range 59 kJ mol−1 wide, with the proposed best approach. By dividing the range into three portions, the sites were classified as having high, medium and low acidity. Hydrogen bonds formation was found to be a contributing factor in determining a low Brønsted acidity

DFT Study of Pt Particle Growth inside β-Zeolite Cages

The preferred location and the corresponding energetics of zeolite-embedded single metal atoms and small metal particles are hot topics within active site optimization and catalyst tuning, even as part of bifunctional materials design. In this context, periodic density functional theory was used to provide insights on the interactions of a platinum atom with the microporous cages of a purely silicious β-zeolite (BEA) framework. Cluster growth was subsequently addressed, up to Pt3@BEA systems, following a one-by-one platinum atom addition; platinum migration between cages was taken into account as well. An unbiased approach was employed, which allowed a wide panorama of structures being considered in addition to a thorough analysis in terms of energetics, cluster geometries, and cavity distortions. Calculations revealed that the optimal interaction geometry for a single platinum atom is realized where two strong Pt-O bonds in almost linear arrangement can form, regardless of the cavity involved. This can cause distortions or even breaking of the zeolite structure, a factor which however is not decisive in determining the energetics of systems with two and three platinum atoms. Platinum migration is associated with energy barriers ranging from 100 to 200 kJ mol-1, depending on the cages. Up to the dimensions considered here, preference for clustering is observed, being the embedded Pt3 systems in almost all cases energetically favored with respect to isolated atoms within the BEA framework

Some analytic results for two-loop scattering amplitudes

We present analytic results for the finite diagrams contributing to the two-loop eight-point MHV scattering amplitude of planar N=4 SYM. We use a recently proposed representation for the integrand of the amplitude in terms of (momentum) twistors and focus on a restricted kinematics in which the answer depends only on two independent cross-ratios. The theory of motives can be used to vastly simplify the results, which can be expressed as simple combinations of classical polylogarithms.Comment: 18 page

Exploring the role of the histidine biosynthetic hisF gene in cellular metabolism and in the evolution of (ancestral) genes: from LUCA to the extant (micro)organisms

Histidine biosynthesis is an ancestral pathway that was assembled before the appearance of the Last Universal Common Ancestor; afterwards, it remained unaltered in all the extant histidine-synthesizing (micro)organisms. It is a metabolic cross-road interconnecting histidine biosynthesis to nitrogen metabolism and the de novo synthesis of purines. This interconnection is due to the reaction catalyzed by the products of hisH and hisF genes. The latter gene is an excellent model to study which trajectories have been followed by primordial cells to build the first metabolic routes, since its evolution is the result of different molecular rearrangement events, i.e. gene duplication, gene fusion, gene elongation, and domain shuffling. Additionally, this review summarizes data concerning the involvement of hisF and its product in other different cellular processes, revealing that HisF very likely plays a role also in cell division control and involvement in virulence and nodule development in different bacteria. From the metabolic viewpoint, these results suggest that HisF plays a central role in cellular metabolism, highlighting the interconnections of different metabolic pathways

A computational and experimental investigation of the anchoring of organosilanes on the halloysite silicic surface

In this work, the effect of halloysite nanotubes alkali activation on its grafting efficiency with organosilanes was studied by Density Functional Theory and experimental investigations. In particular, computational analysis allowed to enlight the structural properties of the organic molecules attached to the silanol groups on halloysite outer surface. The energetics of the reactions showed that the pretreatment with a base is crucial for the modification of the surface due to the appearance of a high number of active sites which lead to thermodynamically favored exothermic processes. Experimental evidences are in good agreement with calculation hypothesis. For instance, the coating efficiency is higher after the alkali activation of the inorganic counterpart for both the investigated organosilanes. The findings here reported are important in order to improve any functionalization protocols for aluminosilicates without variations or loss of the hollow nanotubular morphological features and it paves the ground to halloysite based technological applications in many fields, from nanotechnology to catalysis

The role of gene elongation in the evolution of histidine biosynthetic genes

Gene elongation is a molecular mechanism consisting of an in-tandem duplication of a gene and divergence and fusion of the two copies, resulting in a gene constituted by two divergent paralogous modules. The aim of this work was to evaluate the importance of gene elongation in the evolution of histidine biosynthetic genes and to propose a possible evolutionary model for some of them. Concerning the genes hisA and hisF, which code for two homologous (β/α)8-barrels, it has been proposed that the two extant genes could be the result of a cascade of gene elongation/domain shuffling events starting from an ancestor gene coding for just one (β/α) module. A gene elongation event has also been proposed for the evolution of hisB and hisD; structural analyses revealed the possibility of an early elongation event, resulting in the repetition of modules. Furthermore, it is quite possible that the gene elongations responsible for the evolution of the four proteins occurred before the earliest phylogenetic divergence. In conclusion, gene elongation events seem to have played a crucial role in the evolution of the histidine biosynthetic pathway, and they may have shaped the structures of many genes during the first steps of their evolution

VP36.15: Evaluation of uterine artery pulsatility index and 17β estradiol serum concentration in first trimester pregnancies with oocyte donation

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