Letteratura scientifica selezionata sul tema "Molecular beams. Potential energy surfaces. Chemical kinetics"
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Articoli di riviste sul tema "Molecular beams. Potential energy surfaces. Chemical kinetics"
1
Stevenson, K. P., J. D. Close, P. L. Muiño e R. O. Watts. "Potential Energy Surfaces for Unsaturated Hydrocarbons from Crossed Molecular Beams". Australian Journal of Physics 50, n. 3 (1997): 683. http://dx.doi.org/10.1071/p96079.
Testo completoAbstract (sommario):
The total differential scattering cross sections for several important unsaturated hydrocarbon molecules with common atmospheric gases were measured in a crossed molecular beam apparatus. The experiments show quantum interferences which relate to potential energy surface parameters, such as the well depth and radial minimum. The damping of the quantum features, over contributions from experimental resolutions, provides information on the angular and radial anisotropies present in the potential energy surfaces. We have investigated two areas: (1) the role of the probe partner in determining the interaction strength for a given hydrocarbon target, and (2) the effect of increasing the overall length of the hydrocarbon molecule for a fixed probe. By comparing results for a class of scattering systems, we can identify chemical and physical trends that determine the van der Waals potential energy surfaces of larger molecules. We expect these results to aid in the prediction and interpretation of complementary experimental measurements on the high resolution infrared spectroscopy of weakly bound complexes.
2
Yuan, Daofu, Yafu Guan, Wentao Chen, Hailin Zhao, Shengrui Yu, Chang Luo, Yuxin Tan et al. "Observation of the geometric phase effect in the H + HD → H2+ D reaction". Science 362, n. 6420 (13 dicembre 2018): 1289–93. http://dx.doi.org/10.1126/science.aav1356.
Testo completoAbstract (sommario):
Theory has established the importance of geometric phase (GP) effects in the adiabatic dynamics of molecular systems with a conical intersection connecting the ground- and excited-state potential energy surfaces, but direct observation of their manifestation in chemical reactions remains a major challenge. Here, we report a high-resolution crossed molecular beams study of the H + HD → H2+ D reaction at a collision energy slightly above the conical intersection. Velocity map ion imaging revealed fast angular oscillations in product quantum state–resolved differential cross sections in the forward scattering direction for H2products at specific rovibrational levels. The experimental results agree with adiabatic quantum dynamical calculations only when the GP effect is included.
3
Kholodar, Svetlana A., Ananda K. Ghosh, Katarzyna Świderek, Vicent Moliner e Amnon Kohen. "Parallel reaction pathways and noncovalent intermediates in thymidylate synthase revealed by experimental and computational tools". Proceedings of the National Academy of Sciences 115, n. 41 (24 settembre 2018): 10311–14. http://dx.doi.org/10.1073/pnas.1811059115.
Testo completoAbstract (sommario):
Thymidylate synthase was one of the most studied enzymes due to its critical role in molecular pathogenesis of cancer. Nevertheless, many atomistic details of its chemical mechanism remain unknown or debated, thereby imposing limits on design of novel mechanism-based anticancer therapeutics. Here, we report unprecedented isolation and characterization of a previously proposed intact noncovalent bisubstrate intermediate formed in the reaction catalyzed by thymidylate synthase. Free-energy surfaces of the bisubstrate intermediates interconversions computed with quantum mechanics/molecular mechanics (QM/MM) methods and experimental assessment of the corresponding kinetics indicate that the species is the most abundant productive intermediate along the reaction coordinate, whereas accumulation of the covalent bisubstrate species largely occurs in a parallel nonproductive pathway. Our findings not only substantiate relevance of the previously proposed noncovalent intermediate but also support potential implications of the overstabilized covalent intermediate in drug design targeting DNA biosynthesis.
4
Zhang, Shen, Yue Liang, Xiangqun Qian, David Hui e Kuichuan Sheng. "Pyrolysis kinetics and mechanical properties of poly(lactic acid)/bamboo particle biocomposites: Effect of particle size distribution". Nanotechnology Reviews 9, n. 1 (6 giugno 2020): 524–33. http://dx.doi.org/10.1515/ntrev-2020-0037.
Testo completoAbstract (sommario):
AbstractBamboo particle (BP)-reinforced poly(lactic acid) (PLA) biocomposites were fabricated. The effect of the BP particle size distribution on the pyrolysis and mechanical properties of PLA biocomposites was evaluated. The optimum particle size of BP for improving the tensile strength PLA biocomposites is 200 mesh (16.6–84.5 µm). The pyrolysis mechanism and kinetics were studied according to the Coats–Redfern method. The addition of BP inhibited the pyrolysis process of PLA. The activation energy of biocomposites ranged from 120.7 to 151.5 kJ/mol, which is significantly higher than that of the neat PLA. The pyrolysis mechanisms of biocomposites are attributed to the chemical reaction at low pyrolysis temperature (270–400℃) and ash layer diffusion control at high pyrolysis temperature (400–600℃). Crystallization behavior of biocomposites showed that small BPs in PLA biocomposites generated more cross-linking points in the PLA matrix, which constrained the movement of the molecular chain and acted as an effective nucleating agent in promoting the crystallization process. The pyrolysis behavior and mechanical properties analysis provide critical information for potential large-scale production of the PLA biocomposites.
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Gratzfeld, Dennis, Juliane Heitkämper, Julien Debailleul e Matthias Olzmann. "On the influence of water on urea condensation reactions: a theoretical study". Zeitschrift für Physikalische Chemie 234, n. 7-9 (27 agosto 2020): 1311–27. http://dx.doi.org/10.1515/zpch-2020-1658.
Testo completoAbstract (sommario):
AbstractThe influence of water molecules on the kinetics of urea condensation reactions was studied with high-level quantum chemical methods and statistical rate theory. The study focuses on the production of biuret, triuret, and cyanuric acid from urea because of their relevance as unwanted byproducts in the urea-based selective catalytic reduction (urea-SCR) exhaust after treatment of Diesel engines. In order to characterize the potential energy surfaces and molecular reaction pathways, calculations with explicitly-correlated coupled-cluster methods were performed. It turned out that the reactions proceed via pre-reactive complexes and the inclusion of one or two water molecules into the condensation mechanisms leads to a decrease of the energy barriers. This effect is particularly pronounced in the production of biuret. Due to the pre-reactive equilibria, the rates of the overall reactions can increase or decrease by incorporating water into the mechanism, depending on the temperature and water concentration. Under the conditions of urea-SCR, the studied reactions are too slow to contribute to the observed byproduct formation.
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Nakamura, Masaaki, Po-Yu Tsai, Toshio Kasai, King-Chuen Lin, Federico Palazzetti, Andrea Lombardi e Vincenzo Aquilanti. "Dynamical, spectroscopic and computational imaging of bond breaking in photodissociation: roaming and role of conical intersections". Faraday Discussions 177 (2015): 77–98. http://dx.doi.org/10.1039/c4fd00174e.
Testo completoAbstract (sommario):
Recent experimental and theoretical advances in the study of the dissociation of excited molecules are revealing unexpected mechanisms, when their outcomes are tackled by combining (i) space-time ion imaging of translational features, with (ii) spectroscopic probing of rotational and vibrational distributions; crucial is the assistance of (iii) the quantum chemistry of structural investigations of rearrangements of chemical bonds, and of (iv) the simulations of molecular dynamics to follow the evolution of selective bond stretching and breaking. Here we present results of such an integrated approach to methyl formate, HCOOCH3, the simplest of esters; the main focus is on the rotovibrationally excited CO (v = 1) product and in general on the energy distribution in the fragments. Previous laser studies of dissociation into CO and CH3OH at a sequence of various wavelengths discovered signatures of a roaming mechanism by the late arrival of CO (v = 0) products in time-of-flight ion imaging. Subsequent detailed investigations as a function of excitation energy provided the assessment of the threshold, which opens for triple breakdown into CO and further fragments H and CH3O, as spectroscopically characterized by ion imaging and FTIR respectively. Accompanying quantum mechanical electronic structure calculations and classical molecular dynamics simulations clarify the origin of these fragments through “roaming” pathways involving incipient radical intermediates at energies below the triple fragmentation threshold: a specific role is played by nonadiabatic transitions at a conical intersection between ground and excited states; alternative pathways focalize our attention to regions of the potential energy surfaces other than those in the neighbourhoods of saddle points along minimum energy paths: eventually this leads us to look for avenues in reaction kinetics beyond those of venerable transition state theories.
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Krok-Borkowicz, Małgorzata, Katarzyna Reczyńska, Łucja Rumian, Elżbieta Menaszek, Maciej Orzelski, Piotr Malisz, Piotr Silmanowicz, Piotr Dobrzyński e Elżbieta Pamuła. "Surface-Modified Poly(l-lactide-co-glycolide) Scaffolds for the Treatment of Osteochondral Critical Size Defects—In Vivo Studies on Rabbits". International Journal of Molecular Sciences 21, n. 20 (13 ottobre 2020): 7541. http://dx.doi.org/10.3390/ijms21207541.
Testo completoAbstract (sommario):
Poly(l-lactide-co-glycolide) (PLGA) porous scaffolds were modified with collagen type I (PLGA/coll) or hydroxyapatite (PLGA/HAp) and implanted in rabbits osteochondral defects to check their biocompatibility and bone tissue regeneration potential. The scaffolds were fabricated using solvent casting/particulate leaching method. Their total porosity was 85% and the pore size was in the range of 250–320 µm. The physico-chemical properties of the scaffolds were evaluated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), sessile drop, and compression tests. Three types of the scaffolds (unmodified PLGA, PLGA/coll, and PLGA/HAp) were implanted into the defects created in New Zealand rabbit femoral trochlears; empty defect acted as control. Samples were extracted after 1, 4, 12, and 26 weeks from the implantation, evaluated using micro-computed tomography (µCT), and stained by Masson–Goldner and hematoxylin-eosin. The results showed that the proposed method is suitable for fabrication of highly porous PLGA scaffolds. Effective deposition of both coll and HAp was confirmed on all surfaces of the pores through the entire scaffold volume. In the in vivo model, PLGA and PLGA/HAp scaffolds enhanced tissue ingrowth as shown by histological and morphometric analyses. Bone formation was the highest for PLGA/HAp scaffolds as evidenced by µCT. Neo-tissue formation in the defect site was well correlated with degradation kinetics of the scaffold material. Interestingly, around PLGA/coll extensive inflammation and inhibited tissue healing were detected, presumably due to immunological response of the host towards collagen of bovine origin. To summarize, PLGA scaffolds modified with HAp are the most promising materials for bone tissue regeneration.
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Sharifi, Maryam Seyed, Hamed Douroudgari e Morteza Vahedpour. "Chemical insights into the atmospheric oxidation of thiophene by hydroperoxyl radical". Scientific Reports 11, n. 1 (22 giugno 2021). http://dx.doi.org/10.1038/s41598-021-92221-z.
Testo completoAbstract (sommario):
AbstractThe reaction mechanisms and kinetics of thiophene oxidation reactions initiated by hydroperoxyl radical, and decomposition of the related intermediates and complexes, have been considered herein by using high-level DFT and ab initio calculations. The main energetic parameters of all stationary points of the suggested potential energy surfaces have been computed at the BD(T) and CCSD(T) methods, based on the geometries optimized at the B3LYP/6-311 + g(d,p) level of theory. Rate constants of bimolecular reactions (high-pressure limit rate constants) at temperatures from 300 to 3000 K for the first steps of the title reaction have been obtained through the conventional transition state theory (TST), while the pressure dependent rate constants and the rate constants of the second and other steps have been calculated employing the Rice–Ramsperger–Kassel–Marcus/Master equation (RRKM/ME). The results show that the rate constants of addition to α and β carbons have positive temperature dependence and negative pressure dependence. It is found that the additions of HO2 to the α and β carbons of thiophene in the initial steps of the title reaction are the most favored pathways. Also, the addition to the sulfur atom has a minor contribution. But, all efforts for simulating hydrogen abstraction reactions have been unsuccessful. In this complex oxidation reaction, about 12 different products are obtained, including important isomers such as thiophene-epoxide, thiophene-ol, thiophene-oxide, oxathiane, and thiophenone. The calculated total rate constants for generation of all minimum stationary points show that the addition reactions to the α and β carbons are the fastest among all at temperatures below 1000 K, while the proposed multi-step parallel reactions are more competitive at temperatures above 1200 K. Furthermore, important inter-and intra-molecular interactions for some species have been investigated by two well-known quantum chemistry method, the NBO and AIM analyses. Thermochemical properties such as free energy, enthalpy, internal energy, and entropy for thiophene and hydroperoxyl radical and related species in the simulated reactions have been predicted using a combination of the B3LYP and BD(T) methods.
Tesi sul tema "Molecular beams. Potential energy surfaces. Chemical kinetics"
1
Reeves, Christopher Thomas. "Kinetics and dynamics of adsorption on single crystal semiconductor and metal surfaces". Access restricted to users with UT Austin EID Full text (PDF) from UMI/Dissertation Abstracts International, 2001. http://wwwlib.umi.com/cr/utexas/fullcit?p3035962.
Testo completoCapitoli di libri sul tema "Molecular beams. Potential energy surfaces. Chemical kinetics"
1
Jouvet, C., e D. Solgadi. "Photochemistry of van der Waals Complexes and Small Clusters". In Chemical Reactions in Clusters. Oxford University Press, 1996. http://dx.doi.org/10.1093/oso/9780195090048.003.0007.
Testo completoAbstract (sommario):
In a chemical reaction, the shape of the potential energy surface (PES) dictates the reaction rate and energy disposal in the products. Not only does the dynamics depend crucially upon the features of the surface, but, ultimately one seeks to influence the course of the reaction by preparing selectively certain regions of the surface. For harpooning reactions, the propensity rules for energy disposal in the products (influence of the entrance kinetic energy, effect of the early or late barrier) have been established by Polanyi (1972) and have been used later as guidelines. Here, the surface may easily be modeled in simple terms using long-range electrostatic interaction in the entrance valley. There was, then, need of an experimental method which allows the possibility of observing directly the characteristic regions of this potential energy surface, but also to investigate precisely the surface in other types of reaction. The study of the reactivity of van der Waals complexes is intended to fulfil this purpose. In classical experiments, the surface is obtained by inversion of the experimental data which are differential cross sections and internal energy distribution of the products. This procedure is difficult and not unambiguous. The first step is to determine the correlation between the entrance channel's parameters (kinetic energy, internal energy, angular momentum) and the final states of the products (kinetic energy, internal energy, angular distribution). This requires a precise control of the entrance channel. Therefore, the goal of many experiments is to reduce the initial states to a small subset, and to measure the energy disposal in the products with the greatest accuracy. This was first achieved by controlling the kinetic energy of the reactants in crossed beam experiments. Later, a certain control of the collision geometry was obtained by orienting the molecules or the atomic orbitals in crossed beam experiments or by using prealigned systems in a van der Waals complex: this subject is discussed in Buelow et al. (1986).