1,721,126 research outputs found
Automation of chemical kinetics: Status and challenges
No full textDriven by synergic advancements in high performance computing and theory, the capability to estimate rate constants from first principles has evolved considerably recently. When this knowledge is coupled with a procedure to determine a list of all reactions relevant to describe the evolution of a reacting system, it becomes possible to envision a methodology to predict theoretically the reaction kinetics. However, if a thorough examination of all possible reaction channels is desired, the number of reactions for which a rate constant estimate is needed can become quite large. This determines the need for rate constant estimation automation. In the present work, the status of this rapidly evolving field is reviewed, with emphasis on recent advancements and present challenges. Thermochemistry is the field where automation is most advanced. Entropies, heat capacities, and enthalpies can be determined efficiently with accuracy comparable to experiments for most chemical species containing a limited number of atoms, while machine learning can be used to improve the computational predictions for large chemical species using reduced computational resources. Several approaches have been proposed to automatically investigate the reactivity over complex potential energy surfaces, while rate constants for elementary steps can be determined accurately for several reaction classes, such as abstraction, addition, beta-scission, and isomerization. Kinetic mechanisms can be automatically generated using methodologies that differ for level of complexity and required physical insight. Among the challenges that are still to be met are the estimation of rate constants for intrinsically multireference reaction classes, such as barrierless processes, the containment of the number of reactions to screen in mechanism development, and the integration of the existing automated software. It is suggested that the synergy between experiment and theory should evolve towards a stage where experiments are focused on the estimation of parameters where theoretical tools are least predictive, and vice versa
Ultrastructural age-related changes in the sensory corpuscles of the human genital skin.
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Le terminazioni nervose sensitive cutanee: dati morfologici, immuno-chimici e funzionali.
No full textH-abstractions by O2, NO2, NH2, and HO2 from H2NO: Theoretical study and implications for ammonia low-temperature kinetics
Full text linkUnraveling the low-temperature chemistry of ammonia is still an open challenge in combustion kinetics, yet of primary importance because of the novel combustion concepts operating in these conditions, as well as of the rising interest on ammonia as an energy carrier. In this work, a fundamental investigation of the H-abstraction reactions from H2NO by O2, NO2, NH2, and HO2 was performed. These reactions, which belong to the radical-radical abstraction class, associate a high sensitivity to the key low temperature ammonia combustion parameters, to a high uncertainty in rate constant values. Theoretically, the investigation of reactions belonging to this class is complicated by their intrinsic multireference nature. To address this issue, a structured theoretical methodology that relies heavily on the use of CASPT2 calculations was devised. The predicted rate constants highlighted significant deviations from the rates commonly adopted in the state-of-the-art mechanisms, most often based on analogies and estimations. In order to understand their impact on ammonia low-temperature kinetics, the obtained rates were integrated into a kinetic model, which was used to investigate ammonia oxidation and ignition at low-temperature and oxygen-rich conditions. It was found that O2 and NH2 play the major role, as abstractors, in regulating ammonia oxidation and ignition. In particular, ignition delay time predictions proved extremely sensitive to the adopted rates: modifying each of them within their theoretical uncertainty caused deviations by even an order of magnitude, and totally changed the predictive features of the mechanism. The kinetic analysis highlighted then the need of a targeted optimization of the critical rates, downstream of the present work and within their uncertainty boundaries, to further refine the mechanism capability over a wide range of operating conditions
Quantification of adrenergic nerve fibers in human thymus at various ages
No full textAdrenergic nerve fibers were studied in human thymus at various ages by biochemical and morphological methods and by quantitative analysis of images. The whole thymus was harvested during autopsies in prenatal of six months (n=6), newborn (n=6), infant (n=3), young (n=3), adult (n=3) and elderly (n=3) men. The thymuses were weighed, dissected and studied. Thymic slices were subjected to specific staining for the detection of microanatomical details regarding thymic microenvironment and nerve fibers. Histofluorescence microscopy was used for staining of adrenergic nerve fibers, while immunofluorescence microscopy was employed for staining of neuropeptide Y-like immunoreactivity. Biochemical dosage of proteins and of noradrenaline amount was performed. All morphological results were subjected to the quantitative analysis of images. Our results confirmed that many changes occur with age in the whole weight of the thymus and in thymic protein content and, as a new finding, demonstrated the specific pattern of three independent markers able to characterize sympathetic nerve fibers of the human thymus and its modifications with age: neuropeptide Y-related staining decreases with age, whereas the density of adrenergic nerve fibers remains unchanged. The apparent discrepancy among these three independent markers for sympathetic nerves was discussed. (author abst.
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