DFT Study of Nitroxide Radicals. 1. Effects of solvent on structural and electronic characteristics of 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl

Imidazoline-based nitroxide radicals are often used as spin probes for medium acidity and polarity in different systems. In this work, using the density functional theory (DFT) approach, we have studied how physico-chemical characteristics (geometry, atomic charges and electron spin density distribution) of pH-sensitive spin label 4-amino-2,2,5,5-tetramethyl-3-imidazoline-N-oxyl (ATI) depend on protonation and aqueous surroundings. Our calculations demonstrate that ATI protonation should occur at the nitrogen atom of the imidazoline ring rather than at the amino group. Protonation of ATI leads to a decrease in a spin density on the nitrogen atom of the nitroxide fragment >N-O. For simulation of ATI hydration effects, we have constructed a water shell around a spin label molecule by means of gradual (step-by-step) surrounding of ATI with water molecules (n = 2-41). Calculated spin density on the nitrogen atom of the nitroxide fragment increased with an extension of a water shell around ATI. Both protonation and hydration of ATI caused certain changes in calculated geometric parameters (bond lengths and valence angles). Investigating how structural and energy parameters of a system ATI-(H2O)n depend on a number of surrounding water molecules, we came to the conclusion that a hydrogen-bonded cluster of n ≥ 41 water molecules could be considered as an appropriate model for simulation of ATI hydration effects.Comment: 30 pages, 11 figures, 6 table

Development of the Metal Rheology Model of High-temperature Deformation for Modeling by Finite Element Method

It is shown that when modeling the processes of forging and stamping, it is necessary to take into account not only the hardening of the material, but also softening, which occurs during hot processing. Otherwise, the power parameters of the deformation processes are precisely determined, which leads to the choice of more powerful equipment. Softening accounting (processes of stress relaxation) will allow to accurately determine the stress and strain state (SSS) of the workpiece, as well as the power parameters of the processes of deformation. This will expand the technological capabilities of these processes. Existing commercial software systems for modeling hot plastic deformations based on the finite element method (FEM) do not allow this. This is due to the absence in these software products of the communication model of the component deformation rates and stresses, which would take into account stress relaxation. As a result, on the basis of the Maxwell visco-elastic model, a relationship is established between deformation rates and stresses. The developed model allows to take into account the metal softening during a pause after hot deformation. The resulting mathematical model is tested by experiment on different steels at different temperatures of deformation. The process of steels softening is determined using plastometers. It is established experimentally that the model developed by 89 ... 93 % describes the rheology of the metal during hot deformation. The relationship between the components of the deformation rates and stresses is established, which allows to obtain a direct numerical solution of plastic deformation problems without FED iterative procedures, taking into account the real properties of the metal during deformation. As a result, the number of iterations and calculations has significantly decreased

Effect of contact with an elastic wall on the spectral characteristics of the scattered echo of a contrast microbubble

International audienceA modified Rayleigh-Plesset equation is proposed to model the oscillation of a contrast microbubble attached to an elastic wall. The equation shows that contact with the wall affects the bubble oscillation as if the bubble oscillated in a liquid with a changed (effective) density. As a result, depending on the wall properties, the natural frequency of the attached bubble can be either lower or higher than the natural frequency of the same bubble in an unbounded liquid. Numerical simulations were made to study the spectral characteristics of the scattered pressure of an attached contrast microbubble. It was assumed that the bubble shell properties are described by the Marmottant model and the properties of the wall correspond to walls of OptiCell chambers commonly used in experiments. It has been found that, depending on the value of the driving frequency, the contact with the wall can noticeably increase or decrease the magnitudes of the fundamental component, the second harmonic, and the subharmonic relatively to their values in an unbounded liquid. These findings can be used to distinguish the scattered echoes produced by contrast agents attached to a wall from echoes produced by agents being at a distance from a wall

Effect of an elastic wall on the behavior of encapsulated microbubbles

International audienceThe aim of this work is to study how boundaries with different mechanical properties affect the acoustic response of contrast agent microbubbles. To this end, numerical simulations are performed for two types of walls: a polystyrene (OptiCell) wall and a biological tissue. For each wall, the behavior of contrast microbubbles of three sizes is investigated. The spectral characteristics of the scattered pressure produced by the microbubbles are compared for two cases: the bubble oscillates far away from the wall and the same bubble oscillates in the immediate vicinity of the wall. The results of the simulations allow one to draw the following main conclusions. The effect of the OptiCell wall on the acoustic bubble response is stronger than that of the tissue wall. Changes in the bubble response near the wall are stronger when bubbles are excited above their fundamental resonance frequency. Changes are stronger for smaller bubbles and changes in the 2nd harmonic are stronger than those in the fundamental. The results obtained allow one to gain an insight into conditions under which the effect of an elastic wall on the acoustic response of a contrast agent microbubble is easier to be detected

GIS Ordination Approach to Model Distribution of Shrub Species in Northern Utah

Anthropogenic and natural disturbances represent a serious threat to natural ecosystems dominated by big sagebrush (Artemisia tridentata). Conservation efforts aim to restore original species composition and prevent the invasion of undesirable species. In order to restore the historic plant communities, we need a clear understanding of how species compositions are distributed along environmental gradients. Species ordination is a process of placing plant species along environmental gradients. This study was conducted in Rich County, Utah, where substantial changes in species composition have been documented in recent years. Field data, literature review, multivariate analyzes, GIS and remote sensing techniques, and expert knowledge were used to define environmental variables and their respective suitability ranges of where shrub species may occur along this area. Ordination and CART- statistical analyzes were used to estimate and predict suitability of shrub species along environmental gradients. GIS procedures were used to spatially predict species distribution. Field data and the Southwest Regional Gap Analysis Project data provided useful information to build the model and 20 percent of field data was withheld to cross-validate the findings. Final results showed that the shrub species distribution in the rangelands of Northern Utah, specifically Rich County, might be driven by precipitation and temperature gradients -influenced greatly by elevation. Slope contributing area, NDVI, and solar radiation were statistically significant factors explaining shrub distribution. To our perception, soil moisture availability might be the most explanatory variable behind these findings. In the model validation, the Kappa coefficient was K = 61.3 percent and the overall model accuracy was 74 percent. The location of species distribution areas, in the final map, can be useful to managers in order to define where resources might be allocated to preserve and restore these native rangeland ecosystems

Multicultural and Social Justice Training in Doctoral Counseling Programs: A Phenomenological Study

Multicultural and social justice are central values of the counseling profession. However, it is unclear how doctoral training advances counselor education students\u27 multicultural and social justice competence and what deficits might exist. The researchers addressed this gap by conducting a phenomenological study of multicultural and social justice training experiences with ten doctoral counselor education students. The researchers identified three themes: limited multicultural integration, methods of compensation, and experiencing growth. The researchers also identified subthemes that explained how doctoral students prepared themselves to be multicultural and social justice competent through self-study, collaborative learning, and role-taking experiences despite often perceiving gaps in their coursework. Finally, the researchers discuss implications and suggestions for future research

Micromechanical model for isolated polymer-colloid clusters under tension

Binary polymer-colloid (PC) composites form the majority of biological load-bearing materials. Due to the abundance of the polymer and particles, and their simple aggregation process, PC clusters are used broadly by nature to create biomaterials with a variety of functions. However, our understanding of the mechanical features of the clusters and their load transfer mechanism is limited. Our main focus in this paper is the elastic behavior of close-packed PC clusters formed in the presence of polymer linkers. Therefore, a micromechanical model is proposed to predict the constitutive behavior of isolated polymer-colloid clusters under tension. The mechanical response of a cluster is considered to be governed by a backbone chain, which is the stress path that transfers most of the applied load. The developed model can reproduce the mean behavior of the clusters and is not dependent on their local geometry. The model utilizes four geometrical parameters for defining six shape descriptor functions which can affect the geometrical change of the clusters in the course of deformation. The predictions of the model are benchmarked against an extensive set of simulations by coarse-grained-Brownian dynamics, where clusters with different shapes and sizes were considered. The model exhibits good agreement with these simulations, which, besides its relative simplicity, makes the model an excellent add-on module for implementation into multiscale models of nanocomposites.National Science Foundation (U.S.) (CAREER Award DMR-1054671)MIT International Science and Technology Initiatives (MISTI-Germany)United States. Dept. of Transportation. UTC Center for Highway Pavement Preservatio

Hydrological and associated biogeochemical consequences of rapid global warming during the Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene Thermal Maximum (PETM) hyperthermal, ~ 56 million years ago (Ma), is the most dramatic example of abrupt Cenozoic global warming. During the PETM surface temperatures increased between 5 and 9 °C and the onset likely took < 20 kyr. The PETM provides a case study of the impacts of rapid global warming on the Earth system, including both hydrological and associated biogeochemical feedbacks, and proxy data from the PETM can provide constraints on changes in warm climate hydrology simulated by general circulation models (GCMs). In this paper, we provide a critical review of biological and geochemical signatures interpreted as direct or indirect indicators of hydrological change at the PETM, explore the importance of adopting multi-proxy approaches, and present a preliminary model-data comparison. Hydrological records complement those of temperature and indicate that the climatic response at the PETM was complex, with significant regional and temporal variability. This is further illustrated by the biogeochemical consequences of inferred changes in hydrology and, in fact, changes in precipitation and the biogeochemical consequences are often conflated in geochemical signatures. There is also strong evidence in many regions for changes in the episodic and/or intra-annual distribution of precipitation that has not widely been considered when comparing proxy data to GCM output. Crucially, GCM simulations indicate that the response of the hydrological cycle to the PETM was heterogeneous – some regions are associated with increased precipitation – evaporation (P – E), whilst others are characterised by a decrease. Interestingly, the majority of proxy data come from the regions where GCMs predict an increase in PETM precipitation. We propose that comparison of hydrological proxies to GCM output can be an important test of model skill, but this will be enhanced by further data from regions of model-simulated aridity and simulation of extreme precipitation events