scholarly journals Selenium– and tellurium–halogen reagents

2018 ◽  
Vol 3 (12) ◽  
Author(s):  
Tristram Chivers ◽  
Risto S. Laitinen
Keyword(s):  
Oxidation State ◽  
Halogen Bond ◽  
The Other ◽  
Oxidation States ◽  
Alkyl Aryl ◽  
Organic Selenium ◽  
Formal Oxidation State ◽  
Wide Range ◽  
Inorganic And Organic

Abstract Selenium and tellurium form binary halides in which the chalcogen can be in formal oxidation states (IV), (II) or (I). They are versatile reagents for the preparation of a wide range of inorganic and organic selenium and tellurium compounds taking advantage of the reactivity of the chalcogen–halogen bond. With the exception of the tetrafluorides, the tetrahalides are either commercially available or readily prepared. On the other hand, the low-valent species, EX2 (E = Se, Te; X = Cl, Br) and E2X2 (E = Se, Te; X = Cl, Br) are unstable with respect to disproportionation and must be used as in situ reagents. Organoselenium and tellurium halides are well-known in oxidation states (IV) and (II), as exemplified by REX3, R2EX2 and REX (R = alkyl, aryl; E = Se, Te; X = F, Cl, Br, I); mixed-valent (IV/II) compounds of the type RTeX2TeR are also known. This chapter surveys the availability and/or preparative methods for these widely used reagents followed by examples of their applications in synthetic inorganic and organic selenium and tellurium chemistry. For both the binary halides and their organic derivatives, the discussion is subdivided according to the formal oxidation state of the chalcogen.

scholarly journals Metrical Oxidation States of 1,4-Diazadiene (DAD) Derived Ligands

2020 ◽  
Author(s):  
F.J. de Zwart ◽  
Bente Reus ◽  
Annechien A.H. Laporte ◽  
Vivek Sinha ◽  
Bas de Bruin
Keyword(s):  
Transition Metal ◽  
Metal Complexes ◽  
Oxidation State ◽  
Transition Metal Complexes ◽  
Coordination Mode ◽  
Chemical Reactivity ◽  
Accurate Determination ◽  
Oxidation States ◽  
Wide Range ◽  
Ligand Bond

The conventional method of assigning formal oxidation states (FOS) to metals and ligands is an important tool for understanding and predicting chemical reactivity, in particular in catalysis research. For complexes containing redox-noninnocent ligands, the oxidation state of the ligand can be ambiguous (i.e. their spectroscopic oxidation state can differ from the formal oxidation state), and thus frustrates the assignment of the oxidation state of the metal. A quantitative correlation between empirical metric data of redox active ligands and their oxidation states using a metrical oxidation state (MOS) model has been developed for catecholate and aminophenolate derived ligands by Brown. In the present work, we present a MOS model for 1,4-diazabutadiene (DAD<sup>n</sup>) ligands. The model is based on a similar approach as reported by Brown, correlating the intra-ligand bond lengths of the DAD<sup>n</sup> moiety in a quantitative manner to the MOS using geometrical information from X-ray structures in the Cambridge Crystallographic Data Center (CCDC) database. However, accurate determination of the MOS of these ligands turned-out to be dependent the coordination mode of the DAD<sup>2-</sup> moiety, which can adopt both a planar <i>κ<sup>2</sup></i>-<i>N<sub>2</sub></i>-geometry and a <i>η<sup>4</sup></i>-<i>N<sub>2</sub></i>-<i>C<sub>2</sub></i> π-coordination mode in (transition) metal complexes in its doubly reduced, dianionic enediamide oxidation state. A reliable MOS model was developed taking the intrinsic differences in intra-ligand bond distances between these coordination modes of the DAD<sup>2‒</sup> ligand into account. Three different models were defined and tested using different geometric parameters (C=C→M distance, M-N-C angle, M-N-C-C torsion angle) to describe the C=C backbone coordination to the metal in the <i>η<sup>4</sup></i>-<i>N<sub>2</sub></i>-<i>C<sub>2</sub></i> π-coordination mode of the DAD<sup>2‒</sup> ligand. Statistical analysis revealed that the C=C→M distance best describes the <i>η<sup>4</sup></i>-<i>N<sub>2</sub></i>-<i>C<sub>2</sub></i> coordination mode, using a cut-off value of 2.46 Å for π-coordination. The developed MOS model was used to validate the oxidation state assignment of elements not contained within the training set (Sr, Yb and Ho), thus demonstrating the applicability of the MOS model to a wide range of complexes. Chromium complexes with complex electronic structures were also shown to be accurately described by MOS analysis. Furthermore, it is shown that a combination of MOS analysis and FOD calculations provide an inexpensive method to gain insight into the electronic structure of singlet spin state (S = 0) [M(trop<sub>2</sub>dad)] transition metal complexes showing multireference character.<br>

scholarly journals A compact furnace for in situ X-ray absorption spectroscopy: design, fabrication and study of cationic oxidation states in Pr6O11 and NiO

2021 ◽  
Vol 28 (2) ◽  
pp. 455-460
Author(s):  
Suchinda Sattayaporn ◽  
Somboonsup Rodporn ◽  
Pinit Kidkhunthod ◽  
Narong Chanlek ◽  
Chutarat Yonchai ◽  
...  
Keyword(s):  
Oxidation State ◽  
Absorption Spectroscopy ◽  
Heating Temperature ◽  
Reduction Process ◽  
Oxidation States ◽  
Quartz Tube ◽  
Edge Structure ◽  
X Ray ◽  
X Ray Absorption

A well designed compact furnace has been designed for in situ X-ray absorption spectroscopy (XAS). It enables various heat ramps from 300 K to 1473 K. The furnace consists of heaters, a quartz tube, a circulated refrigerator and a power controller. It can generate ohmic heating via an induction process with tantalum filaments. The maximum heating rate exceeds 20 K min−1. A quartz tube with gas feedthroughs allows the mixing of gases and adjustment of the flow rate. The use of this compact furnace allows in situ XAS investigations to be carried out in transmission or fluorescence modes under controlled temperature and atmosphere. Moreover, the furnace is compact, light and well compatible to XAS. The furnace was used to study cationic oxidation states in Pr6O11 and NiO compounds under elevated temperature and reduced atmosphere using the in situ X-ray absorption near-edge structure (XANES) technique at beamline 5.2 SUT-NANOTEC-SLRI of the Synchrotron Light Research Institute, Thailand. At room temperature, Pr6O11 contains a mixture of Pr3+ and Pr4+ cations, resulting in an average oxidation state of +3.67. In situ XANES spectra of Pr (L 3-edge) show that the oxidation state of Pr4+ cations was totally reduced to +3.00 at 1273 K under H2 atmosphere. Considering NiO, Ni2+ species were present under ambient conditions. At 573 K, the reduction process of Ni2+ occurred. The Ni0/Ni2+ ratio increased linearly with respect to the heating temperature. Finally, the reduction process of Ni2+ was completely finished at 770 K.

A Study of [Cr-O6]-based rutile analogues by means of EELS

2008 ◽  
Vol 1148 ◽  
Author(s):  
Angel M. Arevalo-Lopez ◽  
Elizabeth Castillo-Martinez ◽  
Miguel Ángel Alario-Franco
Keyword(s):  
Energy Loss ◽  
Linear Relationship ◽  
Electron Energy ◽  
Oxidation State ◽  
Electron Energy Loss Spectroscopy ◽  
Oxidation States ◽  
Formal Oxidation State ◽  
Electron Energy Loss

AbstractWe present in here the study by means of electron energy loss spectroscopy (EELS) of several rutile based oxides, having in common the presence of octahedrally oxygen coordinated chromium, [Cr-O6], in three different formal oxidation states: Cr4+ in CrO2, a regular rutile; Cr3+ in CrOOH, a H bonded orthorrombic distorted rutile and in CrTaO4, a metal disordered rutile and Cr2+ in CrTa2O6 an ordered trirutile structure. A linear relationship is observed between the formal oxidation state of chromium in all these rutile oxides and the separation between the Cr-L2,3 and O-K energy loss peaks.

scholarly journals Metrical Oxidation States of 1,4-Diazadiene (DAD) Derived Ligands

2020 ◽  
Author(s):  
F.J. de Zwart ◽  
Bente Reus ◽  
Annechien A.H. Laporte ◽  
Vivek Sinha ◽  
Bas de Bruin
Keyword(s):  
Transition Metal ◽  
Metal Complexes ◽  
Oxidation State ◽  
Transition Metal Complexes ◽  
Coordination Mode ◽  
Chemical Reactivity ◽  
Accurate Determination ◽  
Oxidation States ◽  
Wide Range ◽  
Ligand Bond

The conventional method of assigning formal oxidation states (FOS) to metals and ligands is an important tool for understanding and predicting chemical reactivity, in particular in catalysis research. For complexes containing redox-noninnocent ligands, the oxidation state of the ligand can be ambiguous (i.e. their spectroscopic oxidation state can differ from the formal oxidation state), and thus frustrates the assignment of the oxidation state of the metal. A quantitative correlation between empirical metric data of redox active ligands and their oxidation states using a metrical oxidation state (MOS) model has been developed for catecholate and aminophenolate derived ligands by Brown. In the present work, we present a MOS model for 1,4-diazabutadiene (DAD<sup>n</sup>) ligands. The model is based on a similar approach as reported by Brown, correlating the intra-ligand bond lengths of the DAD<sup>n</sup> moiety in a quantitative manner to the MOS using geometrical information from X-ray structures in the Cambridge Crystallographic Data Center (CCDC) database. However, accurate determination of the MOS of these ligands turned-out to be dependent the coordination mode of the DAD<sup>2-</sup> moiety, which can adopt both a planar <i>κ<sup>2</sup></i>-<i>N<sub>2</sub></i>-geometry and a <i>η<sup>4</sup></i>-<i>N<sub>2</sub></i>-<i>C<sub>2</sub></i> π-coordination mode in (transition) metal complexes in its doubly reduced, dianionic enediamide oxidation state. A reliable MOS model was developed taking the intrinsic differences in intra-ligand bond distances between these coordination modes of the DAD<sup>2‒</sup> ligand into account. Three different models were defined and tested using different geometric parameters (C=C→M distance, M-N-C angle, M-N-C-C torsion angle) to describe the C=C backbone coordination to the metal in the <i>η<sup>4</sup></i>-<i>N<sub>2</sub></i>-<i>C<sub>2</sub></i> π-coordination mode of the DAD<sup>2‒</sup> ligand. Statistical analysis revealed that the C=C→M distance best describes the <i>η<sup>4</sup></i>-<i>N<sub>2</sub></i>-<i>C<sub>2</sub></i> coordination mode, using a cut-off value of 2.46 Å for π-coordination. The developed MOS model was used to validate the oxidation state assignment of elements not contained within the training set (Sr, Yb and Ho), thus demonstrating the applicability of the MOS model to a wide range of complexes. Chromium complexes with complex electronic structures were also shown to be accurately described by MOS analysis. Furthermore, it is shown that a combination of MOS analysis and FOD calculations provide an inexpensive method to gain insight into the electronic structure of singlet spin state (S = 0) [M(trop<sub>2</sub>dad)] transition metal complexes showing multireference character.<br>

scholarly journals Advantages of Surface Ablation in Excimer Laser Surgery

2020 ◽  
pp. 1-7
Author(s):  
Fouad Chraibi
Keyword(s):  
Excimer Laser ◽  
Laser Surgery ◽  
Current Trend ◽  
Laser Surface ◽  
The Other ◽  
Corneal Surface ◽  
Corneal Dystrophies ◽  
Wide Range ◽  
The One

In this article, we review the advantages of corneal laser surface surgery to explain the current trend toward this technique. The excimer laser in corneal surface surgery can be used for two reasons. On the one hand, to correct a refractive error by photorefractive keratectomy (PRK) and its two variants; Laser epithelial keratomileusis (LASEK) and Epithelial Laser-assisted in situ keratomileusis (Epi-lasik). On the other hand, it is used to perform superficial corneal capacities photoablation, among other uses, by Phototherapeutic keratectomy (PTK). PRK is regaining popularity thanks to its better safety profile, mitomycin C usage and improvement of photoablation profiles. Lasek and Epilasik are less frequently performed nowadays. PTK is an important element in the techniques used to improve visual acuity in a wide range of corneal conditions such as corneal scars, recurrent erosion syndrome and corneal dystrophies.

scholarly journals Biopolymer colloids for controlling and templating inorganic synthesis

2014 ◽  
Vol 5 ◽  
pp. 2129-2138 ◽  
Author(s):  
Laura C Preiss ◽  
Katharina Landfester ◽  
Rafael Muñoz-Espí
Keyword(s):  
Nucleation And Growth ◽  
Inorganic Materials ◽  
The Other ◽  
Shell Structures ◽  
Ex Situ ◽  
Inorganic Synthesis ◽  
Wide Range ◽  
Synthetic Approaches ◽  
Soft Templates

Biopolymers and biopolymer colloids can act as controlling agents and templates not only in many processes in nature, but also in a wide range of synthetic approaches. Inorganic materials can be either synthesized ex situ and later incorporated into a biopolymer structuring matrix or grown in situ in the presence of biopolymers. In this review, we focus mainly on the latter case and distinguish between the following possibilities: (i) biopolymers as controlling agents of nucleation and growth of inorganic materials; (ii) biopolymers as supports, either as molecular supports or as carrier particles acting as cores of core–shell structures; and (iii) so-called “soft templates”, which include on one hand stabilized droplets, micelles, and vesicles, and on the other hand continuous scaffolds generated by gelling biopolymers.

scholarly journals Anticonvulsant vs. Proconvulsant Effect of in situ Deep Brain Stimulation at the Epileptogenic Focus

2021 ◽  
Vol 15 ◽  
Author(s):  
Ping Chou ◽  
Chung-Chin Kuo
Keyword(s):  
Deep Brain Stimulation ◽  
Brain Stimulation ◽  
Rational Design ◽  
The Other ◽  
Anticonvulsant Effect ◽  
Epileptogenic Focus ◽  
Kindling Effect ◽  
Wide Range ◽  
Deep Brain

Since deep brain stimulation (DBS) at the epileptogenic focus (in situ) denotes long-term repetitive stimulation of the potentially epileptogenic structures, such as the amygdala, the hippocampus, and the cerebral cortex, a kindling effect and aggravation of seizures may happen and complicate the clinical condition. It is, thus, highly desirable to work out a protocol with an evident quenching (anticonvulsant) effect but free of concomitant proconvulsant side effects. We found that in the basolateral amygdala (BLA), an extremely wide range of pulsatile stimulation protocols eventually leads to the kindling effect. Only protocols with a pulse frequency of ≤1 Hz or a direct current (DC), with all of the other parameters unchanged, could never kindle the animal. On the other hand, the aforementioned DC stimulation (DCS), even a pulse as short as 10 s given 5 min before the kindling stimuli or a pulse given even to the contralateral BLA, is very effective against epileptogenicity and ictogenicity. Behavioral, electrophysiological, and histological findings consistently demonstrate success in seizure quenching or suppression as well as in the safety of the specific DBS protocol (e.g., no apparent brain damage by repeated sessions of stimulation applied to the BLA for 1 month). We conclude that in situ DCS, with a novel and rational design of the stimulation protocol composed of a very low (∼3% or 10 s/5 min) duty cycle and assuredly devoid of the potential of kindling, may make a successful antiepileptic therapy with adequate safety in terms of little epileptogenic adverse events and tissue damage.

Oxygen Stoichiometry and Ordering in HTSC and Other Perovskite-Like Solids

1989 ◽  
Vol 156 ◽  
Author(s):  
Miguel Angel Alario-Franco
Keyword(s):  
Oxidation State ◽  
Oxygen Vacancies ◽  
The Other ◽  
Oxidation States ◽  
Oxygen Stoichiometry ◽  
Tetrahedral Coordination ◽  
Subsequent Work ◽  
Ordered Phases ◽  
Cation Coordination ◽  
Marked Tendency

ABSTRACTAs long ago remarked by WADSLEY, perovskite-like solids have a clear tendency to be non-stoichiometric and both the geometrical requirements of cation coordination and the oxidation state can control the way in which non-stoichiometry is accommodated.For example, in the case of iron, which shows a particular preference for coexisting octaedral and tetrahedral coordination and in which oxidation states ranging formally from 2 + to 4 + are common in oxides, a Family of Phases AnFenO3n−1, has been described. It includes the members n=2,3,4 and ∞. When the stoichiometries are different from these integral n values, disordered 1−D intergrowthhs are observed. Subsequent work has shown that all the intermediate compositions, whether ordered or disordered in 1−D, can still become non-stoichiometric by oxidation at high temperatures (T>1200°C in air) and this originates the formation of 3D microdomains. Of these, up to nine different sets have been shown to coexist in the same crystal.Similarly, the copper based HTSC, which have a lot in common with perovskite, have shown a marked tendency for being oxygen deficient. In particular, Ybacuo, Ba2YCu3O7±6 has been shown to exist in a wide oxygen range extending from, at least RO7.2 to RO6 (R<>Ba2YCu3). Within that range, and in spite of an apparent structurally monophasic, and thermodynamically bivariant, behaviour, a succesion of ordered states of the oxygen vacancies in 1,2 and 3 dimensions has been found. This, again, suggests the existence of intermediate ordered phases which, on the other hand, can also be non-stoichiometric. Results of our work along these lines is presented and discussed.

In-situ electrical-resistivity measurements in the high-voltage electron microscope

1978 ◽  
Vol 36 (1) ◽  
pp. 68-69
Author(s):  
W. E. King
Keyword(s):  
Electrical Resistivity ◽  
Electron Microscope ◽  
High Voltage ◽  
Resistivity Measurements ◽  
Voltage Electron ◽  
High Voltage Electron Microscope ◽  
High Voltage Electron ◽  
Wide Range ◽  
Helium Gas

A side-entry type, helium-temperature specimen stage that has the capability of in-situ electrical-resistivity measurements has been designed and developed for use in the AEI-EM7 1200-kV electron microscope at Argonne National Laboratory. The electrical-resistivity measurements complement the high-voltage electron microscope (HVEM) to yield a unique opportunity to investigate defect production in metals by electron irradiation over a wide range of defect concentrations.A flow cryostat that uses helium gas as a coolant is employed to attain and maintain any specified temperature between 10 and 300 K. The helium gas coolant eliminates the vibrations that arise from boiling liquid helium and the temperature instabilities due to alternating heat-transfer mechanisms in the two-phase temperature regime (4.215 K). Figure 1 shows a schematic view of the liquid/gaseous helium transfer system. A liquid-gas mixture can be used for fast cooldown. The cold tip of the transfer tube is inserted coincident with the tilt axis of the specimen stage, and the end of the coolant flow tube is positioned without contact within the heat exchanger of the copper specimen block (Fig. 2).

A very rapid in situ Kikuchi technique to determine relative crystal misorientation

1988 ◽  
Vol 46 ◽  
pp. 830-831
Author(s):  
J. I. Bennetch
Keyword(s):  
Electron Beam ◽  
Analytical Techniques ◽  
Superplastic Forming ◽  
The Other ◽  
Kikuchi Pattern ◽  
Kikuchi Line ◽  
Line Analysis

In a recent study of the superplastic forming (SPF) behavior of certain Al-Li-X alloys, the relative misorientation between adjacent (sub)grains proved to be an important parameter. It is well established that the most accurate way to determine misorientation across boundaries is by Kikuchi line analysis. However, the SPF study required the characterization of a large number of (sub)grains in each sample to be statistically meaningful, a very time-consuming task even for comparatively rapid Kikuchi analytical techniques.In order to circumvent this problem, an alternate, even more rapid in-situ Kikuchi technique was devised, eliminating the need for the developing of negatives and any subsequent measurements on photographic plates. All that is required is a double tilt low backlash goniometer capable of tilting ± 45° in one axis and ± 30° in the other axis. The procedure is as follows. While viewing the microscope screen, one merely tilts the specimen until a standard recognizable reference Kikuchi pattern is centered, making sure, at the same time, that the focused electron beam remains on the (sub)grain in question.

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