Three-dimensional electron delocalization: a theoretical study based on the pentacyclo[3.3.1.13,7.01,3.05,7]decane system

1997 ◽  
Vol 75 (2) ◽  
pp. 192-201 ◽  
Author(s):  
Mary S.W. Chan ◽  
Donald R. Arnold
Keyword(s):  
Radical Cation ◽  
Radical Anion ◽  
Structural Changes ◽  
Three Dimensional ◽  
Radical Cations ◽  
Three Dimensions ◽  
Electron Delocalization ◽  
Dimensional Electron ◽  
Frontier Orbitals ◽  
Dft Methods

The concept of electron delocalization, and the possibility of aromaticity in three dimensions has been introduced, but remains relatively unexplored. This work examines the structure and energy of a series of ions generated from pentacyclo[3.3.1.13,7.01,3.05,7]decane (1). The ions considered are the dication (12+), the radical cation (1+•), the radical anion (1−•), and the dianion (12−). The dication belongs to a small group of organic compounds that shows three-dimensional electron delocalization and it has been classified as "three-dimensionally homoaromatic." The relative stability of these ions is estimated by the enthalpy change of isodesmic and disproportionation reactions using energies obtained from HF, MP2, and DFT methods. The structural changes and the reorganization of the frontier orbitals when electrons are successively added to or removed from the neutral molecule (1) are also explored. The results confirm the three-dimensional electron delocalization in 12+ and indicate that it is stable relative to a localized structure with similar features; however, it is highly unstable compared to the radical cation (1+•). The dianion (12−), on the other hand, did not show any evidence for electron delocalization and it is unstable relative to the radical anion (1−•). Keywords: three-dimensional aromaticity, electron derealization, resonance, radical cations, radical anions.

scholarly journals Three-dimensional in-situ imaging of cracks in concrete using diffuse ultrasound

2017 ◽  
Vol 17 (2) ◽  
pp. 279-284 ◽  
Author(s):  
Yuxiang Zhang ◽  
Eric Larose ◽  
Ludovic Moreau ◽  
Grégoire d’Ozouville
Keyword(s):  
Wind Tunnel ◽  
Multiple Scattering ◽  
Structural Changes ◽  
Imaging Technique ◽  
Three Dimensional ◽  
Three Dimensions ◽  
Thick Wall ◽  
Pressure Changes ◽  
Diffuse Ultrasound

Locadiff, an innovative imaging technique based on diffuse waves, has recently been developed in order to image mechanical changes in heterogeneous, geological, or man-made materials. This manuscript reports the on-site application of Locadiff to locate several pre-existing cracks on an aeronautical wind tunnel made of pre-stressed concrete. Using 32 transducers working at ultrasonic frequencies (80–220 kHz) where multiple scattering occurs, we monitor during 15 min an area of 2.5 m×2.5 m of a 35-cm-thick wall. With the wind tunnel in its routine operation, structural changes around the cracks are detected, thanks to their closing or opening due to slight pressure changes. By mapping the density of such microstructure changes in the bulk of the material, locating three pre-existing cracks is properly performed in three dimensions.

Radical ions in photochemistry. 18. The photosensitized (electron transfer) tautomerization of alkenes; the 1,1-diphenyl alkene system

1987 ◽  
Vol 65 (9) ◽  
pp. 2312-2314 ◽  
Author(s):  
Donald R. Arnold ◽  
Shelley A. Mines
Keyword(s):  
Hydrogen Bond ◽  
Electron Transfer ◽  
Radical Cation ◽  
Radical Anion ◽  
Relative Rate ◽  
Radical Cations ◽  
Radical Ions ◽  
Back Electron Transfer ◽  
Ambident Anion ◽  
Determining Factor

The photosensitized (electron transfer) irradiation of several conjugated 1,1-diphenyl alkenes, in acetonitrile with 1,4-dicyanobenzene or 1-cyanonapthalene as electron accepting sensitizer and 2,6-lutidine as base, leads essentially quantitatively to tautomerization to the less stable unconjugated isomer(s). The proposed mechanism for this reaction involves formation of the alkene radical cation and sensitizer radical anion followed by deprotonation of the radical cation, reduction of the resulting radical to the ambident anion by back electron transfer from the radical anion, and reprotonation. There are several steps in this mechanism that could control the ratio of isomers. Evidence is provided that, at least in some cases, it is the relative rate of deprotonation from the isomeric radical cations that is the determining factor. This rate is influenced by the conformation of the radical cation; the carbon–hydrogen bond involved in the deprotonation step must overlap with the singly occupied molecular orbital.

scholarly journals Oxidation of ethidium-based probes by biological radicals: mechanism, kinetics and implications for the detection of superoxide

2020 ◽  
Vol 10 (1) ◽  
Author(s):  
Radosław Michalski ◽  
David Thiebaut ◽  
Bartosz Michałowski ◽  
Mehmet M. Ayhan ◽  
Micael Hardy ◽  
...  
Keyword(s):  
Radical Cation ◽  
Radical Anion ◽  
Radical Cations ◽  
Substantial Effect ◽  
Oxidation Products ◽  
Peroxyl Radicals ◽  
Superoxide Radical Anion ◽  
Biologically Relevant ◽  
Fluorogenic Probes ◽  
New Generation

Abstract Hydroethidine (HE) and hydropropidine ($$\hbox {HPr}^{+}$$ HPr + ) are fluorogenic probes used for the detection of the intra- and extracellular superoxide radical anion ($$\hbox {O}_{ {2}}^{\bullet -}$$ O 2 ∙ - ). In this study, we provide evidence that HE and $$\hbox {HPr}^{+}$$ HPr + react rapidly with the biologically relevant radicals, including the hydroxyl radical, peroxyl radicals, the trioxidocarbonate radical anion, nitrogen dioxide, and the glutathionyl radical, via one-electron oxidation, forming the corresponding radical cations. At physiological pH, the radical cations of the probes react rapidly with $$\hbox {O}_{ {2}}^{\bullet -}$$ O 2 ∙ - , leading to the specific 2-hydroxylated cationic products. We determined the rate constants of the reaction between $$\hbox {O}_{ {2}}^{\bullet -}$$ O 2 ∙ - and the radical cations of the probes. We also synthesized N-methylated analogs of $$\hbox {HPr}^{+}$$ HPr + and HE which were used in mechanistic studies. Methylation of the amine groups was not found to prevent the reaction between the radical cation of the probe and the superoxide, but it significantly increased the lifetime of the radical cation and had a substantial effect on the profiles of the oxidation products by inhibiting the formation of dimeric products. We conclude that the N-methylated analogs of HE and $$\hbox {HPr}^{+}$$ HPr + may be used as a scaffold for the design of a new generation of probes for intra- and extracellular superoxide.

Substituent effects in oxime radical cations. 1. Photosensitized reactions of acetophenone oximes

2003 ◽  
Vol 81 (6) ◽  
pp. 575-585 ◽  
Author(s):  
HJ Peter de Lijser ◽  
Jason S Kim ◽  
Suzanne M McGrorty ◽  
Erin M Ulloa
Keyword(s):  
Electron Transfer ◽  
Steady State ◽  
Radical Cation ◽  
Radical Anion ◽  
Substituent Effects ◽  
Radical Cations ◽  
Ionization Potentials ◽  
Semiquinone Radical ◽  
Am1 Calculations ◽  
Iminoxyl Radical

A variety of ortho-, meta-, and para-substituted (-H, -F, -Cl, -CF3, -CN (meta and para only), -CH3, -OCH3, and -NO2) acetophenone oximes were synthesized and studied using laser flash photolysis (LFP) and steady-state photolysis experiments in acetonitrile with chloranil as the photosensitizer. In addition, semi-empirical (AM1) calculations were performed on the neutral species, the radical cations, and the corresponding iminoxyl radicals. The data was analyzed in terms of the electrochemical peak potentials of the oximes, the quenching rates of triplet chloranil (LFP), the calculated ionization potentials, and the measured conversions of the oximes in the steady-state photolysis experiments. Photolysis of the oximes in the presence of chloranil results in the formation of the chloranil radical anion, which reacts rapidly with the oxime radical cation to form the semiquinone radical and an iminoxyl radical. Evidence for the formation of the chloranil radical anion and the semiquinone radical was obtained from LFP studies. The measured quenching rates from the LFP studies represent the rates of electron transfer from the oximes to triplet chloranil. This data was correlated to various radical and polar substituent constants. The Hammett studies suggest that steric, polar, and radical effects are important for ortho-substituted acetophenone oximes, polar effects are important for para-substituted oximes, and radical stabilization is more important than polar effects for the meta-substituted substrates. The calculated ionization potentials of the oximes show an excellent correlation with the measured quenching rates supporting the electron transfer pathway. On the basis of calculated charge densities, we conclude that the measured substituent effects are transition state effects rather than ground state effects. At this point all of the available data suggests that the conversion of the oximes is controlled by two energetically opposing reactions, namely oxidation of the neutral oxime, which is favorable for oximes with electron-donating substituents, and deprotonation of the oxime radical cation, which is favorable for oximes with electron-withdrawing substituents. The overall result is a reaction with little selectivity as far as substituent effects are concerned.Key words: oxime, radical cation, iminoxyl radical, electron transfer, substituent effect.

Image restoration of thick biological specimens using a through focus series as applied to electron tomography

1994 ◽  
Vol 52 ◽  
pp. 338-339
Author(s):  
Karen F. Han ◽  
John W. Sedat ◽  
David A. Agard
Keyword(s):  
Electron Tomography ◽  
Mass Density ◽  
Three Dimensional ◽  
Three Dimensions ◽  
Dimensional Electron ◽  
Accurate Representation ◽  
Exit Surface ◽  
Ewald Sphere ◽  
Electron Energy Loss ◽  
Simple Fashion

The main focus of our laboratory is the study of higher order chromatin structure using three dimensional electron microscope tomography. Three dimensional tomography involves the reconstruction of an object by combining multiple projection views of the object at different tilt angles. It is thus imperative to obtain an accurate representation of the projected object mass density to reconstruct the object correctly in three dimensions. Due to the effects of electron-specimen interactions and microscope lens aberrations, image intensities are not always related to the projected mass density in a simple fashion. We are using a variety of techniques to interpret collected images. In previous work, we have analyzed image formation for thick (0.3-0.7um) biological specimens by electron energy loss spectroscopy and imaging, as well as Ewald sphere construction analysis. In this work, we have modified existing techniques to restore images with only three focus levels using the exit surface wave reconstruction as a reference for comparison.

convergent-beam diffraction from surfaces

1987 ◽  
Vol 45 ◽  
pp. 30-33
Author(s):  
J. A. Eades ◽  
A. E. Smith ◽  
D. F. Lynch
Keyword(s):  
Reciprocal Lattice ◽  
Three Dimensional ◽  
Grazing Incidence ◽  
Three Dimensions ◽  
Plane Figure ◽  
Transmission Electron ◽  
Convergent Beam ◽  
Beam Patterns ◽  
Beam Diffraction

It is quite simple (in the transmission electron microscope) to obtain convergent-beam patterns from the surface of a bulk crystal. The beam is focussed onto the surface at near grazing incidence (figure 1) and if the surface is flat the appropriate pattern is obtained in the diffraction plane (figure 2). Such patterns are potentially valuable for the characterization of surfaces just as normal convergent-beam patterns are valuable for the characterization of crystals.There are, however, several important ways in which reflection diffraction from surfaces differs from the more familiar electron diffraction in transmission.GeometryIn reflection diffraction, because of the surface, it is not possible to describe the specimen as periodic in three dimensions, nor is it possible to associate diffraction with a conventional three-dimensional reciprocal lattice.

Application of stereological analysis of cell volume to isolated myocytes in culture with and without adrenergic innervation

1990 ◽  
Vol 48 (3) ◽  
pp. 416-417
Author(s):  
Jane K. Rosenthal ◽  
Dianne L. Atkins ◽  
William J. Marvin ◽  
Penny A. Krumm
Keyword(s):  
Cardiac Myocytes ◽  
Structural Changes ◽  
Three Dimensional ◽  
Adrenergic Innervation ◽  
Culture Cell ◽  
Serial Sections ◽  
Newborn Rats ◽  
Primary Culture Cell ◽  
Biological Studies ◽  
Cell Volumes

To comprehend structural changes in cardiac myocytes accompanying adrenergic innervation, it is essential that a three dimensional analysis be performed. To date, biological studies which utilize stereological methods have been limited to cells in tissue and in organs. Our laboratory has utilized current stereological techniques for measuring absolute volumes of individual myocytes in primary culture. Cell volumes are calculated for two distinct groups of cells at 96 hours in culture: isolated myocytes and myocytes innervated with adrenergic neurons (Figure 1).Cardiac myocytes are cultured from the ventricular apices of newborn rats. Cells are plated directly onto tissue culture dishes with or without preplated explants from the paravertebral thoracolumbar sympathetic chain. On day four cultures are photographed and marked for one-to-one cell location. Following conventional fixation and embeddment in eponate-12, the cells are relocated and mounted for microtomy. The cells are completely sectioned at 120nm in their parallel orientation to the surface of the dish (Figure 2). Serial sections are collected on formvar coated slotted grids and are recorded in sequence.

Going Beyond 3-D Imaging: Automated 3-D Montaged image Analysis of Cytological Specimens

1996 ◽  
Vol 54 ◽  
pp. 282-283
Author(s):  
Badrinath Roysam ◽  
Hakan Ancin ◽  
Douglas E. Becker ◽  
Robert W. Mackin ◽  
Matthew M. Chestnut ◽  
...  
Keyword(s):  
Image Analysis ◽  
Structural Changes ◽  
Sampling Methods ◽  
Spatial Clustering ◽  
Three Dimensional ◽  
Field Of View ◽  
Cell Counting ◽  
Depth Of Field ◽  
Tissue Cell ◽  
Tissue Organization

This paper summarizes recent advances made by this group in the automated three-dimensional (3-D) image analysis of cytological specimens that are much thicker than the depth of field, and much wider than the field of view of the microscope. The imaging of thick samples is motivated by the need to sample large volumes of tissue rapidly, make more accurate measurements than possible with 2-D sampling, and also to perform analysis in a manner that preserves the relative locations and 3-D structures of the cells. The motivation to study specimens much wider than the field of view arises when measurements and insights at the tissue, rather than the cell level are needed.The term “analysis” indicates a activities ranging from cell counting, neuron tracing, cell morphometry, measurement of tracers, through characterization of large populations of cells with regard to higher-level tissue organization by detecting patterns such as 3-D spatial clustering, the presence of subpopulations, and their relationships to each other. Of even more interest are changes in these parameters as a function of development, and as a reaction to external stimuli. There is a widespread need to measure structural changes in tissue caused by toxins, physiologic states, biochemicals, aging, development, and electrochemical or physical stimuli. These agents could affect the number of cells per unit volume of tissue, cell volume and shape, and cause structural changes in individual cells, inter-connections, or subtle changes in higher-level tissue architecture. It is important to process large intact volumes of tissue to achieve adequate sampling and sensitivity to subtle changes. It is desirable to perform such studies rapidly, with utmost automation, and at minimal cost. Automated 3-D image analysis methods offer unique advantages and opportunities, without making simplifying assumptions of tissue uniformity, unlike random sampling methods such as stereology.12 Although stereological methods are known to be statistically unbiased, they may not be statistically efficient. Another disadvantage of sampling methods is the lack of full visual confirmation - an attractive feature of image analysis based methods.

Conformational characterization of nucleosome structure by Electron Microscopy

1993 ◽  
Vol 51 ◽  
pp. 194-195
Author(s):  
Gregory J. Czarnota
Keyword(s):  
Electron Microscopy ◽  
Structural Changes ◽  
Physiological State ◽  
Gene Activation ◽  
Three Dimensional ◽  
Macromolecular Complex ◽  
Ultrastructural Studies ◽  
Ionic Environment ◽  
Nucleosome Structure ◽  
Dimensional Reconstruction

Chromatin structure at the fundamental level of the nucleosome is important in vital cellular processes. Recent biochemical and genetic analyses show that nucleosome structure and structural changes are very active participants in gene expression, facilitating or inhibiting transcription and reflecting the physiological state of the cell. Structural states and transitions for this macromolecular complex, composed of DNA wound about a heterotypic octamer of variously modified histone proteins, have been measured by physico-chemical techniques and by enzyme-accessibility and are recognized to occur with various post-translational modifications, gene activation, transformation and with ionic-environment. In spite of studies which indicate various forms of nucleosome structure, all current x-ray and neutron diffraction studies have consistently resulted in only one structure, suggestive of a static conformation. In contrast, two-dimensional electron microscopy studies and three-dimensional reconstruction techniques have yielded different structures. These fundamental differences between EM and other ultrastructural studies have created a long standing quandary, which I have addressed and resolved using spectroscopic electron microscopy and statistical analyses of nucleosome images in a study of nucleosome structure with ionic environment.

Sign in / Sign up

Export Citation Format

Share Document