Accurate Determination of the Reaction Course in HY2 ⇌ Y + YH (Y = O, S): Detailed Analysis of the Covalent- to Hydrogen-Bonding Transition

2013 ◽  
Vol 117 (32) ◽  
pp. 7393-7407 ◽  
Author(s):  
A. J. C. Varandas
Keyword(s):  
Hydrogen Bonding ◽  
Detailed Analysis ◽  
Accurate Determination

Neutron structures of ammonium tetrafluoroberyllate

1999 ◽  
Vol 55 (1) ◽  
pp. 17-23 ◽  
Author(s):  
R. C. Srivastava ◽  
W. T. Klooster ◽  
T. F. Koetzle
Keyword(s):  
Hydrogen Bonding ◽  
Ferroelectric Phase ◽  
Paraelectric Phase ◽  
Accurate Determination ◽  
Mirror Plane ◽  
Accurate Information ◽  
X Ray ◽  
The Past ◽  
Hydrogen Bonding Interactions

It is thought that hydrogen bonding is responsible for the ferroelectricity in ammonium tetrafluoroberyllate, (NH_{4})_{2}BeF_{4}. In the past X-ray data have been collected, but these did not permit accurate determination of the H-atom positions. In order to obtain more accurate information the neutron structures have now been determined for the paraelectric and ferroelectric phases. Going from the paraelectric to the ferroelectric phase, both the BeF_4^{2-} and the NH_4^+ ions rotate and shift from the mirror planes of the paraelectric phase. This results in removal of the mirror-plane symmetry and formation of a superlattice with the a axis doubled. Along the polar c axis, the NH_4^+ ions move towards the BeF_4^{2-} ions within chains of molecules and the chains move slightly relative to one another. The rotations and translations give rise to stronger hydrogen-bonding interactions.

scholarly journals (Metformin-κ2N,N′)(salicylato-κ2O,O′)copper(II) trihydrate

IUCrData ◽  
2018 ◽  
Vol 3 (2) ◽  
Author(s):  
Sandra Julieta Gutiérrez Ojeda ◽  
Ulises Salazar Kuri ◽  
Sylvain Bernès ◽  
Aarón Pérez-Benítez
Keyword(s):  
Hydrogen Bonding ◽  
Diffraction Data ◽  
Accurate Determination ◽  
Title Complex ◽  
Metformin Hydrochloride ◽  
Sacrificial Anode ◽  
Ethanolic Solution ◽  
Hydrogen Bonding Interactions ◽  
Square Planar

The hydrous title complex [systematic name: (1,1-dimethylbiguanide-κ2N2,N4)(2-oxidobenzoato-κ2O,O′)copper(II) trihydrate], [Cu(C7H4O3)(C4H11N5)]·3H2O, was synthesized electrolytically from an ethanolic solution of metformin hydrochloride, acetylsalicylic acid, Pepto-Bismol and a copper sacrificial anode. Diffraction data were collected at 0.56 Å resolution, allowing the accurate determination of H-atom positions in the neutral metformin ligand. Both imine groups in metformin have very similar N=C bond lengths, 1.2978 (17) and 1.3033 (17) Å, and the salicylate dianion behaves as a chelating ligand. The coordination sphere of the copper(II) cation deviates marginally from a square-planar arrangement. In the crystal, short Cu...Cu separations of 3.5476 (3) Å are observed, along with classical hydrogen-bonding interactions.

A quantitative approach to inner shell losses

1978 ◽  
Vol 36 (1) ◽  
pp. 526-527 ◽  
Author(s):  
R.D. Leapman ◽  
P. Rez ◽  
D.F. Mayers
Keyword(s):  
Energy Transfer ◽  
Cross Section ◽  
Cross Sections ◽  
Accurate Determination ◽  
Background Intensity ◽  
Core Excitation ◽  
Quantitative Basis ◽  
Cross Section Differential ◽  
Bound Electrons

Microanalysis by EELS has been developing rapidly and though the general form of the spectrum is now understood there is a need to put the technique on a more quantitative basis (1,2). Certain aspects important for microanalysis include: (i) accurate determination of the partial cross sections, σx(α,ΔE) for core excitation when scattering lies inside collection angle a and energy range ΔE above the edge, (ii) behavior of the background intensity due to excitation of less strongly bound electrons, necessary for extrapolation beneath the signal of interest, (iii) departures from the simple hydrogenic K-edge seen in L and M losses, effecting σx and complicating microanalysis. Such problems might be approached empirically but here we describe how computation can elucidate the spectrum shape.The inelastic cross section differential with respect to energy transfer E and momentum transfer q for electrons of energy E0 and velocity v can be written as

Fast calibration of CBED patterns for quantitative analysis

1993 ◽  
Vol 51 ◽  
pp. 674-675
Author(s):  
M.A. Gribelyuk ◽  
M. Rühle
Keyword(s):  
Reciprocal Lattice ◽  
Accurate Determination ◽  
Incident Beam ◽  
Crystal Thickness ◽  
Structure Model ◽  
Beam Direction ◽  
Transmitted Beam ◽  
Reciprocal Vector ◽  
On Line

A new method is suggested for the accurate determination of the incident beam direction K, crystal thickness t and the coordinates of the basic reciprocal lattice vectors V1 and V2 (Fig. 1) of the ZOLZ plans in pixels of the digitized 2-D CBED pattern. For a given structure model and some estimated values Vest and Kest of some point O in the CBED pattern a set of line scans AkBk is chosen so that all the scans are located within CBED disks.The points on line scans AkBk are conjugate to those on A0B0 since they are shifted by the reciprocal vector gk with respect to each other. As many conjugate scans are considered as CBED disks fall into the energy filtered region of the experimental pattern. Electron intensities of the transmitted beam I0 and diffracted beams Igk for all points on conjugate scans are found as a function of crystal thickness t on the basis of the full dynamical calculation.

Computer-Assisted High-Re Solution TEM

1990 ◽  
Vol 48 (1) ◽  
pp. 162-163
Author(s):  
F.A. Ponce ◽  
H. Hikashi
Keyword(s):  
Signal To Noise Ratio ◽  
Accurate Determination ◽  
Computer Software ◽  
Video Camera ◽  
Image Contrast ◽  
Objective Lens ◽  
Computer Assisted ◽  
Optical Parameters ◽  
Astigmatism Correction

The determination of the atomic positions from HRTEM micrographs is only possible if the optical parameters are known to a certain accuracy, and reliable through-focus series are available to match the experimental images with calculated images of possible atomic models. The main limitation in interpreting images at the atomic level is the knowledge of the optical parameters such as beam alignment, astigmatism correction and defocus value. Under ordinary conditions, the uncertainty in these values is sufficiently large to prevent the accurate determination of the atomic positions. Therefore, in order to achieve the resolution power of the microscope (under 0.2nm) it is necessary to take extraordinary measures. The use of on line computers has been proposed [e.g.: 2-5] and used with certain amount of success.We have built a system that can perform operations in the range of one frame stored and analyzed per second. A schematic diagram of the system is shown in figure 1. A JEOL 4000EX microscope equipped with an external computer interface is directly linked to a SUN-3 computer. All electrical parameters in the microscope can be changed via this interface by the use of a set of commands. The image is received from a video camera. A commercial image processor improves the signal-to-noise ratio by recursively averaging with a time constant, usually set at 0.25 sec. The computer software is based on a multi-window system and is entirely mouse-driven. All operations can be performed by clicking the mouse on the appropiate windows and buttons. This capability leads to extreme friendliness, ease of operation, and high operator speeds. Image analysis can be done in various ways. Here, we have measured the image contrast and used it to optimize certain parameters. The system is designed to have instant access to: (a) x- and y- alignment coils, (b) x- and y- astigmatism correction coils, and (c) objective lens current. The algorithm is shown in figure 2. Figure 3 shows an example taken from a thin CdTe crystal. The image contrast is displayed for changing objective lens current (defocus value). The display is calibrated in angstroms. Images are stored on the disk and are accessible by clicking the data points in the graph. Some of the frame-store images are displayed in Fig. 4.

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