Wyniki wyszukiwania - BazTech

Ograniczanie wyników

Znaleziono wyników: 3

Liczba wyników na stronie

Wyniki wyszukiwania

Wyszukiwano:
w słowach kluczowych: potencjał elektrostatyczny

Sortuj według:

Ogranicz wyniki do:

Molecular models in chemistry education at university and upper secondary school - structure of amides

Kolář Karel, Doležal Rafael, Karásková Natálie, Maltsevskaya Nadezhda V., Křížková Šárka

Chemistry-Didactics-Ecology-Metrology

2019

R. 24, NR 1-2

45--51

Molecular models derived from results of quantum-chemical calculations present an important category of didactic instruments in chemistry education in upper secondary school and, particularly, at university. These models can be used especially as tools for supporting the students’ understanding by visual learning, which can adequately address complexity of many chemical topics, incorporate appropriate didactic principles, as well as utilize the benefits brought up by the actual information technology. The proposed molecular models are non-trivial examples of didactic application of computational chemistry techniques in illustration of electron interactions in amidic group, namely the interaction of the free electron pair on the nitrogen atom with the carbonyl group and also the interaction of atoms in the amide group with other surrounding atoms in the molecule. By these molecular models it is possible to explain acid-base properties of amides applying knowledge of electron density distribution in the molecules and the resulting electrostatic potential. Presentation of the structure and properties of the amides within education is important also for the reason that amidic functions are involved in many important natural substances (e.g. proteins, peptides, nucleic acids or alkaloids), synthetic macromolecular substances (e.g. Silon) or pharmaceutical preparations (e.g. paracetamol). Molecular models then serve to support better understanding of the structure of these substances and, in relation to it, their properties.

Borowce jako centra kwasów Lewisa w oddziaływaniach międzycząsteczkowych : porównanie z wiązaniami wodorowymi

Grabowski S. J.

Wiadomości Chemiczne

2017

[Z] 71, 7-8

447--471

The triel bonds are analyzed and compared with the hydrogen bond interaction. The triel bonds belong to the class of interactions that are named as the σ-hole and π-hole bonds. The σ-hole bond is an interaction between the σ-hole characterized by the positive electrostatic potential and the electron rich regions such as lone electron pairs, π-electron systems, in other words, centers paying a role of Lewis bases. The σ-holes may be observed for elements of the 14–18 groups of the periodic system and the corresponding interactions with Lewis bases are named; tetrel, pnicogen, chalcogen, halogen and aerogen bonds, respectively. On the other hand, π-holes also characterized by the positive electrostatic potential are observed for centers in planar molecules or planar fragments of molecules in regions above those planes. π-holes may be attributed to triel centers (13th group of the periodic system). The boron and aluminium trihydrides and trihalides are examples of molecules where triels are characterized by π-holes. The mechanism of the triel bond formation is very similar to the mechanism of the formation of the hydrogen bond. It is the Lewis acid – Lewis base interaction where the electron charge transfer from the base unit to the acid one is observed. Next there is outflow of the electron charge from the triel center to the other parts of the Lewis acid unit; in other words the positive charge of the triel center increases as a result of complexation. The triel bonds are often very strong and often they possess characteristics of typical covalent bonds; this is confirmed by the QTAIM (Quantum Theory of Atoms in Molecules) and NBO (Natural Bond Orbital) approaches. For example, for the triel bonds the bond paths between the triel center and the Lewis base center are observed with the bond critical points (BCPs) attributed to those paths. Similarly for the A-H…B hydrogen bonds the H…B bond paths are observed. The parameters of those BCPs often indicate the covalent character of the triel bonds and analogously those characteristics for H-bonds may also indicate the covalent character of the latter interactions. It is very interesting that the triel bonds are observed experimentally in the real systems; for example in crystal structures. The triel center which is trivalent and possesses the trigonal configuration is hypovalent; it means that the octet rule is not obeyed here because of the valence electrons´ deficiency (the triel center possesses six valence electrons in such species). Thus it may interact with one Lewis base ligand reaching rather stable octet and tetrahedral configuration. If the trivalent triel center interacts with two Lewis base ligands thus it may lead to the configuration of the trigonal bipyramid with the hypervalent and pentavalent triel center. These kinds of the triel species occur in crystal structures that are described here.

Electrostatic potential of surface charge – error of the measurements made with electrostatic field mill voltmeter

Grabarczyk Z.

Przegląd Elektrotechniczny

2012

R. 88, nr 12b

255-257

This paper presents numerical analysis of the error of the measurement of the electric potential of electrostatic surface charge distributed on the thin dielectric plane. The virtual measurement was made with the field mill voltmeter, calibrated with metallic plate which electric potential was established in respect to the earthed filed mill voltmeter.

W artykule przedstawiono numeryczną analizę błędu pomiaru potencjału ładunku elektrostatycznego na powierzchni płaskiego, cienkiego dielektryka, wykonywanego bezkontaktowym, młynkowym miernikiem strumienia indukcji elektrycznej, przeskalowanego przy użyciu rozległej, płaskiej elektrody metalowej o zadanym potencjale elektrycznym względem uziemionego miernika. Wykazano, że taki pomiar, stosowany w przeciwwybuchowej ochronie antystatycznej, może być obarczony błędem ujemnym rzędu 45 – 55 %. (Błąd pomiaru potencjału ładunku powierzchniowego, wykonanego bezkontaktowym woltomierzem młynkowym).