The chemical bond is always considered from different points of view, depending on the classes of compounds or on the chemical and physical aspects to be examined. In both cases, descriptions of the chemical bond are chosen that are appropriate for the particular research or application. Therefore, there are significant differences in the understanding as what constitutes chemical bonding. This is acceptable in practice but proves to be a hindrance for true interdisciplinarity. The concept of the chemical bond offers a firm basis upon which to forge links not only within chemistry but also to all related sciences. The general desire and the growing necessity for interdisciplinary collaboration requires a careful treatment of these concepts and, if possible, a tightening and standardization to a level that is widely acceptable and beneficial. In the age of tremendously fast development of computers and computer science, we believe that the electron localization function (ELF) provides a new description of the chemical bond for almost all classes of compounds. Its graphical language earns it the ultimate qualification for enhanced interdisciplinari ty.Keywords: bond theory -electron localization function * structure elucidation The Electron Localization Function-ELFChemistry-as we know it-without the Pauli principle is inconceivable. This principle is deeply rooted in so many chemical concepts that the connection is frequently neglected; for instance, the systematics of the periodic table, steric hindrance, and the covalent bond. The Pauli principle itself is also presented in various ways. In books on quantum mechanics it is often described as a property of the wave function, which needs to be antisymmetric with respect to interchange of electrons. From textbooks on chemistry one recalls mostly the fact that orbitals can be occupied by at most two electrons. The Pauli principle is reflected in the expression for the total energy of a system both in an energy-lowering part (the exchange term) and a larger energy-raising contribution, which can be found indirectly in the kinetic energy. The following explanation is usually used : The energetically higher canonical orbitals occupy the same space as the ones lower in energy but they must have [ "I This work was supported by the Swiss National Foundation under Grant Nos.additional nodal planes (in order to meet the criterion of orthogonality) and therefore a steeper behavior. Their appearance strongly resembles a growing precipitous landscape. The slope is expressed mathematically by the derivative; the latter can be found in the formula for the kinetic energy T [Eq. (I)], where pi are the orbitals and ni their occupation numbers.The Pauli principle has been used as a basis for the explanation of many chemical concepts. For example, Gillespie's explanation of his rules."l In this article we will show that most of these chemical concepts can be described very well with the electron localization function (ELF). The relationship of ELF with older concepts will be emphasiz...
Die Gesichtspunkte, unter denen die Frage nach der chemischen Bindung gestellt wird, hängen davon ab, welche Verbindungsklassen oder welche chemisch‐physikalischen Aspekte behandelt werden sollen, d.h. man wählt jeweils die zum Forschungs‐ und Anwendungshintergrund passenden Beschreibungsschemata für die chemische Bindung aus. So existieren bedeutende Unterschiede im Verständnis dessen, was chemische Bindung ist. Dieser Umstand führt zu Trennungen, die vielleicht aus praktischen Gründen verständlich, aber im Sinne einer echten Interdisziplinarität sehr hinderlich sind. Dabei bieten gerade die Konzepte der chemischen Bindung eine ungeheuer tragfähige Basis, um nicht nur innerhalb der Chemie, sondern auch zu allen benachbarten Wissenschaften Brücken zu schlagen. Der allgemeine Wunsch nach interdisziplinärer Zusammenarbeit und die zweifellos gewachsene Notwendigkeit hierfür erfordern eine pflegliche Behandlung dieser Konzepte und, wenn möglich, eine Straffung und Vereinheitlichung, die möglichst vielen akzeptabel erscheint und hilfreich wird. Wir glauben, daß die Elektronenlokalisierungsfunktion (ELF) in der Ära einer lawinenartigen Computer‐ und Informatikentwicklung eine neue Beschreibung der chemischen Bindung für nahezu alle Stoffklassen ermöglicht und durch ihre Bildsprache entscheidend zu einer erweiterten Interdisziplinarität beitragen sollte.
Many forms of chemical bonding are exhibited by Al in intermetallic and Zintl phases. This is revealed by the calculation of the electron localization function for the series Al, CaAl2, SrAl2, BA14, CaAl2Si2, and Si. With this procedure it is possible to distinguish clearly between metallic, covalent multicenter, and homo‐ and hetero‐nuclear single bonding.
Ca14Si19 is an overlooked binary phase in the Ca/Si system with a novel type of twodimensional silicon framework (R3c, a = 867.85(6), c = 6852.8(8) pm, Z = 6). The basic building units are 3,3,3‐barrelanes Si11 which are linked by Si3 bridges to form a twodimensional silicon framework leaving space for interstitial calcium atoms. The thickness of the silicon layers is about 800 pm. The compound is a semiconductor with a band gap of about EG = 0.1 eV and a diamagnetic moment of χmole = −9 · 10−4 cm3mol−1. According to the relatively high linking of silicon atoms the reaction with air and moisture is fairly slow.
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