Valency modern conception

We focus in this Section on particular aspects relating to the direct interpretation of valence bond wavefunctions. Important features of a description in terms of modern valence bond concepts include the orbital shapes (including their overlap integrals) and estimates of the relative importance of the different structures (and modes of spin coupling) in the VB wavefunction. We address here the particular question of defining nonorthogonal weights, as well as certain aspects of spin correlation analysis. 

The ability of olefins and acetylenes to function as ligands towards transition metals has aroused considerable interest in the last ten years from both the theoretical and practical viewpoints. Elucidation of the nature of the bonding between the unsaturated hydrocarbon and the metal atom has made an important contribution to the modern concept of valency, and has led to a better understanding of the role metallic catalysts play in chemical reactions. 

Once the modern conception of the atom, as outlined above, had been accepted, it became clear that the earlier views as to the mechanism of valency required x-evision. 

To systematize the structural problems of the so-called compounds of higher order he reconsidered valency. Thus, he advanced the idea of Hauptvalenz and Nebenvalenz, which could be said to contain the image of modern ionic and covalent bonding theory. Furthermore, he visualized the valence bond not as small sticks existing around atoms in a plane, as conceived by Kekul6, but as positions taken by bond arms on the surfaces of spherical atoms in three dimensions. Could this idea not be a precursor to the modern conception of bond orbitals ... 

The central idea of this book is the development of a general theory of reactions which will include both inorganic and organic reactions. The fundamental view upon which this theory is based is the addition theory according to which when two or more substances react a primary addition is the first step. This theory is not new. It has been used in more or less isolated cases for a number of reactions and may have been suggested as of general applicability. As far as the writer is aware, however, this is the first time that it is published in an extended form with modern conceptions of chemical structures, which themselves rest upon the development of valence views. 

The modem theory of valency is not simple—it is not possible to assign in an unambiguous way definite valencies to the various atoms in a molecule or crystal. It is instead necessary to dissociate the concept of valency into several new concepts—ionic valency, covalency, metallic valency, oxidation number—that are capable of more precise treatment and even these more precise concepts in general involve an approximation, the complete description of the bonds between the atoms in a molecule or crystal being given only by a detailed discussion of its electronic structure. Nevertheless, these concepts, of ionic valency, covalency, etc., have been found to be so useful as to justify our considering them as constituting the modern theory of valency. 

The implications of the foregoing concept have profoundly influenced modern trends in polymer research. If polymers owe their differences from other compounds to the extent and arrangement of their primary valence structures, the problem of understanding them is twofold. It is necessary in the first place to provide appropriate means, both experimental and theoretical, for elucidating their macromolecular structures a[Pg.3]

Valency, as the principle is called today, is one of the fundamental concepts in chemistry. In more modern terms, the valence of an atom equals the number of bonds that an atom has for combining with other elements hydrogen, for example, has one, while other elements have more. 

This graduate-level text presents the first comprehensive overview of modern chemical valency and bonding theory, written by internationally recognized experts in the held. The authors build on the foundation of Lewis- and Pauling-like localized structural and hybridization concepts to present a book that is directly based on current ab initio computational technology. 

Our principal goal has been to translate the deepest truths of the Schrodinger equation into a visualizable, intuitive form that makes sense even for beginning students, and can help chemistry teachers to present bonding and valency concepts in a manner more consistent with modern chemical research. Chemistry teachers will find here a rather wide selection of elementary topics discussed from a high-level viewpoint. The book includes a considerable amount of previously unpublished material that we believe to be of broad pedagogical interest, such as the novel Lewis-like picture of transition-metal bonding presented in Chapter 4. 

Due to the simplicity and the ability to explain the spectroscopic and excited state properties, the MO theory in addition to easy adaptability for modern computers has gained tremendous popularity among chemists. The concept of directed valence, based on the principle of maximum overlap and valence shell electron pair repulsion theory (VSEPR), has successfully explained the molecular geometries and bonding in polyatomic molecules. 

As an outgrowth of an advanced Organic Chemistry course which Jack developed at Cornell, he prepared a 117 page chapter on Modern Electronic Concepts of Valence, which was published in Gilman s Advanced Treatise on Organic Chemistry, Volume II, 1938. 

We overview our valence bond (VB) approach to the ir-electron Pariser-Parr-Pople (PPP) model Hamiltonians referred to sis the PPP-VB method. It is based on the concept of overlap enhanced atomic orbitals (OEAOs) that characterizes modern ab initio VB methods and employs the techniques afforded by the Clifford algebra unitary group approach (CAUGA) to carry out actual computations. We present a sample of previous results, sis well sis some new ones, to illustrate the ability of the PPP-VB method to provide a highly correlated description of the ir-electron PPP model systems, while relying on conceptusilly very simple wave functions that involve only a few covalent structures. 

There are now quite a sizeable number of high superconductors, and in the plot [33] shown in Fig. 15 it is apparent that the Cu-O distance is a useful parameter to describe their superconducting properties. A connection can be made between these Cu-O distances and the number of holes in the CuOj layer by consideration of the bond valence sum, v, at the copper atoms. This concept dates from Pauling s idea of the electrostatic bond sum rule and has been refined in the intervening period [34]. Modern usage defines it as the sum of the bond strengths over all bonds coordinated to copper in the following way. The bond valence sum, u = E, exp(r, - r(,)/0.37, where the parameter depends on the... 

Classical valence bond (VB) theory is very successful in providing a qualitative explanation for many aspects. Chemists are familiar with the localized molecular orbitals (LMO) and the classical VB resonance concepts. If modern accurate wave functions can be represented in terms of such well-known concepts, chemists intuition and experiences will give a firm theoretical basis and the role of the computational chemistry wUl undoubtedly expand. 

For reasons we will explore in the next chapter, elements with a particular type of valence configuration all show very similar chemical behavior. Thus groups of elements, such as the alkali metals, show similar chemistry because all the elements in that group have the same type of valence-electron arrangement. This concept, which explains so much chemistry, is the greatest contribution of the wave mechanical model to modern chemistry. 

Abstract The wave function of Coulson and Fischer is examined within the context of recent developments in quantum chemistry. It is argued that the Coulson-Fischer ansatz establishes a third way in quantum chemistry, which should not be confused with the traditional molecular orbital and valence bond formalisms. The Coulson-Fischer theory is compared with modern valence bond approaches and also modern multireference correlation methods. Because of the non-orthogonality problem which arises when wave functions are constructed from arbitrary orbital products, the application of the Coulson-Fischer method to larger molecules necessitates the introduction of approximation schemes. It is shown that the use of hierarchical orthogonality restrictions has advantages, combining a picture of molecular electronic structure which is an accord with simple, but nevertheless empirical, ideas and concepts, with a level of computational complexity which renders praetieal applications to larger molecules tractable. An open collaborative virtual environment is proposed to foster further development. 

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