Foreword
2006; Wiley; Volume: 28; Issue: 1 Linguagem: Inglês
10.1002/jcc.20544
ISSN1096-987X
Autores Tópico(s)Chemistry and Chemical Engineering
ResumoChemical bonding has been a traditional chemical territory ever since the chemical community amalgamated in the 17th century, and even before. The modern charter of this territory is Gilbert Newton Lewis, who started the electronic structure revolution in chemistry. About 90 years ago Gilbert Newton Lewis published, in the Journal of the American Chemical Society, a paper entitled “The Atom and the Molecule,” in which he proposed a general theory of chemical bonding based on the newly discovered particle, the electron. The paper introduced two effective concepts: One, to be called later by Irving Langmuir the “Octet Rule,” defined the bonding capability of an atom in terms of an upper bound number of electrons around a given atom in a molecule. The other defined a unit of bonding, the “Electron Pair Bond.” Lewis also invented an ingeniously portable symbol for this bond, the electron dot structure, e.g., H:H, which enabled to construct known and new molecules, to reason about their properties and reaction mechanisms; in brief, to construct a chemical universe. Thus after the “compositional revolution” of Dalton, who defined the atoms as the quantum weights of elementary bodies of chemical matter, there came the “electronic structure revolution” ushered by the Lewis concept. And like in any other revolution, now, from a distance of 90 years, we recognize that some of the reasoning applied by Lewis were wrong, quite a few of us tend to quibble with his Octet Rule, we certainly know to day of other “modes of bonding,” and the use of quantum mechanical gown of the bonding concept has tended to associate it with physics rather than with chemistry. Despite all these modifications and qualifiers, the work of Lewis has had a tremendous impact on the evolution of chemistry wherein the bond concept remains to be a central guiding theme. What is so striking about the Lewis bonding model is that it was formulated 11 years before the introduction of quantum mechanics into chemistry in 1927 by Heitler and London (HL). In fact, the HL work and the subsequent quantum mechanical treatments by London and Pauling were a quantum mechanical formulation of the Lewis concept. Even the delocalized molecular orbital picture, of Mulliken and Hund, was shown to be reducible to the Lewis concept by applying localization criteria to the many electron wave-function of molecules. Thus quantum chemistry provided a solid support for the Lewis concept of electron pair bonding. And while the chemical community accepts the ‘verdict’ that the chemical bond can only be correctly understood to day in the framework of quantum mechanics, yet the Lewis concept remains the most widely used model in contemporary chemistry. The strength of the Lewis hypothesis rested on a wide-ranging experimental foundation, following the maxim coined by Roald Hoffmann: “Experimental trends are noted. A theory is constructed that does not merely rationalize but makes verifiable predictions. When these predictions fail the theory can be enriched by re-examination. Chemistry advances.” Indeed, such as this was the Lewis theory, and hence, his ideas and applications have resurfaced in modern approaches to the chemical bond. Thus, the Lewis notion outlined a vast chemical territory and affected most profoundly the mental map of chemistry for generations ahead. As a tribute to Lewis and to his great intellectual achievement, and as recognition that “chemical bonding” is a chemical terrain that needs to be nurtured by the chemical community, this issue of the Journal of Computational Chemistry is dedicated to chemical bonding in general and to Lewis in particular. The issue involves two parts: essay and regular papers. The first part involves nine essays, written about Lewis, his work, and others in the context of issues associated with chemical bonding and its ramifications in chemistry. The writers of the essays were given a good deal of freedom to express their opinions. While the editors may not agree with all the opinions expressed in the essays, we note that all make interesting reading material and reflect the sometimes polyphonic culture of chemical thought. The first essay by Ana Simoes looks at the birth of the Lewis concept of electron pair bonding, and how this idea was appropriated by the American founders of quantum chemistry. The essay discusses the evolution of the Lewis notion within the context of a discovery process, and characterizes Lewis as an architect, a builder of a cathedral with ideas that crossed disciplines. The second essay by Don Truhlar provides a rich perspective on issues in valence bond theory. With more than 400 references, the essay discusses the physical significance of semilocal bonding orbitals, the capability of valence bond concepts to explain systems with multireference character, the use of valence bond theory to provide analytical representations of potential energy surfaces for chemical dynamics. In Don's essay it becomes apparent that valence bond theory is a vital and evolving part of chemical thought. Mary Jo Nye discusses the post quantum mechanical developments, when physical chemists began incorporating the new theory in blend with the Lewis bond concept in their studies of activation energies, transition states, and chemical reactions for simple atomic and molecular systems. The essay examines the circumstances of the collaborative work of Michael Polanyi and Henri Eyring, two of the founders of chemical dynamics and transition state theory, and its impact on subsequent generations. The essay by Jean-Paul Malrieu, Nathalie Guihéry, and Carmen Jiménez Calzado argues that quantum chemistry has given the fantastic Lewis idea a surprisingly poor status. The essay reflects the tension still at work between MO and VB theories and shows how the theories are easily abridged by projecting correlated MO wave functions unto VB structures. This allows the authors to demonstrate that electron correlation effects do not really alter the integrity of electron pair bonding. The Lewis concept emerges intact from the highest levels of theory. Werner Kutzelnigg devotes his essay to Hückel and his theory (HMO theory) and mentions also Otto Schmidt, the inventor of the free-electron model, and the tension between the two scientists. The author analyzes the basic assumption of the HMO theory and highlights the aspects that make it still a powerful conceptual tool in theoretical chemistry. Many application of the theory are discussed and the author concludes that the HMO model is not merely an approximate solution of the Schrödinger equation, but rather a framework for understanding the chemical bond. Ron Gillespie's essay discusses the notion of the bond in historical context and emphasized its advantages and disadvantages. The bond is then viewed within the perspective of electron density and quantum theory of atoms in molecules (QTAIM) and valence bond theory. The author reaches the conclusion that it is more practical to consider molecular geometry than attempting to define the “undefinable” object called a bond. The author then discusses the valence shell electron pair repulsion (VSEPR) model, based on the Lewis electron pair concept and the Pauli exclusion principle, and shows its wide applicability to the understanding of molecular geometry. Some new modifications of the VSEPR model are reviewed. Richard Bader, Jesus Hernandez-Trujillo, and Fernando Cortes-Guzman discuss the Lewis concept within the context of Bader's QTAIM. They show that the Lewis concept is recovered in the properties of the electron pair density and in the topology of the Laplacian of the electron density. Using these physical properties the authors show that there is a rigorous physical basis behind the chemical concepts of atoms and bonds in molecules. Sason Shaik's essay begins with the question, asked by historians of science: is it true that chemistry is a science without a “territory?” The essay tries to answer this question by looking back at the roots of the notion of chemical bonding, and by analyzing the three key papers of Lewis from the years 1913, 1916, and 1923. The author argues that it was Lewis' work that defined the quantum unit for construction of a chemical universe, for ushering the electronic structure revolution, and for charting a vast chemical territory that is still teeming with exciting problems. In their essay, Gernot Frenking and Andreas Krapp look at the important roles played by heuristic concepts in chemistry. The authors liken these concepts to “unicorns,” which while not real bring order and good fortune. Bonding models share with unicorns the feature that their presence seems natural although nobody has even seen them. The authors mention some of these concepts, e.g., aromaticity, classical bonding pictures, etc., and then focus on the concepts derived from energy decomposition analysis (EDA) and its application to chemical bonding. Quite a few surprises await the reader. The second part of the issue involves 28 papers dedicated to various aspects of the chemical bond and to the methods used to probe its properties. The large variety of papers exemplifies the enormous efforts that have been made to follow the famous admonition of Charles Coulson: “Give us insight, not numbers.” Two papers by Klaus Ruedenberg and Mike Schmidt and by T. Bitter, Klaus Ruedenberg, and Eugen Schwarz focus on the understanding for the physical origin of a two-center chemical bond. The reader may wish to read in this context the essay by Bader and coworkers. Numerous papers deal with various energy and charge partitioning techniques. Two recent methods of EDA are described in the contributions by Dimitri Fedorov and Kazuo Kitaura, and by Angel Martin Pendás, M. Blanco and E. Francisco. Matthias Bickelhaupt, Miquel Solà, and Célia Fonseca Guerra employ an energy partitioning analysis for addressing the question about the ionic and covalent character of alkalimetal fluoride compounds. Michel Rafat and Paul Popelier discuss their attempt to use atom–atom partitioning of molecular energies for deriving force–field parameters. The evaluation of the electrostatic energy in the benzene dimer, using the effective fragment potential method, is presented in the paper by Lyudmila Slipchenko and Mark Gordon. The role of radial nodes of atomic orbitals for chemical bonding across the periodic table is discussed by Martin Kaupp. Three papers demonstrate the use of valence bond theory for chemical bonding models. Recent methodology developments in the field are described in the paper by Philippe Hiberty and Sason Shaik. The essay of Truhlar makes a good companion to this paper. Chemical bonding in dioxygen, an archetypical and enigmatic molecule for classical bonding models, is described by Peifeng Su, Lingchun Song, Wei Wu, Philippe Hiberty, and Sason Shaik using modern VB models. John F. Beck and Yirong Mo introduce their study of hydrogen bonding using the block-localized wave function method. Reading these paper shows that VB theory is a phoenix rising from its ashes. Not surprisingly, several papers focused on the evergreen topics delocalization and aromaticity. A definition of delocalization indices to describe aromaticity based on the QTAIM, is discussed in the contribution by Marcos Mandado, María González-Moa, and Ricardo Mosquera. Patrick Bultinck, Robert Ponec, and Ramon Carbó-Dorca employ two novel approaches for investigating aromaticity in linear polyacenes. The σ and π contributions to the induced magnetic fields in benzene, cyclobutadiene, and other simple hydrocarbons containing double or triple bonds are discussed in the work by Thomas Heine, Rafael Islas, and Gabriel Merino. The very popular natural bond orbital (NBO) method, which is closely associated with the Lewis bonding model, is the focus of two theoretical studies: One, by Clark Landis and Frank Weinhold, who discuss the important question about valence and extra-valence orbitals, and the other by Lionel Goodman and Ronald Sauers, who show that diffuse orbitals may corrupt the NBO results. Bond orders have been among the most popular theoretical concepts for characterizing covalent bonds since Robert Mulliken introduced the first definition, which had numerous offsprings until today. A personal account of the development of bond order and valence indices is presented by Istvan Mayer. Secondary orbital interactions (SOI) have been invoked in the past to explain stereochemical preferences for some pericyclic reactions but the SOI model has been questioned in recent studies. Chaitanya Wannere, Ankan Paul, Rainer Herges, Ken Houk, Fritz Schaefer, and Paul Schleyer discuss the results of a careful analysis of transition states for pericyclic reactions in a search for the existence of SOI effects. Modern methods of Hilbert space partitioning are discussed in the paper by Robert Ponec and Joaquin Chaves, who use domain averaged Fermi holes in conjunction with the electron localization function. In search of Lewis structures, Anthony Scemama, Michel Caffarel, and Andreas Savin define electron pairs by locating domains of the three-dimensional space, which maximize the probability of containing electron pairs with opposite spins. Igor Alabugin and Mariappan Manoharan suggest in their contribution that rehybridization is a general mechanism that determines chemical bonding and chemical reactions. The origin of the sigma trans promotion effect in transition metal complexes is examined by Dmitry Khoroshun, Djamaladdin Musaev, and Keiji Morokuma. Several papers focus on analyzing chemical bonds in particularly interesting compounds. The electronic structure and bonding situation in CO, which is a challenge for chemical bonding theory is analyzed by Gernot Frenking, Christoph Loschen, Andreas Krapp, Stefan Fau, and Steven Strauss, by use of modern methods of quantum theory. The origin of the two-electron/four-center CC bond in dimers of tetracyanoethylene anion is analyzed in the work of Iñigo Garcia-Yoldi, Fernando Mota, and Juan Novoa. The bonding situation in triple-decker sandwich complexes with P6 as middle ring is discussed by Dandamudi Rani, Dasari Prasad, John Nixon, and Eluvathingal Jemmis. A comprehensive analysis of chemical bonding in boron clusters is presented by Dmitry Zubarev and Alexander Boldyrev. Finally, two theoretical studies by Gabriel Merino, Miguel Méndez-Rojas, Alberto Vela, and Thomas Heine and by B. Sateesh, Srinivas Reddy, and Narahari Sastry present theoretical studies of compounds containing planar tetracoordinate carbon atoms. A bountiful issue is waiting for the reader! We hope that this special issue of the Journal of Computational Chemistry will demonstrate to the reader that we are still far from understanding the nature of the chemical bond and much work lies ahead for the chemical community. From a chemical point of the view, chemical bonds characterize the diversity of chemical matter in the universe. Attempting to grasp the essence of chemical bonding in terms, which are accessible to human minds is equal to understanding the material world, an eternally challenging task for scientists. And while there is plenty of work for historians of chemistry to trace the roots and historical development of chemical bonding models, the field itself is exploding with new problems to be solved! The dialog of chemists with the notion of the chemical bond continues…
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