Chemical theory, description

However, in contrast to such whimsical and playful description, some metaphors developed into serious definitions that became essential to rigorous chemical classification and explanation. Indeed, the electrophilic/nucleophilic language is an example from the development of chemical theory which is the subject of later chapters of this book. Let us consider three other examples of metaphor-tumed-convention that dominated eighteenth- and nineteenth-century chemistry. 

The elements of stereoisomerism considered thus far consist of a point occupied by an atom and a line wholly occupied by one or more bonds. The ligands are joined directly to these elements. This emphasis on bonding relationships appears to be proper, as the distinction between constitutional and steric isomerism similarly depends on established patterns of connectedness. From this point of view it seems less than satisfactory if direct connectedness between specific atoms is assumed, when chemical theory envisions no such localized bond. This situation prevails in the description of ir complexes such as the metallocenes. Initially (44a), the 1,2- (22) or 1,3-heteroligated ferrocenes were considered to... 

The second section, on computer algebra, details chemical applications whose emphasis is on the mathematical nature of chemistry. As chemical theories become increasingly complex, the mathematical equations have become more difficult to apply. Symbolic processing simplifies the construction of mathematical descriptions of chemical phenomena and helps chemists apply numerical techniques to simulate chemical systems. Not only does computer algebra help with complex equations, but the techniques can also help students learn how to manipulate mathematical structures. 

The abundance of a solute is its concentration, and this characteristic can be reported with an intimidating variety of terms that you must master because the accurate description of solutions is central to chemical theory and laboratory practice. 

In principle, quantum-chemical theory should be able to provide precise quantitative descriptions of molecular structures and their chemical properties however, due to mathematical and computational complexities this seems unlikely to be realized in the foreseeable future. Thus, researchers need to rely on approximate methods that have now become routine and have found wide applications. In many cases, errors due to the approximate nature of quantum-chemical calculations and the neglect of the solvation effects are largely transferable within structurally related series (Karelson and Lobanov, 1996). Thus, relative values of calculated descriptors can be meaningful even though their absolute values are not directly applicable. 

The merging of mathematical graph theory with chemical theory is the formalization of what most chemists do in a more or less intuitive mode. Chemists currently use graphical images to embody chemical information in compact form which can be transformed into algebraic sets. Chemical graph theory provides simple descriptive interpretations of complicated quantum mechanical calculations and is, thereby, in-itself-by-itself an important discipline of study. 

If one organic compound has dominated the historical literature of the last few years, that compound must be benzene. Most probably, this is because its structure in some respects marks a transition from the most austere form of classical organic chemistry, in which carbon was tetravalent and tetrahedral, to a continuing series of changes from oscillating molecules, through partial valencies to MO descriptions, and Huckel s rules of aromaticity. It is the case par excellence of a single substance whose history intersects all major streams of chemical theory - except perhaps the periodic law - and which also has enormous industrial and economic importance. 

In the MO theory, the most reliable approach for the study of reaction pathways perhaps is CASSCF [12, 13], but multi-VBSCF is essentially at the same level with CASSCF [14]. Since a VB wave function can be expanded into the combination of numerous Slater determinants that are used to define configurations in the MO theory, the VB theory provides a very compact, accurate description for chemical reactions. While both MO and VB theories have their own advantages as well as disadvantages, in our opinions, there are some areas where the VB theory is particularly superior to the MO theory 1) the refinement of molecular mechanics force field 2) the development of empirical or semi-empirical VB approaches 3) the impact of intermolecular charge transfer or intramolecular electron delocalization on the structure and properties 4) the validation of classical chemical theories and concepts at the quantitative level 5) the elucidation of chemical reactions and excited states intuitively. 

If molecular densities were determined on the basis of Eq. (3.38), atomic densities might be evaluated by contraction of those results. Equation (3.38) provides a derivation of the previously mentioned conditional density of Eq. (3.4). This point hints at a physical issue that we note. As we have emphasized, the potential distribution theorem doesn t require simplified models of the potential energy surface. A model that implies chemical formation of molecular structures can be a satisfactory description of such molecular systems. Then, an atomic formula such as Eq. (3.35) is fundamentally satisfactory. On the other hand, if it is clear that atoms combine to form molecules, then a molecular description with Eq. (3.38) may be more convenient. These issues will be relevant again in the discussion of quasi-chemical theories in Chapter 7 of this book. This issue comes up in just the same way in the next section. 

It is interesting that Eq. (4.98), p. 97, offers a discrete-state partition function for the description of the inner-sphere contribution to the thermodynamics. But the discrete coordinate is an occupation number for a precisely defined configurational region, and parameters required for this discrete-state partition function are obtained by molecular-level calculations. Therefore, molecular realism isn t the first casualty of these theories, although strong approximations are typically accumulated after the formulation of quasi-chemical theories. 

The clear liquid yielded saltpeter upon drying. The juxtaposition of these two methods—distillation vs. precipitation —relayed Geoffroy s hope that the laws of rapports could mediate between the different chemical procedures to shape a uniform terrain of chemical theory. By going through a variety of processes that produced corrosive sublimates, Geoffrey illustrated that the same order of rapports could explain all the varieties of chemical operations in a consistent manner. The theory of the process was the same, no matter what the variations in practice were. Through the example of corrosive sublimates, Geoffroy thus presented concisely and powerfully the descriptive, predictive, and explanatory functions of his table of rapports. 

Chemistry has not progressed so far as physics, for some parts of physics have now become essentially theoretical sciences rather than, descriptive. It is not possible to obtain a sound knowledge of chemistry simply by learning theoretical chemistry. Even if a student were to learn all the chemical theory that is known, he still would not have a knowledge of the science, because a major part of chemistry, the discussion of the special properties of individual substances, has not yet been well incorporated into chemical theory. Therefore these chemical facts must be taught in the chemistry course. 

In your study of chemistry you should make a continual effort to correlate the descriptive facts with the system of chemical theory. For example, the facts about the element phosphorus and its compounds may be more easily remembered if, while studying them, you think about their possible explanation in terms of atomic and molecular structure and also about the way in which they differ from the corresponding facts about other elements, especially in relation to the position of the elements in the Periodic Table, as described in Chapter 5. 

The need to memorize some of the facts of descriptive chemistry and to correlate them with chemical theory. 

The fundamental principles underlying the planning of the present book have been expressed in the preface of my earlier textbook, General Chemistry, An Introduction to Descriptive Chemistry and Modern Chemical Theory, published three years ago. The first two paragraphs of the Preface of General Chemistry" summarize these principles ... 

These ring collision events are now a familiar part of the kinetic theory description of dynamic processes in simple dense fluids. A brief comparison of the theory for the velocity autocorrelation function with that for the chemically reacting fluid will help motivate our description. Recent developments in the theory of the velocity autocorrelation function have arisen out of an attempt to understand the slow t power law decay observed by Alder and Wainwright in a computer simulation of a dense hard-sphere fluid. This work also showed that the translational motion of a small hard sphere in a fluid of similar hard spheres has a significant collective (hydro-dynamic) component. On the theoretical side, this type of behavior was discussed from the kinetic theory point of view in terms of the ring collision events described above and provided a microscopic basis for the introduction of collective effects. In addition, it was shown that mode... 

Pauling, L., General Chemistry An Introduction to Descriptive Chemistry and Modern Chemical Theory W. H. Freeman San Francisco and London, 1953. 

Since the introduction of the electronegativity as chemical potential with changed sign is considered to be a fundamental observable for the characterization of the equilibrium states of the electronic systems in interaction, the chemical reactivity description in terms of quantum statistics and algebraic theory is considered to be a fundamental step in elucidating the tendencies of evolution to and from the equilibrium states, admitted by an electronic system (finite) and also of the afferent critical states. 

It would be desirable to apply analytical expressions for the activity coefficient, which are not only able to describe the concentration dependence, but also the temperature dependence correctly. Presently, there is no approach completely fulfilling this task. But the newer approaches, as for example, the Wilson [13], NRTL (nonrandom two liquid theory) [14], and UNIQUAC (universal quasi-chemical theory) equation [15] allow for an improved description of the real behavior of multicomponent systems from the information of the binary systems. These approaches are based on the concept of local composition, introduced by Wilson [13]. This concept assumes that the local composition is different from the overall composition because of the interacting forces. For this approach, different boundary cases can be distinguished ... 

Section II summarizes the chemical theory most important for the discussion of ozone in the troposphere, and typical tropospheric ozone concentrations are presented in Section III. Section IV contains a short description of the methods which are currently used to measure tropospheric ozone and problems and methods of tropospheric ozone trend determination are discussed in Section V. The present knowledge of trends of surface ozone in different parts of the world is summarized in Sections VI-IX and ozone trends of the free troposphere are summarized in Section X. Section XI includes the smnmaty and some conclusions. 

Plate Theory elution equation equation that gives the concentration of a given analyte in the last plate of the column (adjacent to the detector) as a function of the initial concentration in the first plate before elution has begun, the total mobile phase volume required for analyte elution, the number of theoretical plates N, and basic physico-chemical properties of the system (the partition coefficient) see Equation [3.11] as the Plate Theory description of a chromatographic peak like that in Figure 3.2. 

In this abstract we discuss only those aspects of our work which fall under the last head. The relation between the Mbssbauer parameters AEq (quadrupole split) and 6 (the chemical shift) on the one hand and current chemical theory (especially ligand field theory) is now so well understood that it is possible to predict the way they will change as chemical features are altered, and how they are related to the results of other physical measurements. We here present a brief description of the theoretical features and the way they explain certain experimental results reported previously. 

It will be pointed out later in this chapter that the description of compounds as aggregates of ions is an approximation. The electronic structure of molecules and crystals usually described as ionic involves only a partial transfer of electrons from the metal atoms to the nonmetal atoms. Nevertheless, the discussion of ionic valence in relation to the argononic electron configurations, as given above, is an important and useful part of-chemical theory. 

Density-functional theory, even with rather crude approximations such as LDA and GGA, is often better than Hartree-Fock LDA is remarkably accurate, for instance, for geometries and frequencies, and GGA has also made bond energies quite reliable. Therefore, the aura of mystery appeared around DFT (see discussion of this by Baerends and Gritsenko [367]). The simple truth is not that LDA/GGA is particularly good, but that Hartree-Fock is rather poor in the two-electron chemical-bond description. This becomes clear when one considers the statistical two-electron distribution, which is usually cast in terms of the exchange-correlation hole the decrease in probabihty to find other electrons in the neighborhood of a reference electron, compared to the (unconditional) one-electron probabihty distribution [337]. 

It has been the aim of the aathois, in writing the present treatise, to place before the reader a irly complete and jet a dear and succinct, statement of the facts of Modem Chemistij, whilst at the same time entering so far into a discussion of Chemical Theory as the size of the wori and the present transition state of the sdence permit. Special attention has been pud to the acenrate description of the more important processes in technical diemislxj, and to the careful representation of the most approved forms of apparatus employed. As an instance of thi the authors may refer to the chapter on the Maou cture of Sulphuric Add. For valuable information on these points they are indebted to many friends both in this country and on the Continent. 

Some of the contributions address the calculation of intermolecular forces at a fimdamental level, while the majority are concerned with appHcations, ranging from water clusters, through smfaces, to crystal structures. Sza-lewicz, Patkowski and Jeziorski provide a timely review of how perturbation theory can be used to address intermolecular forces in a systematic way. In particular, they describe a new version of symmetry-adapted perturbation theory, which is based on a density functional theory description of the monomers. The interpretation of bonding patterns for both intra- and intermolecular interactions is addressed in Popelier s review, which focuses on quantum chemical topology. He suggests a novel perspective for treating several of the most important contributions to intermolecular forces, and explains how these ideas are related to quantum delocalization. 

---