A Review of Basic Bonding Concepts

In this section we present a number of basic concepts associated with chemical bonding and organic structure. Most of this material should be quite familiar to you. We use this section to collect the terminology all in one place, and to be sure you recall the essentials we will need for the more advanced model of bonding given in Sections 1.2 and 1.3. For most students, a quick read of this first section will provide an adequate refresher. 

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In the past bond analysis was frequently limited to calculating gross redemption yield, or yield to maturity. Today basic bond math involves different concepts and calculations. These are described in several of the references for chapter 3, such as Ingersoll (1987), Shiller (1990), Neftci (1996), Jarrow (1996), Van Deventer (1997), and Sundaresan (1997). This chapter reviews the basic elements. Bond pricing, together with the academic approach to it and a review of the term structure of interest rates, are discussed in depth in chapter 3. 

The molecular orbital (MO) is the basic concept in contemporary quantum chemistry. " It is used to describe the electronic structure of molecular systems in almost all models, ranging from simple Hiickel theory to the most advanced multiconfigurational treatments. Only in valence bond (VB) theory is it not used. Here, polarized atomic orbitals are instead the basic feature. One might ask why MOs have become the key concept in molecular electronic structure theory. There are several reasons, but the most important is most likely the computational advantages of MO theory compared to the alternative VB approach. The first quantum mechanical calculation on a molecule was the Heitler-London study of H2 and this was the start of VB theory. It was found, however, that this approach led to complex structures of the wave funetion when applied to many-electron systems and the mainstream of quantum ehemistry was to take another route, based on the success of the central-field model for atoms introduced by by Hartree in 1928 and developed into what we today know as the Hartree-Foek (HF) method, by Fock, Slater, and co-workers (see Ref. 5 for a review of the HF method for atoms). It was found in these calculations of atomic orbitals that a surprisingly accurate description of the electronic structure could be achieved by assuming that the electrons move independently of each other in the mean field created by the electron cloud. Some correlation was introduced between electrons with... 

In the 1920s it was found that electrons do not behave like macroscopic objects that are governed by Newton s laws of motion rather, they obey the laws of quantum mechanics. The application of these laws to atoms and molecules gave rise to orbital-based models of chemical bonding. In Chapter 3 we discuss some of the basic ideas of quantum mechanics, particularly the Pauli principle, the Heisenberg uncertainty principle, and the concept of electronic charge distribution, and we give a brief review of orbital-based models and modem ab initio calculations based on them. 

In Section IV-VI we systematically discuss the acidity, basicity and, where appropriate, the hydrogen-bonding of nitrones, nitriles and thiocarbonyls. Since this review very much relies on physical measurements and the discussion of acid-base reactions, Section HI provides a brief introduction to proton affinity and a short summary of the acid-base concept and the quantitative measure of basicity. 

The purpose of this review is to discuss some recent computational studies of radical cations in the context of qualitative concepts of classical physical organic chemistry. In particular, we will demonstrate how such basic, well-understood concepts such as conjugation and electronic state or even more fundamental notions of structure, bonding, and mechanism can lead to new and interesting effects in radical cation chemistry, which are quite different than what is usually expected in the chemistry of neutral compounds. We will also discuss how these effects need to be taken into consideration to understand the chemistry of radical cations. This relatively broad scope means that this review will necessarily be limited to a focused discussion rather than a comprehensive review of the different aspects of radical cation chemistry. Thus, we will concentrate on computational results from our own laboratory, and will discuss experimental data only in the context of the calculational data. A number of recent reviews5 and book chapters6 provide much more detail on aspects that cannot be covered in this limited contribution. 

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