Lewis structure defined

When writing a Lewis structure we restrict a molecule s electrons to certain well defined locations either linking two atoms by a covalent bond or as unshared electrons on a sm gle atom Sometimes more than one Lewis structure can be written for a molecule espe cially those that contain multiple bonds An example often cited m introductory chem istry courses is ozone (O3) Ozone occurs naturally m large quantities m the upper atmosphere where it screens the surface of the earth from much of the sun s ultraviolet rays Were it not for this ozone layer most forms of surface life on earth would be dam aged or even destroyed by the rays of the sun The following Lewis structure for ozone satisfies fhe ocfef rule all fhree oxygens have eighf elecfrons m fheir valence shell... 

Bond paths are observed between bonded atoms in a molecule and only between these atoms. They are usually consistent with the bonds as defined by the Lewis structure and by experiment. There are, however, differences. There is only a single bond path between atoms that are multiply bonded in a Lewis structure because the electron density is always a maximum along the internuclear axis even in a Lewis multiple bond. The value of pb does, however, increase with increasing Lewis bond order, as is shown by the values for ethane (0.249 au), ethene (0.356 au), and ethyne (0.427 au), which indicate, as expected, an increasing amount of electron density in the bonding region. 

Bond paths are normally found in cases in which there is a bond as defined by Lewis. There is only one bond path for a multiple bond irrespective of the bond order. The bond order is, however, reflected in the value of pbcp. Bond paths are also found in molecules for which a single Lewis structure cannot be written. 

Figure 3.43 The pi-conjugative non-Lewis energy (l NL(7t)) for benzene (17) along a distortion coordinate (cf. Fig. 3.42). Corresponding values for m-butadiene (2, crosses) are shown for comparison. (The cusp-like behavior arises because Lnl171 is defined with respect to a different idealized Lewis structure on each side of A R = 0.)...

A bond separation reaction takes any molecule comprising three or more heavy (non-hydrogen) atoms into the set of simplest (two-heavy-atom) molecules containing the same bonds. The only requirement is that bonding must be defined in terms of a single valence (Lewis) structure or set of equivalent valence structures. This in turn guarantees that the bond separation energy is unique. ... 

An important feature of the BOVB method is that the active orbitals are chosen to be strictly localized on a single atom or fragment, without any delocalization tails. If this were not the case, a so-called "covalent" structure, defined with more or less delocalized orbitals like, e.g., Coulson-Fischer orbitals, would implicitly contain some ionic contributions, which would make the interpretation of the wave function questionable [27]. The use of pure AOs is therefore a way to ensure an unambiguous correspondence between the concept of Lewis structural scheme and its mathematical formulation. Another reason for the choice of local orbitals is that the breathing orbital effect is... 

The VB structures can be defined in different ways according to the desired level of accuracy, but all levels agree on the principle that the active orbitals should be strictly localized on their specific atom or fragment, and not allowed to delocalize in the course of the orbital optimization process. This latter condition is important for keeping the interpretability of the wave function in terms of Lewis structures, but also for a correlation-consistent description of the system throughout a potential surface. 

Before we explore the problem space for a simple proton transfer reaction, we need to understand the basics of bonding and define a consistent nomenclature. In order to use the electron flow paths, you first need to be able to keep track of atoms and electrons— write Lewis structures correctly and easily. 

A provisional Lewis structure may contain the correct bonding framework, but the distribution of the valence electrons may not be the one that gives the maximum stability. The correct stereochemistry is predicted by the valence shell configuration using VSEPR theory, as shown in Chapter 6. A concept called formal charge (FC) can be used to predict which structure of a number of alternative structures is the most reasonable for a Lewis structure. The formal charge (FC) on any atom in a Lewis structure can be defined as ... 

So far we have discussed the geometry of molecules having only one central atom. The overall geometry of molecules with more than one central atom is difficult to define in most cases. Often we can only describe the shape around each of the central atoms. For example, consider methanol, CH3OH, whose Lewis structure is... 

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