Chemical bonding central atom

Since the concepts of atoms and bonds are central to chemical understanding, approaches based on atom-additivity and bond-additivity are very appealing. Due to their simplicity, they were used in the early days for actual calculations, but nowadays they continue to be employed for interpretative purposes. Needless to say, their accuracy can be surpassed by methods based on quantum mechanics. As with field-free isolated molecules, early models used to estimate second- and third-order macroscopic nonlinear responses considered such simple schemes. In the following, we describe methods that treat either chemical bonds or atoms as the central quantities for evaluating the bulk NLO responses. The philosophy consists in incorporating in the description of these central constructs the effects of the surroundings. In this way the connection with more elaborate methods, such as the oriented gas model that focuses on one molecule with local field factor corrections, or with the crystalline orbital approach that reduces the system to its unit cell, is more obvious. In what follows, a selection of such schemes is analyzed and listed in Table VII. 

Structural (topological) Databases The structural databases play a central role in chemistiy because they contain information on chemical stmctures. Examples of this type are CAS registry, National Cancer Institute (NCI) database, Crystallographic Stracture Database (ICSD), CSD, Protein Data Bank (PDB), etc. The structure databases are usually designed to store chemical stmctinal information representing the chemical bonds and atoms in such a way to use them for computational operations, such as structure search, data mining, etc. 

However, one of the most successfiil approaches to systematically encoding substructures for NMR spectrum prediction was introduced quite some time ago by Bremser [9]. He used the so-called HOSE (Hierarchical Organization of Spherical Environments) code to describe structures. As mentioned above, the chemical shift value of a carbon atom is basically influenced by the chemical environment of the atom. The HOSE code describes the environment of an atom in several virtual spheres - see Figure 10.2-1. It uses spherical layers (or levels) around the atom to define the chemical environment. The first layer is defined by all the atoms that are one bond away from the central atom, the second layer includes the atoms within the two-bond distance, and so on. This idea can be described as an atom center fragment (ACF) concept, which has been addressed by several other authors in different approaches [19-21]. 

Atoms of these elements have empty J-orbitals in the valence shell. Another factor—possibly the main factor—in determining whether more atoms than allowed by the octet rule can bond to a central atom is the size of that atom. A P atom is big enough for as many as six Cl atoms to fit comfortably around it, and PC15 is a common laboratory chemical. An N atom, though, is too small, and NC15 is unknown. A compound that contains an atom with more atoms attached to it than is permitted by the octet rule is called a hypcrvalent compound. This name leaves open the question of whether the additional bonds are due to valence-shell expansion or simply to the size of the central atom. 

VSEPR theory works best when predicting the shapes of molecules composed of a central atom surrounded by bonded atoms and nonbonding electrons. Some of the possible shapes of molecules that contain a central atom are given in Figure 7.11, along with the chemical formulas of molecules that have that shape. 

In the case of covalent compounds, crystal-field theory is a poor model for estimating electric field gradients because of the extensive participation of ligand atomic orbitals in the chemical bonds. MO calculations are a much better choice, since the corresponding interactions are considered, and realistic (noninteger) population numbers are obtained for the central metal as well as the ligand atomic orbitals. 

Mica and other layered minerals differ from talc because metal atoms lie between their layers producing some chemical bonding. Also, their layers are usually stronger because A1 replaces (partially or fully) the central Mg layer of Figure 11.3. 

There are several important chemical species that consist of four atoms and have a total of 24 valence-shell electrons. Some of the most common isoelectronic species of this type are C032-, N03 , S03, and P() j (known as the metaphosphate ion). Because four atoms would require a total of 32 electrons for each to have an octet, we conclude that eight electrons must be shared in four bonds. With four bonds to the central atom, there can be no unshared pairs on that atom if the octet rule is to be obeyed. Therefore, we can draw the structure for CO, 2 showing one double C=0 bond and two single C-O bonds as... 

The main characteristic of cluster-type indices is that all bonds are connected to the common, central atom (star-type structure). The third-order cluster molecular connectivity index (3yc) is the first, simplest member of the cluster-type indices where three bonds are joined to the common central atom [102-104, 111-113,152-154,166,167,269]. The simplest chemical structure it refers to is the non-hydrogen part of ferf-butane. This index is then calculated using Eq. (43) ... 

Fig. 14. Normalized averaged intramolecular distances plotted as a function of the position of bead j for a star with 12 arms with a total of 472 bonds. The beads are labeled as negative from -N to 0 (the central atom) on the first arm and as positive up to on the second arm. The three locations of the first bead i corresponds to i -N, at the free end of the first arm (circles) i=-N /2, at the midpoint of the first arm (squares) and i=0, at the central branch point (triangles). Reprinted with permission from [158]. Copyright (1997) American Chemical Society...

A great deal of experience informs us that carbon atoms frequently form molecules in which they are simultaneously linked to four other atoms, hi contrast, hydrogen atoms are usually linked to only one other atom. It follows that the methane molecule is structured with a central carbon atom to which are bonded the four hydrogen atoms. In such a structure, only carbon-hydrogen (C-H) bonds exist. We can represent methane in the following way, in which the solid lines are symbols for chemical bonds ... 

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