Identical atoms

Intensive data reduction is an efficient inetl iod of managing large datasets. Generally, hasl i codes are used within chemical information processes such as molecule identification and recognition of identical atoms [9S]. 

The hardness parameters for MM+ are the dissociation energies of the nonbonded interactions of two identical atoms divided by 1.125, 8=Dq/1.125. The values of Dq used are ... 

Structure Identical atoms attached directly to phosphorus Non-identically substituted phosphorus ... 

Figure 7.12 Formation of two molecular orbitals from two identical atomic orbitals...

The example of COj discussed previously, which has no vibrations which are active in both the Raman and infrared spectra, is an illustration of the Principle of Mutual Exclusion For a centrosymmetric molecule every Raman active vibration is inactive in the infrared and any infrared active vibration is inactive in the Raman spectrum. A centrosymmetric molecule is one which possesses a center of symmetry. A center of symmetry is a point in a molecule about which the atoms are arranged in conjugate pairs. That is, taking the center of inversion as the origin (0, 0, 0), for every atom positioned at (au, yi, z ) there will be an identical atom at (-a ,-, —y%, —z,). A square planar molecule XY4 has a center of symmetry at atom X, whereas a trigonal planar molecule XYS does not possess a center of symmetry. 

FIGURE 3.7 The arrangements of atoms that give rise to polar and nonpolar molecules. In these VSEPR formulas, A stands for a central atom, X for an attached atom, and E for a lone pair. Identical atoms are... 

In this part of the chapter, we begin with molecular solids and distinguish them from network solids. Then we examine metallic solids, which, if consisting of a single element, are built from identical atoms stacked together in orderly arrays. The structures of ionic solids are based on the same kinds of arrays but are complicated by the need to take into account the presence of ions of opposite charges and different sizes. 

The Helium Molecule and Molecule-ion.—The simplest example of a molecule containing a three-electron bond is the helium molecule-ion, in which a Is eigenfunction for each of two identical atoms is involved. The two unperturbed states of equal energy are He He+ and He-+ He. The formation of this molecule might be represented by the equation He Is2 >5 + He+ Is 5 —>- He (Is + ls) 2 Three dots in a horizontal line placed between the two atomic symbols may be used to designate a three-electron bond He He+. 

It has been pointed out4 that bonds between non-identical atoms may be considered to resonate between a covalent and an ionic structure, the bond in this way having partially covalent and partially ionic character. The resonance energy of this effect, which is usually essentially the same for a given bond in different molecules, is included in the values given for the bond energies in the nonresonating molecules discussed. 

In order to obtain information as to the extent to which the bond strength S of an orbital can be considered a qualitative measure of its bondforming power and also as to the quantitative relation between the energy of a bond and the strengths of the bond orbitals involved in its formation, we have now carried out the thorough discussion of various sp and spd one-electron bonds between identical atoms. 

Figure 7-1. Planar representation of the "three-point attachment" of a substrate to the active site of an enzyme. Although atoms 1 and 4 are identical, once atoms 2 and 3 are bound to their complementary sites on the enzyme, only atom 1 can bind. Once bound to an enzyme, apparently identical atoms thus may be distinguishable, permitting a stereospecific chemical change.

Each nucieus of a hydrogen moiecuie has a charge of+1. Consequently, both nuclei attract electrons equally. The result is a symmetricai distribution of the eiectron density between the atoms. Each nucleus of a fluorine molecule has a charge of +9, and again the eiectrons experience the same net attraction toward both nuclei. In a chemical bond between identical atoms, the two nuclei share the bonding eiectrons equally. 

Check a provisional Lewis structure by determining that all the valence electrons are assigned. Also check that identical atoms—the four F atoms in this example—all have the same electron distribution. 

Multiple pathways leading to the same product channel can also be observed in a reaction when there are a sufficient number of identical atoms, thereby allowing different intermediate structures to yield the same products. In these cases, the mechanisms in the two pathways are often quite similar, but involve differing positions of identical atoms on the reactants. The different pathways often involve formation of ring intermediates in which the rings have different sizes. A simple example of this class is the photodissociation of vinyl chloride [9]... 

The possibility of multiple pathways arising from identical atoms becomes greater as the total number of atoms increases because the possibility to form... 

Like the previous reaction, multiple pathways in the HO2 - - O3 reaction arise because of the participation of identical atoms. However, in this case the two... 

The final example of a reaction in which multiple pathways arise from the participation of identical atoms is the reaction HCCO + O2. This reaction demonstrates another general feature of pathway competition when unsaturated species are involved, namely, the possibilities for formation of ring intermediates of different sizes. 

CH2O H2 + CO. In contrast to the previous three systems, this pathway competition cannot be studied by isotopic substitution because the identical atoms are found in only one product (H2). Formnately, the two contributing pathways can be separated because their product state distributions are markedly different. 

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