Chlorine electronic structure

In earlier chapters we recognized that strong chemical similarities are displayed by elements which are in the same vertical column of the periodic table. The properties which chlorine holds in common with the other halogens reflect the similarity of the electronic structures of these elements. On the other hand, there is an enormous difference between the behavior of elements on the left side of the periodic table and those on the right. Furthermore, the discussions in Chapter 15 revealed systematic modification... 

The electronic structure of the chlorine atom (3s-3p ) provides a satisfactory explanation of the elemental form of this substance also. The single half-filled 3p orbital can be used to form one covalent bond, and therefore chlorine exists as a diatomic molecule. Finally, in the argon atom all valence orbitals of low energy are occupied by electrons, and the possibility for chemical bonding between the atoms is lost. 

As we saw in Chapter 19, chlorine represents the other extreme in chemical reactivity. Its most obvious chemical characteristic is its ability to acquire electrons to form negative chloride ions, and, in the process, to oxidize some other substance. Since the tendency to lose or gain electrons is a result of the details of the electronic structure of the atom, let us try to explain the chemistry of the third-row elements on this basis. 

Chlorine is one of the strongest oxidants whether it is in the elementary form or as oxidised anions, with oxidation states of +l (hypochlorites) to +VII (perchlorates). The chloride ion with an oxidation state of -I is very stable (octet electronic structure) only hydrochloric acid is dangerously reactive, linked to its strongly acidic character. This explains the nature of the dangerous reactions which have already been described and have caused a large number of accidents. The accidental aspect is aggravated by the fact that the derivatives mentioned in this paragraph are much used. 

However, there are some cases when an unpaired electron is localized not on the n, but on the o orbital of an anion-radical. Of course, in such a case, a simple molecular orbital consideration that is based on the n approach does not coincide with experimental data. Chlorobenzothiadiazole may serve as a representative example (Gul maliev et al. 1975). Although the thiadiazole ring is a weaker acceptor than the nitro group, the elimination of the chloride ion from the 5-chlorobenzothiadiazole anion-radical does not take place (Solodovnikov and Todres 1968). At the same time, the anion-radical of 7-chloroquinoline readily loses the chlorine anion (Fujinaga et al. 1968). Notably, 7-chloroquinoline is very close to 5-chlorobenzothiadiazole in the sense of structure and electrophilicity of the heterocycle. To explain the mentioned difference, calculations are needed to clearly take into account the o electron framework of the molecules compared. It would also be interesting to exploit the concept of an increased valency in the consideration of anion-radical electronic structures, especially of those anion-radicals that contain atoms (fragments) with available d orbitals. This concept is traditionally derived from valence-shell expansion through the use of d orbital, but it is also understandable in terms of simple (and cheaper for calculations) MO theory, without t(-orbital participation. For a comparative analysis refer the paper by ElSolhy et al. (2005). Solvation of intermediary states on the way to a final product should be involved in the calculations as well (Parker 1981). 

The complex is dimeric and has a chlorine-bridged structure in which each tetramethylcyclobutadiene molecule is bound to a nickel atom by its four 7r-electrons (71). The nuclear magnetic resonance spectrum of the complex... 

As a univalent ion of medium size, CN , in simple compounds like K+CN , behaves as a chlorine ion, especially if the positive ion has a rare-gas or an 18-electron structure, and covalent bonds cannot be formed. If, however, the positive ion is one of the transition elements, covalent bonds are formed by the lone electron-pair of the CN ion. 

In a molecule of chlorine, with the electronic structure Cl Cl , the covalent radius of chlorine may be described as representing roughly... 

For example, [MoC+,]3 has a total of thirty-nine valence electrons from the molybdenum 4d5 5s and six chlorine 3p5 configurations, and the —3 charge on the ion. Its electron configuration is therefore cr12 n24 (2/2 )3. Thus the MO theory of these ML6 complex ions confirms that the electrons of prime importance are those occupying the t2g and eg levels on the metal, as predicted by crystal-field theory. However, the MO theory points the way to the more accurate calculation of electronic structure and properties. 

The features of the electronic structure of aryl-substituted pyrazolines influence their chemical properties. For example, in the case of 3-substituted 7V-phenyl-pyrazolines 100 reactions of formylation, acylation, nitration, sulfonation, azocoupling and other electrophilic processes involve the para position of the 7V-phenyl ring, with formation of compounds 101 [103, 104, 105, 106, 107, 108, 109, 110, 111, 112, 113]. On the other hand, some electrophilic reactions, including nitration, bromination, chlorination, formylation and azocoupling, for 3-unsubstituted pyrazolines 102 occur at position 3, yielding heterocycles 103 and in some cases as a mixture with 104 [108, 114, 115] (Scheme 2.26). This fact provides evidence for orbital control of these reactions. 

The halogenation reaction of ethylene has been modeled by many researchers [170, 172-176], For chlorination in apolar solvents (or in the gas phase), the formation of two radical species requires the use of flexible CASSCF and MRCI electronic structure methods, and such calculations have been reported by Kurosaki [172], In aqueous solution, Kurosaki has used a mixed discrete-continuum model to show that the reaction proceeds through an ionic mechanism [174], The bromination reaction has also received attention [169,170], However, only very recently was a reliable theoretical study of the ionic transition state using PCM/MP2 liquid-phase optimization reported by Cammi et al. [176], These authors calculated that the free energy of activation for the ionic bromination of the ethylene in aqueous solution is 8.2 kcalmol-1, in good agreement with the experimental value of 10 kcalmol-1. 

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