Delocalization, of electrons

The meaning of the word aromaticity has evolved as understanding of the special properties of benzene and other aromatic molecules has deepened. Originally, aromaticity was associated with a special chemical reactivity. The aromatic hydrocarbons were considered to be those unsaturated systems that underwent substitution reactions in preference to addition. Later, the idea of special stability became more important. Benzene can be shown to be much lower in enthalpy than predicted by summation of the normal bond energies for the C=C, C—C, and C—H bonds in the Kekule representation of benzene. Aromaticity is now generally associated with this property of special stability of certain completely conjugated cyclic molecules. A major contribution to the stability of aromatic systems results from the delocalization of electrons in these molecules. 

In orbital terms, as represented in Figure 11.10, benzyl radical is stabilized by delocalization of electrons throughout the extended tt system formed by overlap of the p orbital of the benzylic carbon with the tt system of the ring. 

The electrostatic potential map of 5/ phenoxide ion on Learning By Modeling displays the delocalization of electrons into the ring. 

In aromatic diazonium compounds containing an ionized hydroxyl group ( —O-) in the 2- or 4-position, it is necessary to consider delocalization of electrons and, therefore, two mesomeric structures (1.7a-1.7b) (see Sec. 4.2). This fact has implications for nomenclature compounds of this type are considered as quinone derivatives following IUPAC Rule C-815.3 (Exception) compounds of this class are called quinone diazides. As a specific compound 1.7a-1.7b is indexed in Chemical Abstracts as 4-diazo-2,5-cyclohexadien-l-one. If reference is made specifically to mesomeric structure 1.7b, however, it is called 4-diazoniophenolate. 

In the ionization of the p-nitroanilinium ion, the free base is stabilized by delocalization of electrons involving the canonical structure 19. An analogous structure is not possible for... 

The second important characteristic of aromatic isocyanates is that they are much more reactive than aliphatics due to delocalization of electron density into the aromatic ring. Of the mesomer structures shown in Scheme 4.3, if the... 

According to molecular orbital theory, the delocalization of electrons in a polyatomic molecule spreads the bonding effects of electrons over the entire Energy molecule. 

Delocalization of electrons is important in chemistry. Electron delocalization is a major factor of the stabilities and the reactivities of molecules. The delocalization occurs through the interaction of an occupied orbital with a vacant orbital (Scheme 13). The two electrons occupy the stabilized orbital. There are no electrons in the destabilized orbital. The stabilization results from the interactions between the occupied and unoccupied orbitals. 

K Relaxation mechanism was also proposed [130-133], The % relaxation originates from cyclic delocalization of % electrons in the double bond through the hyperconjugation with a bonds on the saturated ring atoms under control of the orbital phase property [134, 135],... 

Aromatic rings containing, s/j2-hybridized silicon atoms also constitute members of the silene family. Interest in these compounds largely focuses on the possible delocalization of electron density in the rings, the extent to which they display aromatic character, and their relative stabilities. Raabe and Michl6 have reported much data, which will not be repeated here other computational studies are reported in Section III.D. A few interesting additions to our knowledge of these systems have been reported in recent years. 

Bis(ethylenediamine)gold(III) chloride reacts with /3-diketonates in aqueous base via Schiff base condensation to form complexes of gold(III) with a 14-membered macrocyclic tetraaza ligand such as (292).1711-1713 The X-ray structure showed the cation to be nearly planar. Delocalization of -electrons within the six-membered /3-diketonate rings was indicated by the observed pattern of C—C and C—N distances. Open-chain tetraaza ligand complexes in which condensation of only one /3-diketonate has occurred, can be isolated as intermediates in this reaction. They may be used for further condensation with a different /3-diketonate. Oxidation of this complex with trityl tetrafluoroborate introduces a double bond in position C2C3.1712,1714... 

Selenoketones are less stable than thioketones and cannot be obtained as monomers unless sterically hindered by alkyl groups or stabilized by electronic delocalization of electron rich substituents. Some stable selenoketones are reported in Table 1. 

P)GeFc2 compounds, suggesting delocalization of electron density from the porphyrin to the ferrocenyl group. The conclusion is that the stability of Ge—C bonds is higher in the ferrocenyl complexes than in the alkyl complexes165. 

Many distibines and dibismuthines have lighter colors in solutions or melts than in the solid state. Crystals of these thermochromic distibines or dibismuthines consist of linear chains of the dimetal compounds with short intermolecular metal-metal contacts. Delocalization of electrons along the chains is possibly responsible for the bathochromic shift between fluid and solid phases. Usually, the /raor-conformation is adopted by the tetraorganodimetal compounds in the solid state. (CF3)4As2 shows the /ra r-conformation also in the gas phase. Photoelectrospectroscopic measurements on Me4Sb2 revealed the presence of gauche- (12%) and trans- (88%) conformed in the gas phase.52... 

In a review, Gorelik51 has shown that magnetic, structural, and energetic properties are determined by the electronic structure of cyclic conjugated systems, which are stabilized by a cyclic delocalization of electrons. Chemical reactivity cannot serve satisfactorily as a general criterion of aromaticity. 

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