Aromaticity sigma

Group Branch Extension of Aromaticity Sigma/Rho Fragment Interaction Special ortho effects Crowding Intramolecular Hydrogen Bonding... 

Mechanistically this reaction is described as a concerted pericyclic [3,3] sigma-tropic rearrangement. A carbon-oxygen bond is cleaved and a carbon-carbon bond is formed. In a subsequent step the initial product 4 tautomerizes to the stable aromatic allylphenol 3 ... 

III. SUBSTITUENT EFFECTS IN AROMATIC SYSTEMS A. Sigma Values from Reactivity Studies... 

In any event, between 1951 and 1975, no papers appeared on polysilane high polymers. However, linear permethylpolysilanes of the type MelSiMezhiMe were prepared and studied, especially by Kumada and his students,(5) and cyclic polysilanes were being investigated in several laboratories.(6,7) Studies of the permethyl-cyclosilanes, (Me2Si)n where n = 4 to 7, showed that these compounds exhibit remarkable delocalization of the ring sigma electrons, and so have electronic properties somewhat like those of aromatic hydrocarbons.(6)... 

Sobolewski AL, Domcke W, Dedonder-Lardeux C, Jouvet C (2002) Excited-state hydrogen detachment and hydrogen transfer driven by repulsive (l)pi sigma states a new paradigm for nonradiative decay in aromatic biomolecules. J Phys Chem Chem Phys 4 1093—1100... 

Antisymmetric matrix, non-adiabatic coupling, vector potential, Yang-Mills field, 94-95 Aromaticity, phase-change rule, chemical reaction, 446-453 pericyclic reactions, 447-450 pi-bond reactions, 452-453 sigma bond reactions, 452 Aromatic transition state (ATS), phase-change rule, permutational mechanism, 451-453... 

The effect of a substituent on the aromatic substitution reaction is similar to its effect on electrophilic side chain reactions, but not precisely parallel. Thus the Hammett relationship using the usual sigma or substituent constants gives considerable scatter when applied to aromatic substitution. The scatter is probably due to an increased importance of resonance effects in the nuclear substitution reaction as compared with the side chain reactions. 

Table 3.2. Lewis (L) and non-Lewis (NL) energy contributions and NBO occupancies for sigma and pi bonds of the simplest alkane (CH4), alkene (C2H4), and alkyne (C2H2) species, with aromatic benzene (C fD includedfor...

Finally, we briefly compare the NBO donor-acceptor picture of geminal delocalization with Dewar s sigma-aromaticity concept and the associated pi-complex... 

One can also localize aromatic systems by applying the localization procedure to 7r electrons and a electrons simultaneously. The sigma electrons will then be localized in the regions of the single bonds. Since the localization energy to be gained from the n electron localizations is less than that from the sigma electron localization, the total localization will be dominated by the latter. 

We note here, that the eclipsed conformation of N204 can be viewed as being a sigma aromatic system geometrically similar to the pi aromatic system napthalene as illustrated below ... 

Arguing as before, we conclude that pi nonbonded attractive interactions are maximized in the SS conformation while sigma nonbonded attractive interactions are maximized in the EE conformation. This conflict between pi and sigma nonbonded attractive interactions is similar to the conflict observed in cis 2-butene in which steric effects resulted in an apparent dominance of sigma over pi aromaticity. 

Scheme 3.2-3. Reactions with conservation of 2e aromaticity. It is the sigma skeleton which is changed, namely from classical in 2a via partially non-classical in 3a and classical in 4b to partially non-classical again in 3b. 4b has a...

In Taylor s tabulation of sigma values228, a+ and a for p-NCP are given identical values of 0.79228. No distinction between these two types of substituent constant is to be expected for p-N02 in connection with electrophilic aromatic substitution. Some slight distinction between them is drawn for m-N02 a = 0.72, a+ = 0.73. 

F. Terrier, Eiectron Deficient Aromatic and Heteroaromatic Base Interactions. The Chemistry of Anionic Sigma Complexes, pp. 78-85. Elsevier, Amsterdam, 1984. 

In order to understand any chemical process, you need to remember that stability is the key. In the case of aromatic systems, resonance is important to stability. To find out more about stability and resonance, we begin by examining the resonance in each of the different sigma complexes that may form, listed below and shown in Figure 7-15. The entering group can attack in one of three relative positions ... 

The PONA results at constant conversion (72 vol%) for the series SIGMA 300, ALPHA 500 and BETA 500 show gasoline olefin content increased from 11.3 percent to 15.9 percent, paraffin content decreased from 47.4 percent to 43.7 percent, naphthene content decreased from 10.3 percent to 9.3 percent and aromatics content remained constant. Research octane increased 2.6 numbers while motor octane increased 1.4 numbers. The LCO yield increased 0.5 volume percent while the coke yield decreased from 7.1 weight percent to 6.5 weight percent. 

Photodegradation rates of ortho derivates present good correlation with the thermodynamic stability of sigma-complexes formed between the aromatic ring and the surface OH-radicals. Rates decrease in the order -OCH3 (guiacol) > -Cl (2-chlorophenol) -H (phenol) > -OH (catechol). ... 

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