Conjugated systems resonance structures

Benzene has already been mentioned as a prime example of the inadequacy of a connection table description, as it cannot adequately be represented by a single valence bond structure. Consequently, whenever some property of an arbitrary molecule is accessed which is influenced by conjugation, the other possible resonance structures have to be at least generated and weighted. Attempts have already been made to derive adequate representations of r-electron systems [84, 85]. 

Our first approach took resort in simple resonance theory [36, 37]. For each conjugated nr-system aU resonance structures were generated, such as those shown in Figure 7-5. 

Allyl radical is a conjugated system in which three electrons are delocalized over three carbons The resonance structures indicate that the unpaired electron has an equal probability of being found at C 1 or C 3 C 2 shares none of the unpaired electron... 

It is apparent from the size of the conjugated system here that numerous resonance possibilities exist in this species in both the radical and the molecular form. Styrene also has resonance structures in both forms. On the principle that these effects are larger for radicals than monomers, we conclude that the difference ep. - ej > 0 for both hemin and styrene. On the principle that greater resonance effects result from greater delocalization, we expect the difference to be larger for hemin than for styrene. According to Eq. (7.23), r j oc > 1. According to Eq. (7.24), i2 < 1. 

Mass spectrometry, infrared spectroscopy, and nuclear magnetic resonance spectroscopy are techniques of structure determination applicable to all organic molecules. In addition to these three generally useful methods, there s a fourth—ultraviolet (UV) spectroscopy—that is applicable only to conjugated systems. UV is less commonly used than the other three spectroscopic techniques because of the specialized information it gives, so we ll mention it only briefly. 

The reported systems are always hydrogenated derivatives or compounds which exhibit full conjugation only in one mesomeric resonance structure, e.g. amides. 

Follow the four-step procedure for the composite model of bonding. Use localized bonds and hybrid orbitals to describe the bonding framework and the inner atom lone pairs. Next, analyze the system, paying particular attention to resonance structures or conjugated double bonds. Finally, make sure the bonding inventory accounts for all the valence electrons and all the valence orbitals. 

An extension of the above method was developed for conjugated it-systems Partial Equalization of Pi-Electronegativity (PEPE)47,48). After calculation of the charge distribution in the a-skeleton, the various resonance structures of a it-system are generated. The 7t-charge distribution is obtained by assigning weights to these... 

V. The Quantum-Mechanical Calculation of the Resonance Energy of Benzene and Naphthalene and the Hydrocarbon Free Radicals," J.Chem.Physics 1 (1933) 362374 Linus Pauling and J. Sherman, "The Nature of the Chemical Bond. VI. Calculation from Thermochemical Data of the Energy of Resonance of Molecules Among Several Electronic Structures," J.Chem.Physics 1 (1933) 606617 and Pauling and Sherman, "The Nature of the Chemical Bond. VH. The Calculation of Resonance Energy in Conjugated Systems," J.Chem.Physics 1 (1933) 679686. 

Silyl cations like 3 and 7 in which the positively charged silicon is part of a n-conjugated system attracted particular interest. The marginally stable silatropylium ion 7, is characterized by a Si NMR resonance at 8 Si = 149 in CD2CI2 at —50 °C, downfield-shifted by 192 ppm compared to the precursor silane.This experimental value is in fair agreement with the calculated silicon NMR chemical shift for the optimized gas phase structure of 7 (8 Si = 159.9, at GIAO/HF/6-311 + G(2df,p)(Si), 6-31G(d) (C,H)). This indicates only small interactions between the cation and dichloromethane, the solvent used for the NMR investigations. 

For compounds that require metabolic activation, avoiding structural features that may provide resonance stabilization of electrophilic metabolites (e.g., conjugated double bonds, or conjugated system/aryl moiety) will decrease the lifetime of the reactive intermediates. 

Dissociation of the 7-OH generates an anion with an extended conjugated n electron system, which will favor absorption of long-wavelength light (Chapter 23) and a blue color. Notice that a large number of resonance structures can be drawn for both the anthocyanin and the dissociated forms. Formation of complexes of Mg2+ or other metal ions with the 4 -0 and adjacent OH groups may also stabilize blue colored forms.d... 

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