Electronic conjugations

Substituents in aromatic rings may influence each other in a number of ways depending on their chemical nature, mutual position, and the presence of other substituents. For example, methyl groups have relatively little electronic interaaion with an aromatic ring and with each other. Their incremental contribution to the lipophilicity of, say xylenes, is additive as expected, and independent from their relative position (ortho vs. meta vs. para). Such groups are thus well behaved in lipophilic fragmental systems. -  

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Another structure with a 10-7r-electron conjugated system is bicyclo[6.2.0]deca-1,3,5,7,9-pentaene. The crystal structure of the 9,10-diphenyl derivative (Fig. 9.6) shows the conjugated system to be nearly planar. " ... 

Ultraviolet (UV) spectroscopy (Section 14.7) An optical spectroscopy employing ultraviolet irradiation. UV spectroscopy provides structural information about the extent of 7r electron conjugation in organic molecules. 

Dibenzo[c,g][l, 2]diazocine (3) can be reduced with zinc in ethanol to the dihydro derivative 9.22 which formally exhibits a lOre-electron conjugation (see also ref 28). The UV and H NMR spectra of 9 indicate some resonance stabilization of the eight-membered ring. 

The effects of cyclic 6n electron conjugation have been found in the optimized geometries of pentazole 17 [102] and hexazine 18 [97], The N=N bond is longer than the isolated double bond in NH=NH. The N-N single bond in the tetrazadiene moiety is shorter than the single bond in NH NH. The bond lengths in 18 are nearly intermediate between those in NH NH and NH=NH. The aromatic character of pentazoles was supported by the effect of electron donating substituents on the thermodynamic and kinetic stabilization [103],... 

Of particular interest when considering ionizable compounds is the difference of lipophilicity between the neutral species and one of its ionic forms, because ionization dramatically alters intramolecular interactions (such as electronic conjugation, internal ionic and hydrogen bonds, polarity, hydrophilic folding, and shielding). In a given solvent system, diff (log is approximately constant for compounds with similar chemical... 

LDMS is particularly well suited for the analysis of porphyrins.35-39 The heme molecule—a 22 rc-electron conjugated protoporphyrin system (Figure 8.1)—is an efficient photo-absorber in the visible and near UV (with an absorption maximum—the Soret band—near 400nm). This feature, concurrently with its low ionization potential, warrants that direct LDMS will possess extremely low limits for heme detection. The uses of IR or UV LDMS for structural characterization of natural porphyrins and their metabolites, synthetic monomeric porphyrins (e.g., used in photodynamic therapy), porphyrin polymers, and multimeric arrays, have been well documented.41148 In addition fast atom bombardment MS has been used to characterize purified hemozoin, isolated from the spleens and livers of Plasmodium yoelii infected mice.49... 

W.-S. Li, D.-L. Jiang, and T. Aida, Photoluminescence properties of discrete conjugated wires wrapped within dendrimeric envelopes "dcndrimer effects on -ir-electronic conjugation, Angew. Chem. Int. Ed., 43 2943-2947, 2004. 

In THF at 20°C, dendritic monomers 3a-3c show absorption bands at 335 and 278 nm, due to the focal 1,4-diethynylbenzene unit and the dendritic wedges, respectively. On the other hand, dendritic macromolecular rods 2a-2c display a strong absorption band in the visible region (400-460 nm), characteristic of an extended electronic conjugation in the backbone. Upon excitation of the conjugated backbone at 425 nm in THF (abs425 nm = 0.01) at 20°C, 2a-2c show a strong blue fluorescence at 454 nm, where the quantum yield ( FL) has been evaluated to be virtually 100%. Of much interest is the fact that the fl>FL value of 2c stays at nearly 100%, even when the solution is concentrated until the... 

Lewis acid catalysis enormously enriches the scope of Diels-Alder reactions, but it is limited to reagents containing Lewis basic sites, i.e. functional groups with lone pairs such as carbonyl, amino, ether or nitro close to the reaction centre. As we have seen in the discussion about the FMO aspects of Lewis acids, the major reason for catalysis is the reduction of the HOMO-LUMO gap. In case of Diels-Alder reactions with normal electron demand, it follows that the coordination of the Lewis acid lowers the LUMO energy of the dienophile. Such interactions are only possible if there is a spatial proximity or an electronic conjugation between the coordinated Lewis basic site and the reaction centre. Fortunately, in nearly every Diels-Alder reaction one of the reagents, mostly the dienophile, meets this requirement. 

Related compounds, cyclo[2.2.3]azine and l,2,3,4-dibenzocyclo[2.2.3]azine, also give ion-radicals with peripheral Jt-electron conjugate systems (Gerson et al. 1973, Matsumoto et al. 1996). 

Fig. 8. (a) Homoaromatic lit-electron conjugation in silyl cation 3. (b) Allyl-type resonance in Sit-electron radical 26. ... 

Other structural features of unsaturated-5(4//)-oxazolones can be deduced from crystallographic data including the effect on planarity of substituents on the exocyclic double bond. For example, 4-benzylidene-5(4//)-oxazolones show a completely planar conformation that favors strong electronic conjugation in both (2)990 isomers. The same effect has been reported for other 4-arylidene-... 

Proton loss to an eight-it-electron conjugated system... 

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