Heteroconjugated systems

A number of disaccharides with 1,3-, 1,4- and 1,6-linkages have been investigated in a continuing study on the use of hydroxy protons in conformational analysis of saccharides in aqueous solution by NMR. The intramolecular N-H-- 0 hydrogen bonds formed by a number of heteroconjugated systems (P-enaminones, ketohydrazones, and nitrosoenamines) were studied by X-ray, IR and NMR spectroscopic, and quantum-mechanical calculation. ... 

FIGURE 3.1. (a-d) Alternant hydrocarbons (e, f) nonalternant hydrocarbons (g) an alternant heteroconjugated system (h) a nonalternant heteroconjugated system. 

The treatment of heteroconjugated systems which follows can be applied to both Hiickel and anti-Hiickel systems in precisely the same way. 

In this case, then, the difference in energy between the contributing p electrons in the heteroconjugated system and that of those in the AH anion is not Scti but 2S(Xi. The difference in total n energy is given as before by... 

The difference AE in 7r-bond energy between the AH anion and the heteroconjugated system is then... 

In our discussion of heteroconjugated systems, we have so far assumed that the distribution of the electrons is the same as it is in the corresponding isoconjugate hydrocarbons. This is not in fact the case. In an even AH, the... 

The effect of such a substituent can be deduced from that of the isoconjugate one by the procedures used to study heteroconjugated systems (Sections 3.18 and 3.19). 

FIGURE 6.12. Effect of replacing atom r in a conjugated hydrocarbon by a heteroatom (a) HOMO and LUMO in the hydrocarbon (b) HOMO and LUMO in the heteroconjugated system. 

The majority of odd, heteroconjugated systems are isoconjugate with carbanions. Here introduction of heteroatoms has very drastic effects on the spectrum, effects which have been used extensively in the dye industry in general, and the photographic dye industry in particular, to modify the... 

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