Talk:Conjugated system

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Article Grading: The following comments were left by the quality and importance raters: (edit · refresh) The discussion of spiroconjugation within the larger context of conjugation in organic chemistry is well-placed. The discovery of an organic molecule spiroconjugated in 3D as the glitter network in the 21st century, is comparable in significance to the discovery of 2D conjugation in the ordinary graphene grid in the 20th century.Hexagonite 13:17, 28 September 2007 (UTC)

In addition to the concept of ordinary conjugation, which is the most important aspect of electron delocalization in organic chemistry, there are also the concepts of (1) hyperconjugation, (2) homoconjugation and (3) spiroconjugation. It seems that hyperconjugation has been properly addressed, albeit briefly, with the link to the hyperconjugation page from this current page about conjugated systems.

I think it would be worthwhile to add a page about homoconjugation from which the reference H. Duerr and R. Gleiter, Angew. Chemie Int. Ed. Eng. 17, 559, (1978) may be useful.

I think it would also be worthwhile to add a page about spiroconjugation the concept of which was first introduced simultaneously by R. Hoffmann, A. Imamura and G. Zeiss, J. Am. Chem. Soc., 89, 5215, (1967) and by H.E. Simmons and T. Fukunaga, J. Am. Chem. Soc., 89, 5208, (1967).

Later in 1994 a polyspiroquinoid polymer was proposed that exhibited spiroconjugation delocalized in 1-dimension, as described in M.J. Bucknum and R. Hoffmann, J. Am. Chem. Soc., 116, 11456, (1994). Bucknum et al. later showed in 2004 that the polyspiroquinoid substructure of his and Hoffmann's hypothetical carbon allotrope glitter was the basis for spiroconjugation extended and delocalized into 2-dimensions (spirographite substructure) and into fully 3-dimensions (glitter structure) as described in M.J. Bucknum and E.A. Castro, J. Math. Chem., 36(4), 381, (2004). This paper explained the metallic status calculated for the glitter allotrope of carbon on the basis of the concept of endo-spiroconjugation.

In 2005 Bucknum et al. showed compelling evidence for the existence of the 3D spiroconjugated glitter allotrope of carbon in a comparison of its calculated diffraction pattern with that of the fairly commonly observed, but heretofore uncharacterized form of carbon known as n-diamond, see M.J. Bucknum and E.A. Castro, Mol. Phys., 103(20), 2707, (2005) for details.

The latter report of the synthesis of the spiroconjugated 3D glitter carbon allotrope provides an apt comparison to the corrfesponding 2D graphene carbon sheet allotrope which possesses ordinary conjugation in 2D. The respective 2D (graphene) and 3D (glitter) resonance structures are diagrammed in this paper.

[edit] Cis or Trans?

In the beta-carotine example shown, the double bonds are all cis. In Conjugated linoleic acid, the bonds are trans. Does cis / trans alignment have an effect on the conjugation? David.Throop 18:42, 28 November 2006 (UTC)

It may have an effect (stability, UV absorbance, etc) but does not affect the designation of something as being "conjugated". DMacks 18:54, 28 November 2006 (UTC)

The reason that possible alignments are only cis/trans, rather then having the third bond go out of the page is that due to conjugation three such bonds tend to be in the same plane. The cis and trans alignments have different conjugation-energies, and in this sense they effect conjugation. Cederal 14:41, 17 December 2006 (UTC)

[edit] Tub shaped???

"It is important to note that merely possessing alternating double and single bonds is not necessarily enough for a system to be conjugated. Some cyclic hydrocarbons (such as cyclooctatetraene) do indeed possess alternating single and double bonds. Although the molecule may appear planar looking only at its chemical structure, the molecule is not actually, and typically adopts a "tub shaped" conformation."

Wouldn't "boat-shaped" be a better description? Isn't that the term used in most organic text books?

"By partial rotations about the carbon-carbon single bonds of the ring, the chair conformation can assume another shape called the "boat" conformation."¹

¹2004. Solomons, T. W. Graham & Fryhle, Craig B. Organic Chemistry 8th Ed. John Wiley & Sons Inc. Hoboken, NJ. p. 164

Boat shaped is the more familiar term, though it would have to be a flat bottomed boat with no sides. I've seen boat shaped used to describe hexagonal cyclic compounds but is it also used for compounds with 8 atoms forming the ring? It looks a lot less like a boat without the triangular ends, though it doesn't look a lot like a tub either. Richard001 05:54, 6 May 2007 (UTC)

[edit] Conjugated systems and energy required for excited state

Why do conjugated systems require less energy to reach the excited state? That's what I came here to find, but there's no mention of the reason behind this trend. Thinking about the delocalized nature of the electrons, it seems plausible to say that they have a weaker attachment to their ground states and thus require only a little energy to reach the excited state. Hopefully someone can expand upon this in the article. Richard001 05:58, 6 May 2007 (UTC)