Delocalized rings

Roth H D, Weng H and Herbertz T 1997 CIDNP study and ab initio calculations of rigid vinylcyclopropane systems evidence for delocalized ring-closed radical cations Tetrahedron 53 10 051-70... 

Overall, it can be seen from Figs. 4 and 5 that in predicting the p parameters almost identical results are obtained in five out of the eight cases. Significantly perhaps, these include the more highly localized O- lone pair and C H orbitals. The worst agreement in both molecules is seen for the more delocalized ring 7t orbital. 

This is not strictly true if the ring current contribution to orA,deloc. includes atom A in the delocalized ring. However, the perturbation of an asymmetric charge distribution on the ring current generally seems to be ignored. 

This delocalized ring structure is called aromatic. Structures possessing this feature are frequently associated with substances with distinct aromas and the concept of aromaticity derives from this fact. While in colloquial terms aromatic is commonly applied to describe strongly fragrant compounds, in chemical technical terminology it refers purely to the possession of this type... 

Vibrational assignments for the parent heterocycles are summarized in Table 25. These have been derived from IR and Raman spectra of both the parent heterocycles and the deuterated derivatives. In the case of the vibrations of A symmetry, these have been further supported by photoelectron spectroscopic studies <1971SAA2525>. The variety of factors responsible for the observed differences precludes a complete rationalization as they may variously operate in concert or in opposition. Pertinent factors include the mass and electronegativity of the heteroatom, 7t-electron delocalization, ring geometry, and vibrational coupling of normal modes. The ring modes permit the best qualitative... 

Benzene is a ring of carbon atoms, with hydrogen atoms attached, that forms a unique kind of hybrid orbital with all the p orbitals. The result is a delocalized ring orbital in which all six carbon atoms donate one electron but share all six electrons in the ring. 

A necessity for a delocalized ring current in CCT catalysts explains the requirement that the equatorial ligand be flat. Chelate 17 would have been a good... 

Aromatic hydrocarbons Benzene and similar condensed ring compounds contain delocalized rings of electrons. 

For the purposes of this article, a sandwich compound is regarded as one in which a metal is n bonded to two planar, delocalized rings. Such compounds almost invariably undergo electron-transfer reactions which are, however, rarely uncomplicated. The enhanced reactivity of the more reduced and oxidized sandwich complexes is finding increased synthetic application. 

To summarize, the reactivity of oxazoles closely resembles that of the ftirans, especially in their readiness to undergo ring-opening reactions and [4+2] cycloadditions. The pyridine-like N-atom impedes electrophilic substitution reactions. On the other hand, it facilitates the attack of nucleophiles onto the C-2 atom. Although oxazole is considered to be a heteroarene, because of its six delocalized ring electrons, its aromaticity is low and comparable to that of furan. 

Fig. 10 ELF for porphyrin. In both parts, pmtioiis of ELF = 0.66 iso-surface are shown to highlight different features. The ELF shows the same delocalization features as benzene in Fig. 9. The nitrogen lone pairs show a slightly larger localization basin than NH bonds. NH induces a small interruption in the delocalized ring between N and neighbor C(H) this small interruption disappears in deprotonated porphyrin and in metal-porphyrin...

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