Electron cloud delocalized

Disclike organic molecules consisting of flat-shaped aromatic macrocycles tend to form columnar stacks in the crystalline state as well as in a liquid-crystalline solution [140-148]. As such, ph-thalocyanines, which behave as molecular semiconductors in the solid state, have been investigated in considerable detail [140,141]. The basic structural unit consists of four connected pyrrole rings (see Fig. 21a), and the flat-shaped form allows a 47-electron cloud delocalized over an array of 18 carbon and nitrogen atoms. Met-allophthalocyanines can be prepared by coordinating metals, such as Fe, Ru, Co, Pd, Pt, Pb, Ni, Cu, Zn, Mn, and Cr, in the core see Figure 21b [141]. In addition, the perimeter can be substituted... 

Paracyclophane also has a compact structure in solution, as is readily seen on comparing its partition coefficient with that of p-xylene for the system octanol/water 32>. Rigid superposition of the benzene rings leads to an intramolecular delocalization of the hydrophobic -electron clouds and hence to an increased affinity for the aqueous phase. Accordingly, the logarithm of the partition coefficient is found to be smaller than the value observed for p-xylene, and not twice as large, as would be expected for completely hydrophobic surfaces. 

It is also worthwhile to compare the ferrocenyl ethylene (vinylferrocene) anion-and cation-radicals. For the cyano vinylferrocene anion-radical, the strong delocalization of an unpaired electron was observed (see Section 1.2.2). This is accompanied with effective cis trans conversion (the barrier of rotation around the -C=C- bond is lowered). As for the cation-radicals of the vinylferrocene series, a single electron remains in the highest MO formerly occupied by two electrons. According to photoelectron spectroscopy and quantum mechanical calculations, the HOMO is mostly or even exclusively the orbital of iron (Todres et al. 1992). This orbital is formed without the participation of the ethylenic fragment. The situation is quite different from arylethylene radical cations in which all n orbitals overlap. After one-electron oxidation of ferrocenyl ethylene, an unpaired electron and a positive charge are centered on iron. The —C=C— bond does not share the n-electron cloud with the Fe center. As a result, no cis trans conversion occurs (Todres 2001). 

In metallic solids, the reticular positions are occupied by cations immersed in a cloud of delocalized valence electrons. In sohd Na, for instance (figure 1.2D), the electron cloud or electron gas is composed of electrons of the s sublevel of the third shell (cf table 1.2). Note that the type of bond does not limit the crystal structure of the solid within particular systems. For example, all solids shown in figure 1.2 belong to the cubic system, and two of them (NaCl, Ne) belong to the same structural class Fm3m). 

In this way the notion of a delocalized electron or electron cloud follows naturally from the wave-like characterization of the electron. 

Although adequate from a formal point of view, it suffers from the usual unwieldiness of VB terminology when extensive delocalization exists. A second attempt considers the B-.H4- anion as isoelectromc and isostructural with ethylene. C2H4. Such an km would have a cloud of electron density above and below the B—H plane. — The neutral B,Hf, molecule could then be formally produced by embedding a proton in the electronic cloud above and below the plane of the ion. Although this may... 

The first three observed ionization potentials for thieno[3,2-6]thiophene (3) (8.14, 8.66 and 10.02 eV) correlate favorably with those observed for the isoelectronic naphthalene (8.15, 8.80 and 10.00 eV) and benzo[6]thiophene (8.22, 8.77 and 10.05 eV) molecules, in contrast to the corresponding values for thiophene (8.87, 9.49 eV) and benzene (9.24, 9.24 eV) (73JCS(F1)93). This observation is explained by the fact that the delocalized 7r-electron cloud resulting from changing a benzene 7r-bond to a sulfur atom causes greater perturbation in the small framework involved compared to the same interchange in a more extended conjugated system such as naphthalene or benzo[6 jthiophene. 

It has already been pointed out that delocalization of electron clouds (that is, the spreading of electronic charge over the region of more than one nucleus) often leads to structural stability. Since the resonance concept is one way of handling delocalization, one would expect that if two or more permissible primary structures can be drawn for a molecule, it would be more stable than a hypothetical molecule of the same type with no delocalization. Stability of molecules can be determined experimentally by measuring heats of chemical reactions a stable molecule will give off less heat in a given reaction than would a less stable molecule of the same type. Now classical thermochemistry has been developed so that heats of... 

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