Extended pi system

It is actually possible to simplify the form of the inducing power in terms of the moment representations defined earlier. Assuming an octahedral geometry of the metal-ligating atom system, and considering initially extended pi-system chelates for which the transitions are mainly long- or short-axis polarized, the inducing powers are readily shown to reduce to the form... 

Extended pi systems are thermodynamically stable but kinetically reactive... 

The analysis outlined above can be extended to ethanes where the substituent does not bear lone pairs but rather a pi system. As an illustration, we will consider 1,2-dicyanoethane. 

Figure4.71 Extended basic system. Si,S2 substrates Pi,P2 products Ei,E2 participating enzymes A,B cofactors I external inhibitor to enzyme Ei.

The spectrum of naphthalene is shown above. Its broad bands reflect the absorption of energy by the extended pi-electron system. 

These three views of the ethylene molecule emphasize different aspects of the disposition of shared electron pairs in the various bonding orbitals of ethene (ethylene), (a) The backbone structure consisting of sigma (a) bonds formed from the three sp2-hybridized orbitals on each carbon, (b) The % (pi) bonding system formed by overlap of the unhybridized pz orbital on each carbon. The pi orbital has two regions of electron density extending above and below the plane of the molecule, (c) A cutaway view of the combined sigma and pi system. 

If the dienophile has a substituent that extends its pi system, the best transition state for the reaction maximizes overlap between the two pi systems. To achieve this greater degree of interaction the dienophile must place the substituent underneath the diene, endo. The Diels-Alder reaction of two dienes has such a transition state with an additional interaction between the unfilled diene i )3 MO and the filled diene tl)2 MO (Fig. 12.20). 

In earlier reports Sharp and co-workers have described the cyclisation of diene-conjugated nitrile ylides of the general type (3) (Scheme 1), where both A and B are benzene rings, to form benz[c,e]azepines <93JCS(p1)2961>. This work has now been extended to systems where A or B is a heteroaromatic ring, in particular furan, thiophene and pyridine <94JCS(pi)1193>. 

The substituted dinitroanlline herbicides absorb well into the visible region due to extended pi-electron systems. The Xmav for each (Table I.) is in the 380-400 nm range where extinction coefficients (logio) are 3.0-3.4 for the pi to pi electronic transition. These electronic spectra afforded excellent chromo-phores for absorption of the emitted black-light radiation. 

Table 4 compares the approximations with simulation results for and [A], respectively, for a system with h = 2 and Z2 = 2 for a variety of values of pi and P2- The approximation tends to underestimate and overestimate [A] because of the assumption that arrivals at the machine queue are Poisson. In fact, they are less variable than Poisson. The approximation can be extended to systems where demand is notified in advance (Buzacott et al. 1992), that is, to MRP-Uke systems. 

The longest cumulenylidene ligand reported to date is the heptahexaenylidene complex shown in Figure 13.49. As in the case of extended organic pi systems, the difference in energy between the HOMO and the LUMO decreases as the length of the cumulene ligand increases. ... 

Fullerene is a condensed ring aromatic compound, possessing extended pi-conjugated systems, which has been applied in various research fields, especially in biology. Ogoshi and co-workers have demonstrated that... 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

The PCA can be interpreted geometrically by rotation of the m-dimensional coordinate system of the original variables into a new coordinate system of principal components. The new axes are stretched in such a way that the first principal component pi is extended in direction of the maximum variance of the data, p2 orthogonal to pi in direction of the remaining maximum variance etc. In Fig. 8.15 a schematic example is presented that shows the reduction of the three dimensions of the original data into two principal components. 

The above approach can be extended to tri-,tetra-, penta-, and hexasubstituted fluorobenzenes. Arguing as before, we conclude that the planar geometry of these systems will be more favorable since pi attractive nonbonded interactions between the fluorine atoms will be maximized in the former case. The pi F—F overlap populations for the tri-, tetra-, penta-, and hexasubstituted fluorobenzenes are shown below. 

---