Pi systems

Molecular Orbitals and Pericyclic Reactions of Conjugated Pi Systems... 

As a signature of the star architecture the elastic scattering data of the completely labelled stars exhibit a pronounced peak at z = 1.5 in the scaled Kratky representation. In contrast to the PI systems, presented before, the Kratky plot of the measured scattering curve disagrees strongly with the prediction of Eq. (123). The experimental halfwidth of the peak is nearly only 50% of the theoretically predicted one. 

Investigations on a series of miktoarm-star AB copolymers of PS and poly(2-methyl-l,3-pentadiene), PS(P2MP)3, indicate a different morphological behaviour [113] (Fig. 37), as predicted by Milner s theory [111] (Fig. 38). The discrepancies near the boundaries between different morphologies when compared with corresponding PS/PI systems on the one hand and to theoret-... 

Phase transitions of the PS-fo-PI system have been extensively studied. The morphological transition from the I phase to the G phase proceeds through nucleation and growth. The difference in the geometrical characteristics of these two phases induces considerable local distortion of both morphologies... 

C.H. Brubaker, Michigan State University In the case of the cytochromes, it has been proposed that electron transfer from the iron porphyrin may involve the pi system of the porphyrin and even nearby aromatic rings. Do you think that a similar thing may happen in the case of the reaction between these copper(I) p lastocyanins and the chromium(III) You seem to favor the idea that the important factor is that the Cr(III) be at a site that is reasonably close to the copper center. 

Aliphatic azo compounds usually undergo photoextrusion of nitrogen (cf. chapter 2.3.3). Nevertheless 1,2-diazetidines are formed efficiently when there is a favourable interaction between the pi-systems of the N—N and the C-C bonds (4.85)499). 

The rather confusing conformational dependences in Fig. 3.52 can be rationalized in a simple way. From the total of 7t-a and a-7t stabilizations at their respective coplanar alignments with the pi system, one can confirm that C—H bonds are better overall hyperconjugating groups than C—I bonds. The most favorable hyperconjugative alignment is therefore to place both C—H bonds maximally out-of-plane (i.e., C—F in-plane) at = 0°, whereas a secondary favorable alignment... 

The geometries in Figs. 4.86 and 4.87 suggest an important distinction in the multicenter hapticity character of ligand attachment to the metal atom. Hapticity refers to the number of atoms in a ligand that are coordinated to the metal. In the Ir+ diammine complex (Fig. 4.86(a)), the metal attaches to each of two nN donor lone pairs in simple monohapto (one-center, q1) fashion. However, in the Ir+ complexes with HCCH or CML the metal attaches to the face of the pi bond or three-center allylic pi system in dihapto (two-center, r 2) or trihapto (three-center, q3) fashion, respectively. The hapticity label q" therefore conveniently denotes the delocalized n -center character of the donated electron pair(s) and the geometry of the resulting coordination complex. 

PCMODEL Version 8 became available in 2002. New features in version 8 include support for different and improved force fields along with the MMX, MM3, MMFF94, Amber, and Oplsaa force fields currently supported. The atom limit has been increased to 2500 atoms, and support for reading and writing PDB and SDF files has been added. Transition-metal complexes can be built with explicit sigma bonding, lone-pair coordination, and pi-system coordination. Parameters are available for all transition metals. 

We first consider pi nonbonded interactions in difluoromethane and employ the dissection shown below. The appropriate interaction diagram for the pi system only is shown in Fig. 13. From this diagram it is obvious that only the symmetric... 

Fig. 13. Interaction of the pi fluorine lone pairs with the methylene pi system in CH2F2...

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. 

The sigma nonbonded interaction between the two substituents fall into pattern d of Scheme 1. Here, unlike the case of 1,2-difluoroethane, we conclude that there will be a preference for the syn conformation due to the sigma nonbonded interaction of the pi systems of the substituents. This will be counteracted by the inherent preference of any ethane molecule for the staggered geometry and a compromise is expected to be reached in the gauche conformation, barring adverse steric effects. 

We shall now examine molecules of the type CH3COX, where X can be a halide, a hydroxyl group, an alkoxy group, or an amido group. We shall first consider the conformation preference, i. e. staggered vs. eclipsed, imposed on such molecules by the presence of lone pairs or pi systems on the group X, e. g. X=F. 

We are now prepared to consider the construction of the composite pi system of methyl vinyl ether from the pi system of fragment B and oxygen lone pair AO. The orbital interactions which obtain in this union are depicted in Fig. 30. On the basis of the principles outlined before, we can see that the pz-03 interaction is more stabilizing in the Cs conformation. As a result, the more crowded structure, Cs, will be lower in energy than the Ts conformation. 

Heteroatoms (such as O, S, N) that are double-bonded to other atoms in a ring can t donate lone-pair electrons to the pi system because their p-electrons are already involved in the double bond. Single-bonded heteroatoms can donate a single lone-pair to the pi system but not two, because one lone-pair must be in an unhybridized p orbital orthogonal (at 90 degrees) to the sp ring plane. 

Water-soluble derivatives of polythiophene have been made allowing counterions bound to the polymer backbone to self-dope with the protons (e.g., lithium and sodium ions) injecting electrons into the pi-system. Thus, combinations of sodium salts and proton salts (e.g., prepared from poly-3-(2-ethanesulfonate)thiophene) have been prepared that are both water-soluble and conducting. 

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