Reorganization, of electrons

Electrocyclic reaction (Section 30.3) A unimolecular peri-cyclic reaction in which a ring is formed or broken by a concerted reorganization of electrons through a cyclic transition state. For example, the cyciization of 1,3.5-hexatriene to yield 1,3-cyclohexadiene is an electrocyclic reaction. 

Diboracyclopropane 1A may serve as an example to illustrate the principles discussed above. With the carbon atom tetrahedrally coordinated by two hydrogen and two boron atoms its classical structure is well described by the Lewis formula in Scheme 3.2-1. Hyperconjugation between the CH bonds and the formally empty p orbitals at the boron atoms leads to only a relatively minor reorganization of electron density compared with that suggested by the Lewis formula. 

Although the principle has turned out to have many exceptions [315], as discussed later, it has served as an inspiration for many researchers interested in uncovering the links between the reorganization of electron momentum densities and chemical bonding. 

In this respect, the CNDO and INDO methods have met with very little success, the values usually being too high by 3.5 eV to 5 eV. The error here seems too large even if provision is made for the possible reorganization of electrons in the molecule. 

A reaction involving concerted reorganization of electrons within a closed loop of interacting orbitals. Cycloadditions are one class of pericyclic reactions, (p. 692)... 

Because radical reactions always involve the reorganization of electron pairs, we can choose whether to show what happens to either or both of the members of... 

Even more so, the aceheptylene, as starting material for the reduction process, was shown to be best described as a peripheral system coupled with an inner perturbation. Consequently, the distribution of electrons in the dianion as proposed in 23 c would minimize the reorganization of electrons in the course of the reduction process. 

It is well known [112] that the electrical conductivity of PANI can be widely varied by acidic protonation. With increase in the MMT amount, the conversion (Table 4) to PANI decreased. Since the proton concentration in the media was kept fixed (2 mol/1) in all the feed, the extent of protonation of PANI was expected to increase with decreasing PANI content in the system. Protonation in PANI would lead to the formation of radical cations by an internal redox reaction causing reorganization of electronic structure to produce semiquinone radical cations and accordingly the conductivity would be enhanced. 

SAMPLE SOLUTION (a) When using curved arrows to represent the reorganization of electrons, begin at a site of high electron density, preferably an atom that is negatively charged. Move electron pairs until a proper Lewis structure results. For nitrate ion, this can be accomplished in two ways ... 

There seems to be another element of asynchronicity associated with bond formation in D-A reactions. The formation of the new double bond and the lengthening of the reacting dienophile bond seem to run ahead of the formation of the new a bonds. For example, in the MP4SDTQ/6-31G TS for the reaction of butadiene and ethene, the new a bonds are only 22% formed at the TS. The same picture emerges by following the transformations of the orbitals during the course of the reaction. The transfer of TT-electronic characteristics from the dienophile tt bond to the product TT bond seems to occur ahead of the reorganization of electrons to form the two new CT bonds. 

Chapter 29 discusses pericyclic reactions—reactions that occur as a result of a cyclic reorganization of electrons. In this chapter, you will learn how the conservation of orbital symmetry theory explains the relationships among reactant, product, and reaction conditions in a pericyclic reaction. 

We consider now processes characterized by bonding changes taking place through a concerted reorganization of electron pairs within a closed loop of interacting orbitals, which have been studied through the ISM [50]. The example to be considered here is the 3,3-shift in 1,5-hexadiene... 

Ab Initio Molecular Dynamics. Chemical reactions involve the reorganization of electrons about the nuclei involved in the bond changes. The ab initio molecular dynamics scheme developed by Car and Parrinello[36] permits an accurate de-... 

---