Electron, delocalization nonbonding

TT-electron interactions between the aromatic ring and adjacent conjugated free electron pairs, either nonbonding electron pairs or double bonds of substituents, improve electron delocalization, which in turn increases absorption intensity, that is, color strength. 

For example, by combining the previous statements, Bingham formulates the following rule. Electron delocalization will, therefore, stabilize trans conformations relative to the corresponding cis structures when only bonding or nonbonding molecular orbitals are occupied. Cis conformations will be stabilized (less destabilized) relative to trans when antibonding molecular orbitals are also filled. ... 

The intramolecular flexibilities of poly(1,4-phenylene oxide), polyi2,6-dimethyl-1,4-phenylene oxide), poly(2-methyl-6-phenyl-1,4-phenylene oxide), and poly 2,6-diphenyl-1,4-phenylene oxide) are evaluated through estimation of the resistance to rotation about the Cj 4—0 bonds in their backbones. A 6-12 potential is used to account for the van der Waals interactions between nonbonded atoms and groups encountered during the backbone rotations, while the twofold intrinsic potential to rotation about the C14—0 bonds resulting from the -electron delocalization is also included. 

To delocalize nonbonded electrons or electrons in 7C bonds, there must be p orbitals that can overlap. This may mean that the hybridization of an atom is different than would have been predicted using the rules first outlined in Chapter 1. 

Resonance effect is an energy stabilization due to delocalization of electrons in the bond network of the molecule and can be attributed to a mesomeric effect, i.e. the delocalization of Jt electrons on the jr orbital network, a hyperconjugation effect, i.e. a delocalization of a electrons in a ji orbital aligned with the o bond, and secondary mesomeric effects, such as repulsion of the ir electrons by nonbonded electrons on a substituent or solvent, or by time-dependent effects due to polarizabilities (for the last, the term electromeric effect is sometimes used). 

The size difference between carbon and sulfur atoms leads to relatively inefficient overlap of -tr-orbi-tals in the C=S bond. Consequently, thiocarbonyl compounds are in general highly reactive and have a tendency to di-, oligo- or poly-merize. This is particularly true for thioaldehydes, thioketones, and thio-ketenes. In contrast, thioamides (1) are usually perfectly stable and can be handled without problems. This stability can be understood in terms of a pronounced resonance interaction between the C =S TT-bond and the nonbonding electron pair on nitrogen. The analogous electron delocalization prevails in thiolactams. ... 

In a reactant molecule RY and an appropriate reference molecule R°Y, the primary steric effect of R is the direct result of differences in compressions, which occur because R differs from R° in the vicinity of the reaction center Y. A secondary steric effect involves the differential moderation of electron delocalization by nonbonded compressions. 

As the MOs increase in energy, the number of nodes increases and the number of bonding interactions decreases, and they alternate from being symmetric to asymmetric. When there is an odd number of molecular orbitals, one must be a nonbonding molecular orbital. Molecular orbital theory and contributing resonance structures both show that electrons are delocalized and that electron delocalization makes a molecule more stable. 

CT is the charge transfer or electron delocalization interaction energy. It results from the interaction caused by electron transfer from the highest occupied molecular orbital (HOMO) of Ma to the lowest unoccupied molecular orbital (LUMO) of Mb, and from the HOMO of Mb to the LUMO of Ma, and higher-order coupled interactions. This interaction is always attractive and highly directional. In hydrogen-bonded structures, a net electron transfer always occurs from the molecule with the nonbonded (lone-pair) electrons, or weakly bonded (tt) electrons to the molecule with a highly polarized A-H bond. 

Hence the Al-Al bond should have a bond order of 1 In addition, there are two 7T electrons around the ring, satisfying the (4 + 2) rule. Thus the Al anion has been called the first all-metal aromatic species. Finally, the bonding picture put forth by the resonance theory four a bonds, one 7t bond, and two lone pairs, are in agreement with the description provided by the molecular orbital theory five occupied delocalied bonding orbitals (one of which is r-type) and two filled delocalized nonbonding orbitals. 

The p-methoxytrityl and to a lesser extent the trityl function are currently used primarily as protecting groups for the primary 5 -hydroxyl of nucleosides. Under mild reaction conditions, only minor tritylation of a secondary hydroxyl or base amino function will occur. If it seems surprising that tritylation of the exocyclic amino function is difficult, especially since the trityl group has been noted to be useful for the protection of the amino function of amino acids, it should be remembered that the base amino function (with its delocalized nonbonding electron pair) is significantly less nucleophilic than a primary alkyl amine. The secondary hydroxyl(s) will... 

It might also be expected that enamines would be less basic than the corresponding saturated tertiary amines as a consequence of the delocalization of the nonbonded electron pair of the nitrogen. The older literature (/—/), which involved measurements in aqueous or partly aqueous solution, led to the opposite conclusion. This unexpected increase in basicity was rationalized in terms of an equilibrium between the enamine and the quaternary iminium hydroxide ... 

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