7t electron systems

In this mechanism, a complexation of the electrophile with the 7t-electron system of the aromatic ring is the first step. This species, called the 7t-complex, m or ms not be involved directly in the substitution mechanism. 7t-Complex formation is, in general, rapidly reversible, and in many cases the equilibrium constant is small. The 7t-complex is a donor-acceptor type complex, with the n electrons of the aromatic ring donating electron density to the electrophile. No position selectivity is associated with the 7t-complex. 

Flexible six- and eight-7t-electron systems would impose an entropic barrier to concerted 10-7i-electron concerted reactions. Most of the examples, as in the cases above, involve cyclic systems in which the two termini of the conjugated system are held close together. 

In the literature discussing these results, the coincidence of the NN bond lengths in diazonium ions with that in dinitrogen seems always to be regarded with complete satisfaction. In the opinion of the present author this close coincidence is somewhat surprising, firstly because of the fact that in diazonium ions one of the nitrogen atoms is bonded to another atom in addition to the N(2) atom, and secondly because work on dual substituent parameter evaluations of dediazoniation rates of substituted benzenediazonium ions clearly demonstrates that the nx orbitals of the N(l) nitrogen atom overlap with the aromatic 7t-electron system (see Sec. 8.4). 

A Structural characteristic of conducting organic polymers is the conjugation of the chain-linked electroactive monomeric units, i.e. the monomers interact via a 7t-electron system. In this respect they are fundamentally different from redox polymers. Although redox polymers also contain electroactive groups, the polymer backbone is not conjugated. Consequently, and irrespective of their charge state, redox polymers are nonconductors. Their importance for electrochemistry lies mainly in their use as materials for modified el trodes. Redox polymers have been discussed in depth in the literature and will not be included in this review. 

Charge transfer complexes (CT complexes) primarily occur in planar organic molecules with conjugated 7t-electron systems [4]. Examples include ... 

Numerous colorless organic compounds with extended 7t-electron systems can be converted to colored cations or anions with polymethyne chromophors by protonation or deprotonation. The intense coloration of the corresponding salts is usually attributable to the fact that the lone pairs of electrons of the heteroatoms participate in the mesomerism of the conjugated 7t-electron systems [4]. 

Figure 1.6. Net charge densities for 7t-electron system of the colored form of 1 in the ground...

In conjugated systems the it orbitals become delocalized. The classical example is die butadiene molecule, that is usually described by the formula CH2—CH-CHa=CH2> This representation of the molecule does not take into consideration the delocalization of the 7T-electron system formed by the four... 

The electronic spectra of the [2.2]paracyclophanes provide valuable information regarding the extent and mechanisms of transannular electronic interactions in strained 7t-electron systems. A rigid system like the [2.2]paracyclophane molecule is of great value as a model for checking theoretical data with a view to interpretating U V spectra of sterically hindered molecules ( overcrowded compounds ). 

Note Any analyte of suitable PA may be regarded as basic impurity of the reagent gas, and therefore becomes protonated in excellent yield. Heteroatoms and 7t-electron systems are the preferred sites of protonation. Nevertheless, the additional proton often moves between several positions of the ion, sometimes accompanied by its exchange with otherwise fixed hydrogens. [23,24]... 

Hiberty, P.C. The Distortive Tendencies of Delocalized 7t Electronic Systems. Benzene, Cyclobutadiene and Related Heteroannulenes. 153, 27-40 (1990). 

Among 7T-electronic systems acting as proton acceptors, the gold ethyne and benzene complexes are best known. Some of them have been characterized by... 

Unique in carbazole chemistry is a paper describing the formation of isolable spirocyclic salts with the carbazole nitrogen having a valence of four, Although these salts no longer have 14 7t-electron systems, these reactions are included here because the formation of such spiro cells salts as 49... 

Compound 442 (commonly termed DoDHT), which possesses only one 1,3-dithiolylidene unit as a 7t-electron system, has been investigated. 

A detailed comparison of spectral data concluded that borazine has a delocalized 7T-electron system like that of benzene (112) other workers, however, have concluded from spin-coupled calculations that borazine has litde aromatic stabilization as compared to benzene (113). 

Charge-Transfer Compounds. Similar to iodine and chlorine, bromine can form charge-transfer complexes with organic molecules that can serve as Lewis bases. The frequency of the intense uv charge-transfer adsorption band is dependent on the ionization potential of the donor solvent molecule. Electronic charge can be transferred from a 7t-electron system as in the case of aromatic compounds or from lone-pairs of electrons as in ethers and amines. 

Substrates which can undergo partial oxidation are characterized by a 7T-electron system or unshared electrons olefins and aromatics contain the first, methanol, ammonia and sulphur dioxide the second. Alkanes do not contain such electrons. Their selective oxidation appears to demand (thermal or catalytic) dehydrogenation to alkenes as the initial process. 

Both the trans conformation 192 and one of the cis conformations 193 are destabilized by a nonbonded interaction between C-l-H and C-(ll)-H, whereas 192 and 194 are possibly destabilized by an interaction between the nitrogen lone pair and the 7t-electron system of the benzene ring. [A... 

Corrole contains an 1 S-7t-electron system and shows aromaticity similar to the porphyrin chromophore, with strong ring current effects on the peripheral and inner-proton NMR, relatively stable parent mass ion and intermediary C—C bond length between single and double bonds. 

Neighboring group interactions with the vacant p orbital of the carbenium ion center can contribute to ion stabilization via charge delocalization. Such phenomena can involve atoms with unshared electron pairs (w-donors), C—H and C—C hyperconjugation, bent o-bonds (as in cyclopropylcarbenium ions), and 7t-electron systems (direct conjugative or allylic stabilization). Thus, trivalent carbenium ions can show... 

The deactivating effect of the nitro group is largely the result of its mesomeric interaction (-M effect) with the 7t-electron system of the benzene ring which is supplemented by the inductive (-/) effect (2). 

Haase and co-workers investigated electro-optic and dielectric properties of ferroelectric liquid crystals doped with chiral CNTs [495, 496]. The performance of the doped liquid crystal mixture was greatly affected even by a small concentration of CNTs. The experimental results were explained by two effects (1) the spontaneous polarization of the ferroelectric liquid crystal is screened by the 7t-electron system of the CNT and (2) the CNT 7i-electrons trap ionic impurities, resulting in a significant modification of the internal electric field within liquid crystal test cells. 

We found that these more sophisticated spin-coupled calculations, which used larger basis sets with polarization functions on all of the atoms and which allowed the a orbitals to relax, produced a picture of bonding in the 7t-electron system of benzene which is practically identical to that described earlier. As before, we found six equivalent spin-coupled orbitals which are transformed into one another by successive C6 rotations. The overlaps between the orbitals, ordered cpa to cp6 around the ring, are reported in Table 1. In this case, the electron correlation effects incorporated in the spin-coupled model provide an energy improvement over the SCF description of 170 kJ mol - with a further lowering of 20 kJ mol -1 on including spin-coupled ionic structures. 

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