7t-electron delocalization

The 7t-electron delocalization requires proper orbital alignment. As a result, there is a significant barrier to rotation about the carbon-carbon bonds in the allyl cation. The results of 6-31G/MP2 calculations show the perpendicular allyl cation to be 37.8 kcal/mol higher than the planar ion. Related calculations indicate that rotation does not occur but that... 

In order for a substitution to occur, a n-complex must be formed. The term a-complex is used to describe an intermediate in which the carbon at the site of substitution is bonded to both the electrophile and the hydrogen that is displaced. As the term implies, a a bond is formed at the site of substitution. The intermediate is a cyclohexadienyl cation. Its fundamental structural characteristics can be described in simple MO terms. The a-complex is a four-7t-electron delocalized system that is electronically equivalent to a pentadienyl cation (Fig. 10.1). There is no longer cyclic conjugation. The LUMO has nodes at C-2 and C-4 of the pentadienyl structure, and these positions correspond to the positions meta to the site of substitution on the aromatic ring. As a result, the positive chargex)f the cation is located at the positions ortho and para to the site of substitution. 

In contrast to the 1,4-dithiocin system, 1,4-dioxocin (1) is well-known and has been characterized as an olefinic compound by its spectra as well as its chemical behavior.5-6 The reason why 1,4-dioxocin in contrast to 1.4-dihydro-1.4-diazocine (see Section 1.4.) and 4//-l,4-oxazocinc (sec Section 1.12.), does not qualify as a 107r-aromatic species, is the less pronounced tendency of oxygen atoms for 7t-electron delocalization. An X-ray analysis of the 6-substituted 1,4-dioxocin 2 confirms the presumed nonplanar conformation of the 1,4-dioxocin structural element.9 The eight-membered ring exhibits a twisted boat-chair confirmation. 

The anisotropy of the magnetic susceptibility of a cyclic conjugated system, attributable to induced ring currents in its rc-electron network, is one of the important quantities indicative of 7t-electron delocalization. The method used for the calculation of the magnetic susceptibilities of nonalternant hydrocarbons is the London-Hoarau method taken together with the Wheland-Mann SCF technique . The resonance integral is assumed again to be of exponential form but... 

Calculations of the geometry of 106 have been reported by a number of groups238"243. The most accurate data are those from the calculations of Kraka and Cremer based on MP2/DZ+P and CCSD(T)/DZ+P calculations238. The C(l)-C(6) distance in the latter work was found to be 1.572 A, which is clearly longer than the value found for cyclopropane. The C(l)—C(6) bond order is significantly smaller than 1, indicating a partial bond between these atoms. 7t-Electron delocalization, as reflected by the ratio of calculated bond lengths [C(l)—C(2) 1.461, C(2)—C(3) 1.357, C(3)—C(4) 1.446 A], appeared to be... 

Generally speaking, electrically conductive polymers are composed of conjugated polymer chains with 7T-electrons delocalized along the backbone. 

The benzene ring is usually depicted as one of two mesomeric structures (2.1). The double arrow indicates that the true structure of the molecule lies somewhere in between the two drawn structures. It is therefore more accurate to use structure 2.2, since the six C-C bonds of the ring are identical, with the 7t-electrons delocalized over the entire ring. The configuration shown in 2.2 is, however, less convenient for drawing reaction mechanisms. 

Vibrational assignments for the parent heterocycles are summarized in Table 25. These have been derived from IR and Raman spectra of both the parent heterocycles and the deuterated derivatives. In the case of the vibrations of A symmetry, these have been further supported by photoelectron spectroscopic studies <1971SAA2525>. The variety of factors responsible for the observed differences precludes a complete rationalization as they may variously operate in concert or in opposition. Pertinent factors include the mass and electronegativity of the heteroatom, 7t-electron delocalization, ring geometry, and vibrational coupling of normal modes. The ring modes permit the best qualitative... 

As shown in Fig. 1 there are two chances. First, the stability can be reached thermodynamically by lowering the energy of the PC double bond via mesomeric 7t-electron delocalization. Second, the stability can be kinetically obtained by introducing voluminous and bulky bridgehead atoms to shield the reactive center. Substituents for the latter process are 2-butyl, mesityl, or especially tri-2-butylphenyl, the so-called supermesityl group. 

Structures tendency toward bond-length equalization and planarity (if applicable). Bond-length equalization cannot be used as the only criterion for aromaticity because some bond-equalized systems are not aromatic. Moreover, bond-length equalization due to 7t-electron delocalization is found not only in cyclic systems but also in highly conjugated acyclic compounds. 

The preparation of annelated tetrakisdehydro[18]annulenes 349 and 350 and the reference dehydroannulene 351 afforded further insight into the 7t-electron delocalization in annelated annulenes . When a dihydronaphthalene nucleus in 351 was replaced by naphthalene to form the annelated dehydroannulene 350, an appreciable suppression of diatropicity in 350 was observed. Further replacement of the dihydronaphthalene by naphthalene to give the dinaphtho-annelated derivative 349 resulted in a further suppression of the diatropicity in contrast to the increase of diamagnetic ring current in the case of transition from 342 to 340 and from 343 to 341. 

Characteristics Planar molecule with hexagonal C framework trigonal-planar geometry and sp hybridization at C atoms a framework has C-C and C-H single bonds 2p-orbitals (one from each C atom) overlap side-by-side form three rr-bonds six 7t-electrons) delocalized over the entire six C atom ring... 

Interrelation between characteristics of H-bonding and overall changes in 7T-electron delocalization in the chelate chain is shown by the dependence of... 

Evidently, due to the formerly shown interrelations, there is also a good correlation between the H NMR chemical shift of the proton involved in H-bonding and 7T-electron delocalization in the ring expressed by HOMA as shown in Fig. 24. 

Imidazolines (and isomers) can act as dienophiles in Diels-Alder reactions (see CHEC-I). The ylide properties of l,3,4,5-tetramethyl-2-methylene-4-iinidazoline (136) are demonstrated by the reactions of Scheme 67. This compound has the 7t-electrons delocalized over the whole ring and the exocyclic double bond which shows characteristic shortening <93CC1136). 

More precise numerical data, such as heats of combustion, are not available but theoretical methods (see Section 4.38.2.1) suggest that l,6,6aA4-trithiapentalenes exhibit a naphthalene-like 7T- electron delocalization. It should be noted, however, that the resonance... 

Comparing the stabilities of the reactant and the product will explain to us why the forward reaction is favorable. The product is, as we can see, stabilized by a considerable degree of 7T-electron delocalization. This is evidenced by the many resonance structures that we can draw for the cation product ... 

---