N-Electron delocalization

During the last few years, both neutral and cationic 1,3,2-diazaphospholes and NHP have been studied extensively by computational methods. The best part of these studies focused on a discussion of n-electron delocalization and their implication on chemical reactivities and stabilities, the explanation of the unique ionic polarization of exocyclic P-X bonds noted for some species, and the evaluation of structural and spectroscopic properties with the aim of helping in the interpretation of experimental data. 

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

Hence, the anion-radical of diphenyl fulvene acquires a spin-charge distribution dictated by steric shielding at Ph2C node and six-n-electron delocalization in the C5H5 ring. Anion-radicals of sterically congested stilbenes represent examples that are quite different, but at the same time are similar in principle. Let us compare the E structures of stilbene and its congested derivative, namely, a,p-di(tert-butyl)stilbene in their neutral and anion-radical forms (Scheme 3.18). 

It shonld also be mentioned that there are some cases when the cation-selective solvation promotes greater n-electron delocalization in the remaining free anion-radicals (see Section 3.2.5). Staley et al. (1999) and Staley and Kehlbeck (2001) studied the phenomenon for organic dianions too. Snch a solvent effect leads to changes in reactivity of these species. 

The isomerization shown in Scheme 6.32 involves two intermediary dianions and not the direct conversion of the initial anion-radical into the final one. However, what is the driving force for this isomerization The molecule of l,3,6,8-tetra(tcrt-butyl)naphthalene is nonplanar the tert-butyl group and carbon atom at position 1 of the naphthalene skeleton lie off the plane of the molecule. The corresponding anion-radical has the same stereochemical peculiarity (Goldberg 1973). Such bending removes the steric strain but, naturally, decreases the degree of the n-electron delocalization over the neutral molecule. As for the anion-radical, its unpaired electron delocalizes less effectively than the anion-radical of the unsubstituted naphthalene. Bending of the naphthalene skeleton... 

The x-ray structure determination of the 1,2,4-thiaselenazoline (14) shows that there is some n-electron delocalization in the NH2—C=N moiety <90CB69l>. X-ray crystal structure determinations of the 1,3,4-selenadiazolines (15) and (16) have been carried out <92MI 421-01,92MI421-02). 

The structural indices of aromaticity, I, of oxadiazoles (145-148), thiadiazoles (150-153) and selenadiazoles (155, 156) are compared with that of the parent furan (144), thiophene (149) and selenophene (154) (Scheme 11). 1,2,3-Oxadiazole (145) is the least stable among them since all attempts to synthesize this compound were unsuccessful, most likely because of its easy isomerization to the acyclic isomer. At the same time its sulfur analogue (150) possesses good stability and has been synthesized. Its 2,4-diaza- (151), 3,4-diaza- (152) and 2,5-diaza-(153) isomers demonstrate even more the extent of n-electron delocalization. There exists a well-known tendency of decreasing aromaticity depending on the type of pyrrole-like heteroatom S > Se > O. However, there is no uniformity in the change in aromaticity in the horizontal rows, i. e., dependence on heteroatom disposition. 

Structurally related cations are cyclic phosphenium ions 287,727,72S288,728-731 289,732-734 and 290735 with an NPN moiety. No interionic contacts were observed in the crystals of hexafluorophosphate, tetrafluoroborate, and triflate salts of cations 288. The planar ring structure and the ring bond distances being intermediate between single and double bonds lent evidence for n electron delocalization. This, however, is only a weak delocalization and the cations cannot be described as genuine aromatic... 

Although aromatic compounds have multiple double bonds, these compounds do not undergo addition reactions. Their lack of reactivity toward addition reactions is due to the great stability of the ring systems that result from complete n electron delocalization (resonance). Aromatic compounds react by electrophilic aromatic substitution reactions, in which the aromaticity of the ring system is preserved. For example, benzene reacts with bromine to form bromobenzene. 

By combining high-level ab initio calculations with high-resolution infrared spectroscopy, the equilibrium bond lengths in x-frans-butadiene have been determined to an unprecedented precision of 0.1 pm. The values found for the pair of n-electron delocalized double bonds and the delocalized central single bond are 133.8 and 135.4 pm, respectively. The data provide definitive structural evidence that validates the fundamental concepts of n-electron delocalization, conjugation, and bond alternation in organic chemistry. 

The tropylium cation is aromatic because it is cyclic, planar, completely conjugated, and has six n electrons delocalized over the seven atoms of the ring. 

Benzene has six n electrons delocalized in six p orbitals that overlap above and below the plane of the ring. These loosely held n electrons make the benzene ring electron rich, and so it reacts with electrophiles. 

Another potential application of fully converted polydiacetylenes is based upon their unusual non-linear optical properties. Sauteret et al. found that the third order susceptibilities of TCDU and TS increases by about a factor of 600 upon polymerization and become comparable to those of inorganic semiconductors like GaAs or germanium. This is a consequence of the increase of n-electron delocalization upon polymerization. Polymeric diacetylenes can therefore be used as efficient elements for third harmonic generation. In Ref. this effect has been employed for tripling the frequencies of 1.89 pm and 2.62 pm radiation. 

The 1,2-dithiolylium and 1,3-dithiolylium ions (115) and (116) are iso-7r-electronic with the tropylium ion, from which they may be formally derived by replacing double bonds by sulfur atoms. Various calculations and structural data demonstrate that the rings are stabilized by n--electron delocalization. 

Compared to many other fused 1,2,3-triazoles described herein, (11) is remarkably stable as evidenced by its inertness to irradiation and to heat (72JOC2022). This stabilization was rationalized as the consequence of extensive n- electron delocalization in the tricyclic system. 

In the fluorenyl radical [130], the shielding of the 9-carbon is obviously not so effective as in HO—PTM- (p. 364), so allowing immediate hydrogen abstraction from HI. The isomeric ketone [132] is not reduced at all, this being explained by the loss of n-electron delocalization between the quino-methane moiety and the fluorene system that would occur in the reduction of cation [133] caused by twisting of the p-hydroxytetrachlorophenyl group around its bond to C(9) of the fluorene system (steric inhibition of resonance) (177) (p. 384). 

The study of a dilute solution of a polymer allows one to find the structure of an isolated chain. Therefore, it provides the unique opportunity to relate the conformational disorder, curvature of the chain and twist between monomer units, to the electronic properties of the conjugated backbone. From that viewpoint, several theoretical works paid special attention to the effects of twist fluctuations, emphasizing their influence on the n electron delocalization [25—29]. Besides, as some of these solutions exhibit spectacular colour changes as a function of the temperature or solvent quality, they were considered as the relevant systems to characterise the relationship between the n electron delocalization and the local structure of the chain. 

The study of the chain conformation in solution of PPA and poly(2-octyne) is of interest for two reasons. First, these two polymers differ as the PPA solution is red while the poly(2-octyne) solution is transparent, so that if it exists, any relation between the chain stiffness and the n electron delocalization may be expected to have a statistical length larger for PPA than for the poly(2-octyiie). The second reason is that PPA and polystyrene have almost identical monomer unit structures, the unique difference being the conjugated backbone structure of the PPA. 

Pyramidal inversion barriers of arsoles have been calculated using a modified version of CNINDO. Agreement between experimentally and computationally derived values is to within a few kcal mol" over a wide range of barrier heights (10-60 kcal mol" ) for a diverse set of structiu-es that include arsoles and arsindoles. These results are used to argue the aromaticity of arsoles via (4p-2p)n electron delocalization <74JA6904>. 

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