Jt-electron delocalization

Since divalent sulfur is isoelectronic with the carbon-carbon double bond and can participate in Jt-electron delocalization, the anions derived from 1,2- (143), 1,3- (144), and 1,4-dithiepins (145) may be regarded as 1 Orc-electron aromatic systems. According to the prediction by Zahradnik and Parkanyi851 based on Hiickel s molecular... 

Heteroindacenes have been prepared and studied by Hafner and co-workers.198 199 The syntheses of 1,3,5,7-tetra-te/t-butyl-4-azaindacene, its AA-oxide, and l,3,5,7-tetra-tot-butyl-4-phospha-s-indacenes have been recently reported (Scheme 66).200 The 12-jt-electron delocalized systems have been studied by dynamic NMR and X-ray and were subjected to molecular orbital calculations, and there is strong evidence of electron delocalization. However, X-ray crystallographic data for 4-phospha-s-indacene 164 and the 4-7V-oxide 164 show that there is a dual orientation in the crystal this disorder with two different orientations of the molecule does not allow for conclusions regarding bond lengths or delocalization, and the mediated structures show a D2h symmetry rather than C2h with localized double bonds. 

A great number of porphyrin analogs possess circular conjugation pathways, and often exhibit aromaticity comparable with that of the parent system 1. Moreover, the JT-electron delocalization of many porphyrin-like molecules is strongly affected by structural detail, redox chemistry, and prototropic tautomerism. The aim of the present review is to provide a description of porphyrinoid aromaticity and its connection with tautomeric equilibria. Our main focus will be on the physical manifestations of aromaticity, with a special emphasis on NMR spectroscopy. The reactivity of porphyrin analogs, including their coordination chemistry, will be discussed only to the extent it has a bearing on their aromaticity. 

Coordination of metal ions often has a dramatic effect on the n delocalization in porphyrins and porphyrinoids. It has particularly conspicuous influence on the electronic spectra of metalloporphyrins, which show a dependence on the identity of the metal ion, axial ligation, oxidation level, and spin state. In regular porphyrins, metal coordination reduces the number of observed Q bands from four to two, reflecting the higher symmetry of the chromophore relative to the free base. However, detailed quantitative information on the Jt-electron delocalization is more easily accessible from other physical methods. 

Unsubstituted tetrazole behaves as an organic acid with acidity close to that of acetic acid. In general, depending on the substituent in position 5, tetrazole ionization occurs in the range from —0.8 unit of the Hq scale to 7 unit of the pH scale. Proton elimination from 1H- and 277-tetrazoles 24 and 106 results in formation of the tetrazolate anion (tetrazolide) 7 (Scheme 5) characterized by high aromaticity and good Jt-electron delocalization. 

A number of PTh, but also PPy, polyaniline, and additional ICPs based on Jt-electron delocalized systems, bearing different metallocenes of Fe, Ni, Co, Ru, or Os, either as a pendant group or as an inclusion complex, have been synthesized and even proposed as electrode coatings for amperometric sensing. They have been extensively and critically reviewed [86]. It was observed that good conductivity and accessible p-doping potentials are only exhibited either (1) by co-polymers of Fc-substituted Th and unsubstituted Th, or (2) by Th bearing rally one Fc residue. 

Prior to systematic studies of N2F4, the structure and the electron distribution of X2Y4 molecules (including N2F4) were discussed in terms of bond order and electronegativity [1] and by a Huckel MO formalism which considered a- and/or jt-electron delocalization [2]. 

Krygowski TM, Stepien BT (2005) o- and Jt-electron delocalization focus on substituent effects. Chem Rev 105 3482-3512... 

Bodwell GJ, Bridson JN, Cyranski MK et al (2003) Nonplanar aromatic compounds. 8. S)mthesis, crystal stmctures, and aromaticity investigations of the 1, n-Dioxa[n](2,7) P5uenophanes. How does bending affect the cyclic jt-electron delocalization of the pyrene system J Org Chem 68 2089-2098... 

Feixas F, Matito E, Sola M, Poater J (2008) Analysis of Hitckel s [4n + 2] rule through electronic delocalization measures. J Phys Chem A 112 13231-13238 Feixas F, Matito E, Sola M, Poater J (2010) Patterns of Jt-electron delocalization in aromatic and antiaromatic organic compounds in the light of the Hiickel s 4n + 2 rule. Phys Chem Chem Phys 12 7126-7137... 

The crystal structure of sodium czs-bis(dicyanomethylene)squarate 27 tetrahydrated has also been reported in the literature [85]. The ring was demonstrated to be planar with small deviations, and the bond distances of the carbon atoms in the ring are equivalent to a value of 1.422 A, suggesting a significant residual double-bond character. Electron spin resonance (ESR) or EPR measurements confirm that cis-bis(dicyanomethylene)squarate 27 exhibits extension of its Jt-electron delocalization [85]. 

Figure 8.4 shows that the polyaniline that was synthesized is in fact polyaniline in its emeraldine base [27]. Additionally, all samples exhibit peaks around 1,306 cm , which is attributed to the jt-electron delocalization induced in the polymer by protonation. The peak around 1,375 cm is attributed to the C-N stretch in the base. The peak around 822 cm is due to C-H bond out of plane [28]. It can also be seen that the addition of tin oxide mainly caused a physical rather than a chemical reaction between the polyaniline and the tin oxide, as the FTIR spectrum is essentially unchanged. The addition of multiwall carbon nanotubes to the polyaniline matrix material also shows more of a physical mixing rather than a chemical mixing. This explains the results that are obtained in the hydrogen sorption measurements, to be discussed in the next section. The addition of aluminum to the sample, however, clearly shows a chemical reaction between the polyaniline and the aluminum powder. The large peak around 1,150 cm is a sign of increased conductivity and was described as the electron-like band [28]. 

Molar extinction coefficient >10 M cm Extensive Jt electron delocalization use of low photosensitizer dosages... 

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