Paratropicity

The parallelization of crystallites, occurring as a result of fiber drawing, which consists in assuming by crystallite axes-positions more or less mutually parallel, leads to the development of texture within the fiber. In the case of PET fibers, this is a specific texture, different from that of other kinds of chemical fibers. It is called axial-tilted texture. The occurrence of such a texture is proved by the displacement of x-ray reflexes of paratropic lattice planes in relation to the equator of the texture dif-fractogram and by the deviation from the rectilinear arrangement of oblique diffraction planes. With the preservation of the principle of rotational symmetry, the inclination of all the crystallites axes in relation to the fiber axis is a characteristic of such a type of texture. The angle formed by the axes of particular crystallites (the translation direction of space lattice [c]) and the... 

The HNMR spectrum of ZV-deprotonated 5//-dibenzf/ ,/]azepine indicates the presence of a strong paratropic ring current.50... 

The results for [16] annulene are similar. The compound was synthesized in two different ways, both of which gave 103, which in solution is in equilibrium with 104. Above -50°C there is conformational mobility, resulting in the magnetic equivalence of all protons, but at — 130°C the compound is clearly paratropic there are 4 protons at 10.565 and 12 at 5.35 5. In the solid state, where the compound exists entirely as 103, X-ray crystallography shows that the molecules are nonplanar with almost complete bond alternation The single bonds are 1.44-1.47 A and the double bonds are 1.31-1.35 A. A number of dehydro and bridged... 

Both peracyclene (105), which because of strain is stable only in solution, and dipleiadiene (106) are paratropic, as shown by NMR spectra. These molecules might have been expected to behave like naphthalenes with outer bridges, but instead, the outer n frameworks (12 and 16 electrons, respectively) constitute antiaromatic systems with an extra central double bond. 

The fact that many 4 systems are paratropic even though they may be nonplanar and have unequal bond distances indicates that if planarity were enforced, the ring currents might be even greater. That this is true is dramatically illustrated by the NMR spectrum of the dianion of 83 (and its diethyl and dipropyl homologs). We may recall that in 83, the outer protons were found at 8.14-8.67 8 with the methyl protons at —4.25 8. For the dianion, however, which is forced to have approximately the same planar geometry but now has 16 electrons, the outer protons are shifted to about -3 8 while the methyl protons are found at 21 8, a shift of 258 We have already seen where the converse shift was made, when [16]annulenes that were antiaromatic were converted to 18-electron dianions that were aromatic. In these cases, the changes in NMR chemical shifts were almost as dramatic. Heat of combustion measurements also show that [16]annulene is much less stable than its dianion. 

Mullen, K. Meul, T. Schade, R Schmickler, H. Vogel, E.J. Am. Chem. Soc., 1987,109, 4992. This paper also reports a number of other bridged paratropic 12-, 16-, and 20-electron dianions and dications. See also Hafner, K. Thiele, G.F. Tetrahedron Lett., 1984, 25, 1445. 

Using similar methodology, macrocycle 126 was prepared, as well as the unusual monoene 127 [76]. Considerable debate in the literature over the last thirty years has focused on whether dehydrobenzoannulenes are able to sustain induced ring currents [5al. Although fusion of arenes to the annulenic core provides rigidity and stability, this also weakens the diatropicity/paratropicity of the macrocycle significantly. Until quite recently, the number of planar systems available for study was limited however, with the the addition of 123 and 126, the series of alkyne-linked, tribenzo-fused dehydroannulenes is complete from... 

Annulenones containing 15 carbon-atoms may be expected to be diatropic. Since the [13]annulenones were paratropic it was of interest to determine the NMR spectral properties of the next higher members. 

Annulenones containing 17 carbon atoms and 16 -electrons may be expected to be paratropic. 

By a similar synthetic sequence to that adopted for 118, Sondheimer and Ojima41 have synthesised the dimethyl analogue 119. Again this proved to be paratropic, and the ring current is enhanced on deuteronation. 

Reduction of 124, 125, and 126 with either sodium borohydride or lithium aluminium hydride followed by methylation of the unstable alcohol obtained gave the methyl ethers 130,131, and 132 respectively. Comparisons between the NMR spectra of the methyl ethers 130 and 131 and the homoannulenes 127 and 128 indicated that 130 and 131 are paratropic like the 16 7r-electron methyl ether 114. The methyl ether 132 is atropic. 

The very low field resonances of the internal protons may indicate that 140 is paratropic but in the absence of more concrete data no firm conclusions were drawn. Reduction of the annulenone 139 with lithium aluminium hydride-aluminium chloride gave the homoannulene 141. 

The H NMR spectrum showed the paratropic shift expected of protons in an antiaromatic system. Because it is difficult to evaluate the magnitude of paratropic shift in a charged species, the average shift of protons in 3 were com-... 

Is the behavior of fluorenylidene dications, such as 3 or 5 unusual Comparison of the average chemical shifts of fluorenyl cations 7 with the analogous diphenylmethyl cations shows minimal antiaromaticity in the monocations, with paratropic shifts of 0.5 ppm or less (29). The dramatic para-... 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

Dioxocines 218 are in turn paratropic and exist in equilibrium with their 2o — 2ti valence isomers sjn-benzene dioxides 217 (Scheme 80).259 Their chemistry and behavior in magnetic fields have been evaluated.260 261... 

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