Diatropic shift

While the diatropic shift is consistent with all aromatic systems, there is no satisfactory experimental criterion for antiaromaticity 38>. The antiaromatic systems are all paratropic but the degree of paratropicity is confusing. It has therefore been suggested that the extent of the HOMO-LUMO gap (AE) correlates with the paratropicity of the system (see section 6.1) 35,36). A generalized approach can be obtained from the Ramsey shielding formula (Eq. 10)53> ... 

In terms of observable phenomena, the most useful definition of aromaticity is based on the magnetic anisotropy (10, 16-18) of a system as deduced from the XH NMR parameters. Antiaromatic 4n tt systems are expected to exhibit paratropic (high-field) shifts in their NMR spectra (11, 19) in contrast to the diatropic (low-field) shifts of (4n + 2) tt systems. Although the diatropic shifts revealed by (4n + 2) it systems are well-established, the experimental support for the relationship between para-tropicity and antiaromaticity is far less satisfactory (11). 

It follows that aromaticity can be determined from an NMR spectrum. If the protons attached to the ring are shifted downfield from the normal alkene region, we can conclude that the molecule is diatropic, and hence aromatic. In addition, if the compound has protons above or within the ring (we shall see an example of the latter on p. 69), then if the compound is diatropic, these will be shifted upfield. One drawback to this method is that it cannot be applied to compounds that have no protons in either category, for example, the dianion of squaiic acid (p. 69). Unfortunately, NMR is of no help here, since these spectra do not show ring currents. ... 

A bispyridine derivative of dibenzo-hexaaza[18]-annulene 236 was prepared by Bell and Guzzo.275 Fur an building blocks were used in the syntheses of the diatropic compounds 237 and 238.276-278 The chemical shifts of the indicated hydrogens show that the diamagnetic ring current decreases in the order 237 > 238 (Scheme 83). 

All the same, the quantitative determination of the aromaticity and antiaromaticity from the ring current model may be complicated by at least two problems. First, experimentally observable values of magnetic susceptibilities and their exaltations and anisotropies as well as the H-NMR chemical shifts are not necessarily determined exclusively by ring currents hence, all other effects have to be identified and removed. Naturally, for this model to work, the contribution by the ring current must be predominant. Another problem is that the calculated results on ring current intensities for molecules from the diatropic-paratropic border area may vary qualitatively depending on the method of calculation (80PAC1541). 

The diatropism of diazapyraceheptylenes (106), which follows from a comparison of the PMR spectra with that of 105a (Table VI), may be due to the effect of the cyano groups on the electronic configuration of the 7r-system. The chemical shift difference between the inside and outside protons clearly reveals the expected diamagnetic ring current in the annulene derivatives (110). The coupling constants of the seven-membered rings of 110 show that this part of the molecule is planar and delocalized. 

Indolizines are diatropic compounds, i.e. the possession of a ring current may be deduced from the chemical shift of the ring protons. 

Cyclazines are diatropic, i.e. a ring current has been deduced from the chemical shifts of the ring protons (Table 10). Additional nitrogen atoms in peripheral positions give rise to deshielding which is similar to that discussed in Section 3.08.2.3.1. UV and ESR spectra of [2.2.3]cyclazines have been discussed on the basis of MO calculations <78AHC(22)321). 

The reader may gain better appreciation of the many basic differences responsible for the division into different classes of heteronin by comparing certain representative members, directly or through appropriate models, in terms of the information presented in Table II. First, one notes that the classification of oxonin (24a) as atropic, jV-methylazonine (27a) as nondescript, and 1 //-azonine or its anion as diatropic, originally proposed on the basis of NMR chemical shifts (data shown in first three rows), was confirmed by the determination of solvent shift character (S values)38 39 that revealed 1//-azonine to possess significant diatropic influence (comparable to that of naphthalene +1.3538), the V-methyl counterpart to exhibit a far weaker effect in the same direction, and oxonin to be atropic or mildly paratropic under this criterion, its S value being closely similar to that of the family s 8 --electron polyenic model, all-cis-cyclononatetraene (24 X = CH2). Major differences between oxonin and parent azonine are also seen to exist in terms of thermal stability and 13C NMR and UV spectroscopy, all of which serve further to emphasize the close structural similarity of oxonin with n-... 

The NMR information listed in Table IV offers rather striking demonstration of the decisive effect that heteroatom electronegativity has on the development of aromaticity the inner a-proton resonance of the general system depicted in 79 experiences an upheld shift in excess of 8 ppm on passing from atropic 71 to diatropic 81 A particularly interesting observation concerning the importance of frame effects in the... 

The destabilization of the n system caused by out-of-plane distortion leads to a smaller HOMO-LUMO gap and, consequently, to red shifts of electronic absorptions. For instance the positions of the Soret and Qn bands are respectively 417 and 659 nm in lc and446 and 691 nm in 5,10,15,20-tetra-w-butylporphyrin [110]. Interestingly, the reduction of the diatropic ring current caused by distortion is only moderate [33],... 

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