Ring current diamagnetic

The NMR spectrum of this compound shows a diamagnetic ring current of the type expected in an aromatic system. X-ray crystal structures of 1 and its carboxylic acid derivative 2 are shown in Fig. 9.2. Both reveal a pattern of bond lengths very similar to that in naphthalene (see p. 534). ... 

The NMR spectrum of the syn isomer shows evidence of a diamagnetic ring current, based on both the relatively low-field position of the vinylic hydrogens and the upfield shift of the methylene hydrogens. The anti isomer shows much less pronounced shifts. The X-ray crystal structure of the syn isomer shows a moderate level of bond alternation, ranging ftom 1.36 to 1.45 A (Fig. 9.4A). In the anti isomer, bond alternation is more pronounced, vith the double bond in the center ring being essentially a localized double bond (Fig. 9.4B). 

The HNMR spectrum indicates that thiepin 2 can sustain a diamagnetic ring current and hence represents a 1 On-aromatic system. 

In the following sections, systems with various numbers of electrons are discussed. When we look for aromaticity we look for (1) the presence of a diamagnetic ring current (2) equal or approximately equal bond distances, except when the symmetry of the system is disturbed by a hetero atom or in some other way (3) planarity (4) chemical stability (5) the ability to undergo aromatic substitution. 

Figure 5 The diamagnetic ring current effect in benzene.

This lack of a major diamagnetism may be attributed to two factors. First even in the completely delocalized cyclopropenium cation the diamagnetic ring current effects are small and in the range of only 25% of those in benzene as concluded from NMR data146. ... 

Molecular models show that in both compounds H-15a must experience a diamagnetic ring current because it is situated under the pyrrole ring. However, in 190 the diamagnetic effet is contrasted by a van der Waals deshielding effect between electronegative 16a-OH and H-15a, and, as a consequence, this last proton occupies an expected position. 

The chemistry and properties of heteronins 221 (Scheme 82) have been reviewed.264-266 These compounds are thermally stable and possess delocalized planar molecules with strong diamagnetic ring currents.2673 Schleyer and co-workers calculated by ab initio and density functional methods aromatic stabilization energies as well as other properties of heteronins, and found that only 221 anions with X = N and P are aromatic and planar.2676... 

NMR data for aza[18]annulene 234 show a wide separation between the centers of the inner and outer proton lH NMR multiplets (Ad =11 ppm), indicative of a strong diamagnetic ring current.270 271 The bis-dehydro system 235 with 22 jr-electrons in the aromatic ring is diatropic.272... 

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). 

Theoretical NMR "J( C/ C) Scalar Couplings as Probes to Study Diamagnetic Ring Currents... 

The potential of long-range couplings as probes to study diamagnetic ring currents is first exemplified considering the couplings in C70. 

We emphasize that only the Tc-transmitted component of the FC contribution can provide insight into diamagnetic ring currents. Therefore, only long-range... 

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