Paratropic ring current

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

Neutral homoantiaromaticity has also been invoked for 117 by Wilcox and cowork-ers251. A shift difference of 2.25 ppm was found for the methylene protons of 117 which was interpreted as arising from a paratropic ring current and local anisotropies. Support for the importance of the homoantiaromaticity in describing 117 was also suggested from a consideration of its UV and PE spectra. However, we note our earlier caution about the use of PE as a criterion for homoaromaticity. 

Apparently, the first examples of [16]annulenoid porphyrin derivatives were obtained by Eschenmoser and coworkers during their work on corphin (21, Fig. 15), a peripherally reduced porphyrin derivative [91]. Free base corphin has the appropriate oxidation level to adopt a tetraaza[16]annulene structure, but it actually exists as the cross-conjugated tautomer 21. However, some of the complexes of corphin with divalent metals (such as Zn or Pd) were shown to have the symmetrical structure 22, and their H NMR spectra revealed a modest paratropic ring current, comparable with that of [16]annulene [92-94],... 

If the purpose of a calculation is to probe the inherent properties of a molecule as a thing in itself, or of a phenomenon centered on isolated molecules, then we do not want the complication of solvent. For example, a theoretically oriented study of the geometry and electronic structure of a novel hydrocarbon, e.g. pyramidane [6], or of the relative importance of diatropic and paratropic ring currents [7], properly examines unencumbered molecules. On the other hand, if we wish, say, to calculate from first principles the pZa of acids in water, we must calculate the relevant free energies in water [8]. Noteworthy too is the fact that solvation, in contrast to gas phase treatments, is somewhat akin to molecules in bulk, in crystals [9]. Here a molecule is solvated by its neighbors in a lattice, although the participants have a much more limited range of motion than in solution. Rates, equilibria, and molecular conformations are all affected by solvation. Bachrach has written a concise review of the computation of solvent effects with numerous apposite references [10]. 

Paratropic ring current ( H-NMR) An induced ring current within the annulene n-circuit that aligns with the applied magnetic field, resulting in an upfield shift of affected protons. 

Figure 13.12 ACID hypersurfaces and ring currents in a Huckel- (a), and a Mobius-type [16]annulene (b). For technical details see Figure 13.8. The Huckel structure (experimentally known, see text) is antiaromatic, and exhibits a strong paratropic ring current (counter clockwise). The Mobius isomer...

This compound was very unstable and decomposed within a few minutes at 0 . It could be trapped by cyclo-addition reactions with dienes. The alkene protons provided a sharp singlet at 6 4.93 in the H-n.m.r. spectrum, indicating a paratropic ring-current in the eight-membered ring. 

The tetrabenzo[5.5]fulvalene dication 49 and derivatives have been generated experimentally " and studied computationally. The H NMR spectrum of 49 has signals at d 5.07—5.87 with a center of gravity of d 5.41, and the upheld shifts from the neutral precursor, with a center of gravity of <5 7.75 ppm, indicate a substantial paratropic ring current, suggesting the species is antiaromatic. The NICS values and magnetic susceptibility exaltation of 49... 

In the particular case of planar carbocycles in geometries with the highest symmetry consistent with their electronic shell closure, it has been shown that the symmetry-based selection rules for diatropic and paratropic ring currents can be reformulated in the language of orbital angular momentiun [15]. Within pictorial Hiickel theory, a translational (rotational) transition connects occupied and empty orbitals only if their angular momenta differ by one (zero) unit, thereby defining selection rules for orbital current diatropic-ity (paratropicity) in equilateral carbocycles [15]. 

The predictions of this perturbed perimeter model are fully consistent with the maps obtained in full ab initio calculations. The model provides a rationalisation of the success of straightforward 7r-electron counting in HB-BH- and HN - NH-clamped COT (see Scheme 1,11 and 12). Figure 2d and 2f show the induced 7r-current density maps for HB - BH- and HN - NH-clamped COT. The global paratropic ring current characteristic of the flattened 8 TT-electron carbocycle is seen to survive both HB - BH and HN - NH clamping. 

Habenith, R. Engelberts, 1. Fowler, R Steiner, E. van Lenthe, 1. Lezeretti, P. Localisation and reversal of paratropic ring currents in molecules with formal antiaromatic electron counts. Phys. Chem. Chem. Phys. 2004, 6, 289. 

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