Diatropic ring currents

The scientific interest in porphyrin ligands (Fig. 5) derives in part from their ability to accommodate a large series of different elements, often in various oxidation states. On the other hand porphyrins are planar molecules with a delocalized 18 Ti-electron system and a diatropic ring current [25], which makes them interesting for the design of new materials with applications in photochemistry [25-27]. 

Homoaromatic Compounds. When cyclooctatetraene is dissolved in concentrated H2SO4, a proton adds to one of the double bonds to form the homotropylium ion (107). In this species an aromatic sextet is spread over seven carbons, as in the tropylium ion. The eighth carbon is an sp carbon and so cannot take part in the aromaticity. The NMR spectra show the presence of a diatropic ring current H/, is found at 5= - 0.3 at 5.1 6 Hj and H7 at... 

Angular momentum arguments show that the Jt-electron-rich nature of (4 +2) SN heterocycles is the key to their diatropic current. Formal expansion of (4 +2)-Jt-carbocyclic systems by insertion of NSN motifs in every CC bond is predicted to lead to structures that support diatropic ring currents explicit ab initio calculation of magnetic response predicts the 24-center, 30n-electron heterocycle S6N12(CH)6, formally derived from benzene, to be aromatic on the basis of this criterion <2002JA11202>. 

Mapping of the current densities in these and a wide range of hypothetical clamped systems gives a surprising result [14a,b]. Substantial bond alternation can coexist with the characteristic 4-electron diatropic ring current of benzene, but systems with similar alternation can also be found where the current is quenched. 

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],... 

The magnetic properties used to probe aromaticity arise from the presence of a diatropic ring current which tends to push an aromatic molecule out of a magnetic field (calculated property magnetic susceptibility, /). and which exerts NMR shielding on a proton at or above the ring center (calculated property nucleus-independent chemical shift, NICS). NICS values are obtained from the... 

Careful reduction of 41 with lithium metal at low temperature (195 K) leads to a salt that shows a temperature-dependent 111 NMR spectrum, assigned as the dianion (412-)61. As a result of the strong diatropic ring current, the signals of the inner protons are shifted to high field, while an opposite effect is exerted on the peripheral protons (Table 1). 

While both the ipsoeentrie and NICS methodologies loosely define aromaticity as the occurrenee of diatropic ring currents, in ref. 131 molecules a) and b) were examined from the VB (and chemically intuitive) viewpoint of resonance. It was shown that modern VB theory in its SC form strongly suggests that molecule (a) retains some of the resonance between electronic bonding structures observed in benzene, whilst the electronic structure of... 

---