Aromaticity NICS

HOMA, harmonic oscillator model of aromaticity NICS, nucleus independent chemical shift ASE, aromatic stabilization energy. 

Figure 5 Correlations between two aromaticity indices calculated for the five- (empty signs) and six-membered rings of studied tautomers of DNA bases (a) NICS(0) vs HOMA, cc(6) = 0.889 (b) NICS(1) vs HOMA, cc(6) = -0.931. HOMA, harmonic oscillator model of aromaticity NICS, nucleus-independent chemical shift.

Figure 8 Sammon map obtained for the training data set, showing the relative distances between the input variabies [ASE, A, NICS(1)zz, and HOMA] in the original space. The color scale indicates the Euclidean distances between the weight vector of each neuron and the neuron activated by benzene. ASE, aromatic stabilization energy HOMA, harmonic oscillator model of aromaticity NICS, nucleus-independent chemical shift A, magnetic susceptibility exaltation. Reprinted with permission from Aionso and Herradon (2010JCC917). Copyright 2009 Wiiey Periodicais, inc.

The observed planarity and bond length equalization in 1,3,2-diazaphospholenium cations likewise suggest that these compounds have substantial n-electron delocalization and possess possibly aromatic character. Several studies were undertaken to quantify the degree of n-delocalization by computational calculations using the interpretation of population analyses, ELF calculations, evaluation of magnetic criteria [nucleus independent chemical shift (NICS) values], and the... 

We have carried out DFT level investigations to explain the observed stereo and regioselectivities. Concerted nature of the mechanism has been confirmed by the involvement of aromatic but asynchronous transition structures as confirmed by their NICS values in each case [101, 102],... 

It is seen how all indicators of aromaticity correctly predict that a structure close to the TS is the most aromatic species along the reaction path, except HOMA and FLU indices, that are unsuccessful to account for the aromaticity of the TS. The reason for the failure of HOMA and FLU is that both values measure variances of the structural and electronic patterns, respectively, with respect to a reference value. Therefore, HOMA and FLU might fail if they are not applied to stable species because, while reactions are occurring, structural and electronic parameters suffer major changes. It is worth noting that the isotropic NICS(O) and NICS(l) values of the TSs are larger than those of benzene, but this is not the case with the more sophisticated NICS indices. 

Fluorenylidene dications, such as the dications of p- and m-substituted diphenylmethylidenefluorenes, show appreciable antiaromaticity. Evidence of antiaromaticity is demonstrated through H NMR shifts, nucleus independent chemical shifts (NICS), magnetic susceptibility exaltation, A, and (anti)aromatic (de)stabilization energies, ASE. Extension of the research to indenylidenefluorene dications shows that, contrary to expectation, the indenyl cation in these dications is less antiaromatic than the fluorenyl cation. The magnitude of the antiaromaticity is evaluated through comparison to the aromaticity of related dianions and reveals that the fluorenylidene dications are more antiaromatic than the fluorenylidene dianions are aromatic. 

---