Aromaticity NICS value

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],... 

Since antiaromaticity is related to aromaticity, it should be defined by many of the same criteria (31). That is, antiaromatic species should be less stable in comparison to a localized reference system, should demonstrate paratropic shifts in the H NMR spectrum, should have positive NICS values, and positive values of magnetic susceptibility exaltation, A. While the presence of enhanced bond length alternation has been considered as evidence of antiaromaticity (31), the deformation of square cyclobutadiene to rectangular cyclobutadiene to reduce its antiaromaticity suggests that the lack of bond length alternation is also a characteristic of antiaromatic compounds. 

Siloles, germoles, stannoles, and plumboles have been extensively reviewed.180 The aromaticity of these systems is still under scrutiny. In the study, the cyclopentadienyl anion had the highest aromatic character, with a NICS value of —14 and a Bird index of 100. Structures 138 and 139 also showed high and similar aromaticity indices (Scheme 59). For the planar silacyclopentadienyl anion 138, the NICS value is —10.2 and the Bird index 80, while the values for anion 139 are —10.9 and 77, respectively. 

Fragments in compounds 155—157 exhibit aromatic bond delocalization. The lowest aromaticity is calculated for Af-pyridinium cyclopentadienide 157, with the interfragmental C—N bond shorter than the corresponding one in 155 and 158. The phenolate moiety in 159 has a high NICS value (—4.6 ppm), in agreement with the one for deprotonated phenol (—6.2 ppm compared to —9.7 ppm for benzene, as cited),196 while the acceptor pyridinium counterpart has a NICS value of —5.5 ppm, showing aromatic delocalization. 

Hirsch s rule has more limited applicability than the Hiickel rule. However, the 2(n +1)2 concept has been used very successfully to interpret relative fullerene stabilities [66], and to suggest new systems, including neutral and charged non-fullerene carbon [67] and homoaromatic cages [68]. All these species have large NICS values in their centers and satisfy other criteria of aromaticity. 

NICS(O) value (—11.0) is lower than that for the corresponding azadiphosphole (—12.8 see section III.C.2.1), similar to the differences in the furan and pyrrole NICS values (—12.3 and —14.7, respectively, at the same level of theory).The high reactivity of the oxadiphosphole in cycloaddition reactions is in accord with its low aromaticity. 

The computational results point to an aromatic character of the silanaphthalenes 18-20. The chemical shifts which were observed for 1 are similar to those calculated for 19a-c (Table 3). The observed (in 1) and calculated (for 19a-c) <5(29Si) and 5(13C) chemical shifts are similar to those previously reported for other sp2-hybridized Si and C atoms32,8. All the NMR signals are, as expected, in the aromatic region (Table 3). The highly negative NICS values calculated for 18,19a and 20 (Figure 3), which are very similar to those of benzene, also indicate that all the calculated silanaphthalenes (18-20) are aromatic. 

FIGURE 3. Calculated NICS values (ppm) at the ring centres for silanaphthalenes 18, 19a, 20 and related aromatic systems, at GIAO-SCF/6-31+G. The optimized Si—C bond lengths (A) are at B3LYP/6-311+G. Reprinted with permission from Reference 8. Copyright (1997). American Chemical Society... 

Further evidence showing disproportionately high transition state aromaticity comes form NICS values,144,145 Bird indices,148,149 and HOMA142,143... 

Additional confirmation of early development of aromaticity as the reaction progresses comes from plots of NICS values and Bird indices versus the reaction coordinate for the reaction of 61H-S with CH2NO (Figs. 7 and 8), and of 61H-0 with CH2NO2 (figures not shown). These reactions were chosen... 

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