Bird-index, aromaticity

One may consider phospholes to belong to the family of five-membered P-heterocycles pyrrole, furan, and thiophene. A significant difference, however, is that the phospholes described in the literature display only a slight extent of aromaticity. This is well demonstrated by the comparison of the Bird-indexes [32] of benzylphosphole [33], furan, pyrrole, and thiophene (Fig. 1). The Bird-index is an indicator of aromaticity based on the bond-equalizaton. It is the maximum (100) for benzene. 

Fig. 1 The aromaticity of five-membered heterocycles characterized by the Bird-Index [32]...

Sulfur. Thiophene and benzo[ >] thiophene are both aromatic heterocycles, as discussed earlier in this review. Isothiazole is a planar molecule with an aromaticity comparable with those of thiazole and pyrazole, and higher than those of isoxazole and oxazole,122 140 as evaluated on the basis of Bird s aromaticity index A, based upon the statistical degree of uniformity of the bond orders of the ring periphery. Theoretical calculations and experimental data in connection with the aromaticity of isothiazole have been reviewed.141 Thiazole is also viewed as an aromatic molecule, similar to thiophene. It lacks an experimental aromaticity value, but the heat of formation together with bond lengths and angles have been calculated by various computational meth-... 

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. 

The planar form of phosphole is a first-order saddle point on the potential energy surface, 16—24 kcal/ mol above the minimum (at different levels of the theory). ° (The calculated barriers are the highest at the HF level, which underestimates aromatic stabilization of the planar saddle point, while the MP2 results are at the low end.) It has been demonstrated by calculation of the NMR properties, structural parameters, ° and geometric aromaticity indices as the Bird index ° and the BDSHRT, ° as well as the stabilization energies (with planarized phosphorus in the reference structures) ° and NIGS values ° that the planar form of phosphole has an even larger aromaticity than pyrrole or thiophene. 

The geometric aromaticity criteria are related to the observations of the equalized bond length of benzene, which is in contrast with that of the conjugated polyenes, having alternating bond lengths (and bond orders). The extent of the equalization can be measured statistically, and perhaps the most widely used method is that proposed by Bird [66]. The Bird index (BI), which can also be applied for heteroatoms (including... 

Much more stable are the 2- or 4-oxides, i.e., a-pyrones (unsaturated lactones) 20 and y-pyrones, which could, in principle, have aromatic character owing to their zwitterionic resonance structures (20A-20C). However, although protonated pyrones are definitely aromatic, the neutral compounds appear to have very little aromaticity. Bird s aromaticity index I6 for pyrylium is only 65.8% in comparison with benzene, whereas for 4-pyrone it is 37.2% and for 2-pyrone it is only 32.9%, as seen in Table 4 [22], In agreement with Table 2, the ring 0-(C = O) bond in 20A is a type X-Z bond, whereas the ring O = (C-O ) bond in the two other resonance formulas is a Y-Y bond. 

Table 4 Bird s aromaticity index for six-membered heterocycles with one heteroatom...

Surprisingly, the HOMA index classifies pyrrole as being more aromatic than thiophene but the Bird index gives the order thiophene > pyrrole > furan. Both pyrrole and thiophene are much more aromatic than furan and according to the Bird index indole and isoindole are significantly more aromatic than their oxygen and sulfur analogues. 

The HOMA index and Bird aromaticity indices (Is and /A) for five-membered heterocyclic rings are shown in Table 39. The theoretical background to these indices is discussed in Section 2.2.4.2.3 and the derivation of the Bird unified aromaticity index (/A) is given in Section 2.3.4.2.3. 

Table 4 HF/6-31G calculated geometric parameters (a-c In A sum of the bond angle at P In deg) and aromaticity Indexes (Bird Index)...

Bird index calculated from statistical deviation of bond orders from bond length measurements higher values imply higher aromaticity, on a scale of 0—100 <8571409). 

New measures of aromaticity such as Parr and coworkers, absolute and relative hardness <89JA7371> or Bird s aromaticity index <85T1409> have yet to be applied to these systems. 

Rank from 1 (most aromatic highest Bird index) to 3 (least aromatic lowest Bird index). 

Imidazole is the common name used for 1,3-diazole. This is a stable, aromatic ring system, with a Bird Unified Aromaticity Index (Ia) value of 79. Its resonance and tautomeric forms were discussed in Chapter 7 (section 7.1.8). This ring frequently appears in pharmaceuticals fused-ring systems are also common. Thus, the all-important purines are formed from fusion of imidazole to pyrimidines and are the subject of section 9.17. There is no general method for forming the imidazole ring such as the Paal, Knorr, and Hantzsch syntheses we have discussed for other cycles, so specific syntheses must be performed. 

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