Aromaticity indices

However, they are of greater interest as intermediates not only do they undergo a variety of reactions, but the A -oxide entity decreases the aromaticity index " and substantially affects reactivities at other positions. 

The order of aromaticity of these compounds is benzene > thiophene > pyrrole > fiiran, as calculated by an aromaticity index based on bond-distance measurements. This index has been calculated for five- and six-membered monocyclic and bicyclic heterocycles Bird, C.W. Tetrahedron, 1985, 41, 1409 1986, 42, 89 1987, 43, 4725. 

X-Ray diffraction data have been used to study the aromaticity of complex quinolizinium systems, such as the acenaphtho[4,5-c]quinolizinium derivative 36. The rings connected to the molecule by single C-C bonds are more aromatic than those connected by more links as indicated by the homeostatic model assessment (HOMA) aromaticity index <1992AXC2238>. 

There are little data on theoretical chemistry associated with such ring systems. In a series of 5,5-fused heterocycles, the inner salt 1 displays a substantial aromatic character as indicated by the aromaticity index 7(5,5) with a value of 82 <1987T4725>. 

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-... 

Several aromaticity indices (bond lengths, bond orders, Jug and Francois s aromaticity index) indicate that, despite the nonplanarity of the five-membered ring in 2,5-diphenylthiophene-l-oxide (108), this compound is intermediate in aromaticity between the corresponding thiophene 107 and the nonplanar 1,1-dioxide 109 (Scheme 48).149 The theoretical calculations were supported by experimental electrochemical data.150... 

Other congeners of phosphinins—arsenin, antimonin, and bismin—have been shown to be definitely less aromatic than benzene by diverse theoretical treatments that have been reviewed.236 For instance, the Bird aromaticity index for arsenin was found to be 67, compared to 100 for benzene.123 Table 4 summarizes a few parameters used to estimate the aromaticity of heterobenzenes resonance energies... 

Triphospholes can even be planarized fully by using proper substituents. l-(Bis(trimethylsilyl)methyl)-3,5-bis(trimethylsilyl)-l,2,4-triphosphole 21 (see Figure 5) was the first planar phosphole reported. Its Bird aromaticity index is 84, BDSHRT 60. All these... 

For deriving the aromaticity index AN proposed by Pozharskii (85KGS867) the sum of absolute values of all differences between the bond orders of n skeletal bonds including those with equal values of the orders is calculated and it is normalized with respect to the number of those differences equaling that of the dual combinations of n... 

Since the magnetic susceptibility anisotropy Ax is a characteristic attribute of aromatic molecules (66MI1 75MI2), its value could play the role of an aromaticity index... 

This was found indeed to be the case of 2-pyrone and 4-pyrone, which do possess fairly large values of Ax but their origin is determined mainly by local effects (69JA1991). Thus, the conclusion is obvious—the local and nonlocal contributions to Ax (Axlocal and Axnonlocal) must be sorted out and only the latter is to be used as an aromaticity index (73JA7961 74TL2885). 

The hydrogen bonding acceptor ability (log K ) for 3,5-diphenyl-l,2,4-oxadiazole has been determined <89JCS(P2)1355>. An aromaticity index indicates low aromaticity of the parent 1,2,4-oxadiazole <85T1409>. 

A new index of aromaticity has been proposed and applied to neutral and mesoionic five-membered heterocycles. The aromaticity index is based on data from experimentally determined bond lengths which yield statistically evaluated bond orders. A reasonable parallel exists between the aromaticity index and resonance energies <85T1409>. 

An x-ray structure analysis has also been performed for a 4,4 -bis(l,2,3-diazaarsole) <88IZV150> its molecular dimensions result in an aromaticity index not significantly different from that of a 2H-1,2,3-diazaphosphole <90T5697>. 

Some of the more critical properties related to marine fuels include ash content, carbon residue, calculated carbon aromaticity index (CCAI), density, sulfur, total sediment, and viscosity. A description of these properties and the primary reason for their implementation are provided below ... 

It is impractical to determine the cetane number of residual fuels in the ASTM D-613 cetane engine. Because of this, the Calculated Carbon Aromaticity Index and the Calculated Ignition Index were respectively developed by Shell and BR These values can be determined from the following equations where d = Density in kg/m3 59°F (15°C) and v = Viscosity in cSt 122°F (50°C). 

Recent research by Kornilov et al. (91ZOR144) on the aromaticity of 6-nitro-TPs substituted at C-2 revealed that the experimentally detected values (by reversible covalent hydration) proved more sensitive to substituent effects than the modified aromaticity index ANs of Pozharskii did. 

A linear relationship has been shown to exist between structural aromaticity indices and resonance energies (92T335). From this so-called unified aromaticity index, IA, has been proposed. It makes for a more appropriate comparison the aromaticity of heterocycles of different size. 

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