Benzene aromaticity index

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. 

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... 

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. 

Cyclic systems with 4m + 2 carbon atoms are characterized by the aromatic index m. At the lowest level (m = 0) ethylene formally corresponds to a 2-membered ring. The electronic structure shown in Figure 6.1 is consistent with aromatic electron distribution. Benzene (m = 1) is the prototype of properly cyclic aromatics. The molecular o-a-m conserves the sum of atomic angular momenta. The odd couples with the screening function are delocalized around the ring, with zero o-a-m. 

It has been shown that a linear relationship between the present RLHEs and the r[ values (normalized benzene character in PPP-MO theory [51]) holds very well. Very recently, Suresh and Gadre [49] suggested a new local aromaticity index based on the molecular electrostatic potential topography (MESP) of the 7t regions of BHs, which is obtainable from ab initio MO calculations. This index was also found to bear linear correlation with the RLHE values. As a result, one... 

Aromaticity. An aromaticity index, based on deviation of peripheral bond orders,1691 has been applied to pyrazine (89% of that for benzene) and some derivatives.257,376,379,383 The aromaticity of l,s4-dihydropyrazines has been studied.565,1734... 

As listed in Table 33, a-H s of the heterocycles appear downfield and the difference between a- and / -H s are 0.22 ppm for thiophene. If an assumption is made that the difference of a- and /3-H s of benzene is zero because it is fully aromatic and it corresponds to the index of aromaticity of 1.00, then the relative indices of aromaticity may be related to the difference between and 5p.H- A set of indexes may be proposed for thiophene 0.89 (=1.00 — 0.22/ 2). This seems quite reasonable not only because the difference in chemical shift of a- and / -H is likely to originate from the presence of the heteroatom but also because the values are in the range of reported sets of aromaticity indexes <1974AHC( 17)255, 1993AHC(56)303, 1991H(32)127, 1981JST(85)163, 1985T1409>. 

Table 66 Aromaticity indexes for some five-membered-rings and benzene...

The aromaticity of the planar (Dgh) and chair (I>3d) forms of Si He and GegHg and other heteroatom benzene analogues has been the subject of a theoretical study by Schleyer and coworkers. The authors used three different criteria for estimating the aromatic character of the molecules. They are (i) the calculated NICS (Nuclear Independent Chemical Shift) values at the center of the ring, (ii) the ring size adjnsted aromaticity index p which is based on the calcnlated magnetic snsceptibility " and... 

It is an aromaticity index based on the first-order valence connectivity index divided by the number B of bonds of the molecule and normalized on the benzene molecule [Kier and Hall, 1986] ... 

The phosphorus analogues of the cyclopentadienyl anion P (CH)5 have been examined by SINDOl for their aromaticity. For n = 0 and 5 the aromaticity index is comparable to that of benzene. Of the two diphospholides (n = 2), the 1,3-isomer (88%) has a slightly higher relative aromaticity than the 1,2-isomer (86%) <92JOC3694>. 

In contrast, on the basis of a new aromaticity index (Ig) for six-membered heterocycles, 1,2,6-thiadiazin-4-one 1,1-dioxide (60) has been assigned an value of 54.2 (benzene 1 = 100) and is reckoned to have moderate to low aromatic character and is deemed to be more aromatic than pyranones but less aromatic than 4-pyridones <86T89>. 

Fig. 21. Aromaticity index HOMA and its components EN and GEO of the phenyl ring for two conformers of p-dinitroso and p-dimethoxy benzene.

The bond energy E y and the bond length r are related by E y= 1/r thus, there should be a correlation between aromaticity indexes and resonance energies. Accordingly, a unified aromaticity index 7a has been defined (reference benzene 7a=100), and 7a=77 for 1, 86 for 1,2,4- and 100 for 1,3,5-triazine <1992T335>. From molecular dimensions, 7a = 77 has also been calculated for both 4,6-dimethyl- and 4-methyl-6-phenyl-l,2,3-triazine, 73.4 for 4,6-dimethyl-l,2,3-triazine 2-oxide, 76.0 for 4-methyl-6-phenyl-l,2,3-triazine 2-oxide, and 68.95 for 6-methyl-4-phenyl-1,2,3-triazine 1-oxide <1993T8441>. In this sense, N-oxidation is accompanied by a reduction in aromatic character. 

The 7t-electron correlation energy E. in planar heteroatomic molecules follows simple additivity rules. The nondynamical component (ND),i of aromatic compounds is lower than that of their open-chain counterparts and (in comparison with benzene) regarded as another useful aromaticity index <2001GPH(269)11>. 

Since all bonds in benzene are equal in length, the bond alternation ratio and the Julg aromaticity index for benzene is one. The Julg index has been modified to account for resistance to circulation of the 7j-electrons which is a major factor in electronegative heteroatoms . This correction factor A2 is given by Equation (3). 

The nondelocalized Kekule form depends on the ring system. Thus = 35 for five-membered and for fused five- and six-membered rings, and 33.3 for six-membered rings. Since benzene is a fully delocalized system, Khas the value zero and for benzene is 100. Similarly, the aromaticity index (/ ) for thiophene is 66 while those for pyrrole and furan are 59 and 43, respectively. Quantitatively, these values agree with that predicted from their chemical behavior. 

Properties Clear pale yel. liq. or solid sol. in ketones, esters, acetone, benzene, aromatic soivs. insol. in water, alcohol m.w. 631.56 dens. 1.066 m.p. 23 C f.p. 8 C flash pt. 440 F ref. index 1.4710 Toxicology ACGIH TLV/TWA0.1 mg(Sn)/m (skin) ED50 (oral, rat), 3954 mg/kg, (dermal, rabbit) > 2000 mg/kg EDEo (IP, rat) 85 mg/kg toxic poison by ing. and IP routes conosive skin/eye irritant readily absorbed thm skin target organs Gl system, nerves, liver TSCA listed Precaution Combustible reacts with oxidizers Hazardous Decomp. Prods. Heated to decomp., emits acrid smoke and fumes... 

An additional problem arises when the Bird index is considered in its unified form Ia- It is biased by the assumption made in the unifying procedure. The Hiickel delocalization energies (DE) of benzene the cyclopentadienyl anion (2.47 ), naphthalene (3.68 ), and indenyl anion (4.17 8), i.e., for the nominal parents of the A, h, and /s.e, were used in the construction of an unified aromaticity index... 

Figure 1 Correlations between HOMA aromaticity index and resonance substituent constant (cjr) for substituted benzene (Bz), pyrazole (Pz), and imidazole (Im) derivatives. HOMA, harmonic oscillator model of aromaticity. Reprinted with permission from (20nJPCA(ii 5)8571). Copyright 2011 American Chemicai Society.

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