Resonance energy aromaticity estimation

Pyrrole has a planar, pentagonal (C2 ) stmcture and is aromatic in that it has a sextet of electrons. It is isoelectronic with the cyclopentadienyl anion. The TT-electrons are delocalized throughout the ring system, thus pyrrole is best characterized as a resonance hybrid, with contributing stmctures (1 5). These stmctures explain its lack of basicity (which is less than that of pyridine), its unexpectedly high acidity, and its pronounced aromatic character. The resonance energy which has been estimated at about 100 kj/mol (23.9 kcal/mol) is intermediate between that of furan and thiophene, or about two-thirds that of benzene (5). 

In summary, all estimates of resonance energies indicate a decrease in aromaticity in the sequence benzene > thiophene > pyrrole > furan. Similar sequences are also found for the benzo[6] and dibenzo analogues. A somewhat different sequence is found for the benzo[c] fused heterocycles with isoindole > benzo[c]thiophene > benzo[c]furan. As would be anticipated, the resonance energies for the benzo[c] heterocycles are substantially lower than those for their benzo[6] isomers. 

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... 

In line with Beak s finding, pyridin-2-one was estimated to be 31 kJ mol-1 less aromatic than the pyridine, and a similar figure of 25 was estimated for pyridine-2-thione. Subsequent results (73JCS(P2)1080, 76AHC(S1)71) on the pyridin-4-one, quinolin-2-one and isoquinolin-1-one series showed that aromatic resonance energy difference for the pyridin-4-one/4-hydroxypyridine system was very similar to that for the 2-substituted compounds, in contrast to Beak s findings. 

Quantum-chemical calculations for pyrylium including one, two, or three water molecules using DFT and 6-31 + G(d,p) basis set revealed that the aromaticity (estimated by harmonic oscillator stabilization energy, HOSE natural resonance theory, NRT harmonic oscillator model of aromaticity, HOMA and nucleus-independent chemical shifts, NICS) is not influenced by water molecules [82],... 

The dearomatization of benzene requires that a resonance energy estimated at 36 kcal mol-1 be overcome [14]. For this reason, when benzene attacks an electrophile, the end result is typically a substitution product rather than an addition product. The initial cyclohexadienyl cation is unstable to deprotonation by even extremely poor bases, and it therefore loses a proton and returns to an aromatic state. 

Oxadiazole is aromatic, having an estimated resonance energy of 167.4 kJ mol-1. The ring is stable to heat, a property which has been exploited in the production of heat-resistant poly-1,3,4-oxadiazoles (Section 4.23.5.2). 

---