Statistical analyses of quantitative definitions of aromaticity, ASE (aromatic stabilization energies), RE (resonance energies), Lambda (magnetic susceptibility exaltation), NICS, HOMA, I5, and A(J), evaluated for a set of 75 five-membered pi-electron systems: aza and phospha derivatives of furan, thiophene, pyrrole, and phosphole (aromatic systems), and a set of 30 ring-monosubstituted compounds (aromatic, nonaromatic, and antiaromatic systems) revealed statistically significant correlations among the various aromaticity criteria, provided the whole set of compounds is involved. Hence, broadly considered, the various manifestations of aromaticity are related and aromaticity can be regarded statistically as a one-dimensional phenomenon. In contrast, when comparisons are restricted to some regions or groups of compounds, e.g., aromatic compounds with ASE > 5 kcal/mol or polyhetero-five-membered rings, the quality of the correlations can deteriorate or even vanish. In practical applications, energetic, geometric, and magnetic desriptors of aromaticity do not speak with the same voice. Thus, in this sense, the phenomenon of aromaticity is regarded as being statistically multidimensional.

The molecular rearrangement of 5-methyl-4-nitrobenzofuroxan to 7-methyl-4-nitrobenzofuroxan was studied by means of ab initio and density functional theory. Experimentally obtained IR spectra and X-ray data support the applicability of the theoretical methods and allow for a complete assignment of the vibrational modes. The influence of the methyl substituent on the underlying tautomeric reaction was investigated in detail. Trends for the reactivity of 4-nitrobenzofuroxans with substituents in the 5-position were established on the basis of an energy partitioning, providing insight into the driving forces of the Boulton−Katritzky rearrangement. Rate constants were calculated for this reaction using different implementations of variational transition-state theory. In addition a reaction yielding 6-methyl-7-nitrobenzofuroxan was investigated as an alternative to the rearrangement considered. An accurate treatment of relative energies of this competing reaction requires very demanding computational methods which could not be applied here. Therefore, energy corrections were estimated from smaller benchmark systems.