Is azulene's local aromaticity and relative stability driven by the Glidewell–Lloyd rule?

Abstract

The local aromaticity of azulene has been studied to understand their electronic properties. For this purpose, we have used the magnetic criterion through magnetically induced current density maps, ring current strengths, NICS_zz(1), and the bifurcation value of three-dimensional surfaces of NICS_zz. On the other hand, the delocalization criterion was used by calculating the MCI and ELF_π. The results show that the five-membered ring (5-MR) is more aromatic than the seven-membered ring (7-MR) and more aromatic than the free C₅H₅⁻ ring. The opposite case is seen for the seven-membered ring, which is less aromatic than the free C₇H₇⁺. The local aromatic rings in azulene are formed due to an intramolecular electron transfer from the 7-MR to the 5-MR. In addition, the proposed resonance structures that allow explaining the properties of azulene, such as the dipole moment or the relative stability (in comparison to other isomers), show a preference for the formation of 5-MRs; for this reason, it is possible to conclude that the aromaticity and relative stability of azulene is driven by the Glidewell–Lloyd rule.