On the Intrinsic Difference between Topological Aromaticity and So-Called Quasi-Aromaticity

In an attempt to understand the intrinsic difference between topological aromaticity and so-called quasiaromaticity, the authors examined the potential energy and the kinetic energy of electrons in 6-π-acyclic polymethine-like molecules (5–7), benzene (8), and hexatriene (9) as a reference structure. Protonation interrupts π conjugation; the energetic difference from the non-protonated structure depends on the conjugative stabilization energy. Such a difference from the reference system corresponds to extra stabilization. Analysis of this difference showed that benzene increases the kinetic energy by interrupting its conjugation more than hexatriene while polymethine-like molecules increase the potential energy by doing so. This indicates that extra stabilizaion in benzene is produced by lowering the kinetic energy of electrons. In marked contrast with this that in polymethine-like molecules is due to lowering the potential energy. Analogous results were obtained in the ten-π-electron systems (10–13). The authors discussed the inappropriateness of the term “quasi-aromatic” for polymethinic resonance stabilization.