Novel main group-element cyclic molecules and their radical ions: Are they aromatic?

Abstract

More than half a century ago, Erich Huckel in his classical publications forwarded the 4n + 2 rule for closed shell cyclic π systems. In the decades to follow, this intrigueing concept stimulated chemists to synthesize a manifold of then rather breath-taking molecules and molecular ions, slang-termed “aromatic” (greek: aromatikos = spicy smelling). Since then chemistry has progressed tremendously, especially by adopting a great variety of ingenious measurement methods from the physical armoury as well as by using advanced computer technology to perform quantum chemical calculations in ever higher precision for ever larger molecules. Today, highly reactive intermediates can be generated under partly extreme conditions and characterized by their spectroscopic molecular state ’fingerprints’. Concomittantly, simple chemical bonding models are more and more replaced by correlations closer to the reality of molecular states, i.e. the changes of structure with energy and charge. Molecular dynamics become important, especially to gain some insight into the largely unknown microscopic reaction pathways of medium-sized molecules. Examples predominantly from own research efforts are presented for illustration. They comprise e.g. the gasphase generation of silabenzene, tetrahedrane ⇔ cyclobutadiene conversions, perturbations of organic π systems by powerful main group element donor substituents as well as generation and properties of radical cations and anions with special emphasis on ion pairs and their isola-tion. Several total energy hypersurface calculations are presented to rationalize the experimental observations. © 1990, IUPAC.