Archive for March 22nd, 2022

A four-atom molecule exhibiting simultaneous compliance with Hückel 4n+2 and Baird 4n selection rules for ring aromaticity.

Tuesday, March 22nd, 2022

Normally, aromaticity is qualitatively assessed using an electron counting rule for cyclic conjugated rings. The best known is the Hückel 4n+2 rule (n=0,1, etc) for inferring diatropic aromatic ring currents in singlet-state π-conjugated cyclic molecules and a counter 4n rule which infers an antiaromatic paratropic ring current for the system. Some complex rings can sustain both types of ring currents in concentric rings or regions within the molecule, i.e. both diatropic and paratropic regions. Open shell (triplet state) molecules have their own rule; this time the molecule has a diatropic ring current if it follows a 4n rule, often called Baird’s rule. But has a molecule which simultaneously follows both Hückel’s AND Baird’s rule ever been suggested? Well, here is one, as indeed I promised in the previous post.

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