Miquel Solà and Jordi Poater working University of Girona in Spain have put forward an extension of Hirsch’s rule to open-shell spherical species.

The famous Hückel rule allows one to estimate whether or not a planar ring molecule would have aromatic properties. When the molecule has 4N + 2 π-electrons then it follows Hückel’s rule. In 2000 Andreas Hirsch found a rule to predict the aromaticity of fullerenes, known as the 2(N +1)2 rule and now Solà has extended this rule to spherical systems with an open-shell.

They found that spherical compounds with a half filled last energy level, e.g. those with 2N2 + 2N + 1 electrons, are aromatic. This was backed up with computational evidence and showed that for example C 60 19+ and C 60 1- both have strong aromatic character.

Solà speculates that this finding could be an important tool for those working in the in stable high spin molecules such as molecular magnets.

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