{{Short description|Term used in organic chemistry}} In organic chemistry, '''spherical aromaticity''' is formally used to describe an unusually stable nature of some spherical compounds such as fullerenes and polyhedral boranes.
In 2000, Andreas Hirsch and coworkers in Erlangen, Germany, formulated a rule to determine when a spherical compound would be aromatic. They found that those with 2(''n''+1)<sup>2</sup> π-electrons could display aromatic properties, as spherical molecular orbitals are filled when there are 2(''n''+1)<sup>2</sup> π-electrons for some positive integer ''n''. For example, in buckminsterfullerene (C<sub>60</sub>) this happens for the species C<sub>60</sub><sup>10+</sup>, which has 50 π-electrons: 50/2 = 25, which is a perfect square.<ref>{{citation | title = Spherical Aromaticity in ''I''<sub>h</sub> Symmetrical Fullerenes: The 2(''N''+1)<sup>2</sup> Rule | pages = 3915–17 | first1 = Andreas | last1 = Hirsch | first2 = Zhongfang | last2 = Chen | first3 = Haijun | last3 = Jiao | journal = Angew. Chem. Int. Ed. Engl. | year = 2000 | volume = 39 | issue = 21 | doi = 10.1002/1521-3773(20001103)39:21<3915::AID-ANIE3915>3.0.CO;2-O| bibcode = 2000AngCh..39.3915H }}.</ref>
In 2011, Jordi Poater and Miquel Solà expanded Hirsch's rule to open-shell spherical compounds, which have unfilled outer shells but are still aromatic. They found that spherical compounds with 2''n''<sup>2</sup>+2''n''+1 π-electrons with spin S = (n + 1/2) would also display aromatic properties, sometimes more aromatic than comparable closed-shell species. This corresponds to the outer shell being half-filled,<ref>{{citation | title = Open-shell spherical aromaticity: the 2N<sup>2</sup> + 2N + 1 (with S = N + ½) rule | pages = 11647–11649 | first1 = Jordi | last1 = Poater | first2 = Miquel | last2 = Solà | journal = Chemical Communications | year = 2011 | volume = 47 | issue = 42 | doi = 10.1039/C1CC14958J| pmid = 21952479 }}.</ref> and is similar to Baird's rule. For example buckminsterfullerene with one additional electron, (C<sub>60</sub><sup>1–</sup>) is aromatic, with S = 11/2 and a bond-length alternation of 0.2 pm.
== See also == *Fullerene chemistry *Carborane *Boranes
== References == <references />
== External links == * [http://pubs.acs.org/doi/abs/10.1021/cr0300892?journalCode=chreay Spherical Aromaticity: Recent Work on Fullerenes, Polyhedral Boranes, and Related Structures]
{{Chemical bonds}}
Category:Aromatic compounds Category:Physical organic chemistry Category:Supramolecular chemistry