An unusually small (doubly) aromatic molecule: C4.
When you talk π-aromaticity, benzene is the first molecule that springs to mind. But there are smaller molecules that can carry this property; cyclopropenylidene (five atoms) is the smallest in terms of atom count I could think of until now, apart that is from H3+ which is the smallest possible molecule that carries σ-aromaticity. So here I have found what I think is an even smaller aromatic molecule containing only four carbon atoms. And it is not only π-aromatic but σ-aromatic.
Let me go through the analysis (using a CCSD(T)/Def2-TZVPPD calculation, DOI: 10.14469/hpc/10226).
Four carbons contain 16 valence electrons for bonding.
Eight of these are conventional, forming four C-C single bonds around the 4-ring.
Eight are left over, and these partition into a set of six and a set of two.
The set of two are in p-π atomic orbitals and form a 4n+2 (n=0) aromatic system
The set of six are in σ-sp AOs and form a 4n+2 (n=1) aromatic system.
The three σ-MOs all contribute to the central C-C bond, particularly σ3 and σ2 in different ways.
σ2 also reminds of [1.1.1]-propellane, where the two σ-electrons are in effect external to the central C-C bond, but spin coupled to form what might be called a σ exo-bond. There is also similarity to the exo bond in C2.
The dissociation energy of the central bond can be estimated at 28 kcal/mol from the triplet state energy.
Bonding MOs for C4.Click image to load 3D model
π1
σ3
σ2
σ1
So this little molecule carries a lot of diversity in its chemical bonding; an ideal candidate perhaps for a tutorial in bonding theory of organic molecules?
The post has DOI: 10.14469/hpc/10252
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