Henry Rzepa's Blog

Cyclopropenylidene must be the smallest molecule to be aromatic due to π-electrons, with just three carbon atoms and two hydrogen atoms. It has now been detected in the atmosphere of Titan, one of Saturn’s moons[1] and joins benzene, another aromatic molecule together with the protonated version of cyclopropenylidene, C₃H₃⁺ also found there.

The molecule has two π-electrons in the three membered ring and a carbene lone pair in the σ-framework. As with the cyclopropenium cation (C₃H₃⁺), these two electrons make it π-aromatic, as indicated by Hückel’s 4n+2 rule (n=0). I thought it might be fun to show the molecular orbitals containing these two pairs of electrons and then to show the result of a double excitation of the carbene lone pair into the π-system to make a anti-aromatic isomer with four π-electrons. This species is a whopping 209.3 kcal/mol higher in free energy, made up of the double electronic excitation energy topped up by conversion of the stabilizing aromaticity into destabilizing anti-aromaticity. Because of this antiaromaticity, the excited state is in fact a second order saddle point, avoiding anti-aromaticity by asymmetric distortion back down to the ground state and resymmetrisation.

Ground state of Cyclopropenylidene
Click to view 3D model of NBO 10	Click to view 3D model of NBO 8
Doubly excited anti-aromatic state of Cyclopropenylidene
Click to view 3D model of NBO 10	Click to view 3D model of NBO 8

It might be a tiny molecule, but to chemists at least it is very interesting in a historical sense at least. Curiously, the astrophysicists describe it as a “complex molecule”!