Other examples of aromatic molecules:

Cyclopropenyl cation:

(i.e. C₃H₃⁺)

Only 2 p electrons; both can go in the lowest energy MO (orbital with energy a + 2b).

Total p electron energy

= 2´(a +2b)

= 2a +4b

Delocalization energy of cyclopropenyl radical

= (2a+4b) – (energy of 1 double bond, (2 p electrons))

= (2a+4b) –(2a + 2b)

= 2b (same as benzene)

· So C₃H₃⁺ is predicted to be stable.

· It is aromatic with 4n+2 (n = 0) p electrons.

Experimentally:

· Cyclopropenyl cation is found to be relatively stable.

· It can be synthesized as shown above, and isolated.

· The triphenyl derivative has been crystallized and its structure determined by X-ray crystallography. C-C distances all equal, 1.40Å, similar to benzene (1.39Å).

Cyclopentadienyl anion