Ab initio calculations on the isomerization of the 1,3-hexadien-5-yne radical cation to the benzene structure and to nonclassical ion structures.

Ab initio calculations at the MRCI//ROHF/6-31G** level show that the barrier for isomerization of the 1,3-hexadien-5-yne radical cation to nonclassical structures with a five-membered ring and an exocyclic CH bond is about 15 kcal mol(-1). The barrier for a subsequent isomerization to the benzene structure is calculated to be slightly higher.