Unconventional ionic hydrogen bonds: CH+⋯π (CC) binding energies and structures of benzene+(acetylene)1-4 clusters

► Rapid condensation of acetylene onto the benzene cation in the gas phase at 120-140 K. ► Weak binding energies of the four acetylene molecules to the benzene cation (3-4 kcal/mol). ► Associative charge transfer activates the cyclization of three acetylene molecules to form a benzene molecule.