Computational Study of Isopropylbenzenium Ions

A theoretical investigation of the isopropylbenzenium
ion system
has been carried out with structures determined with B3LYP. Energies
are calculated with the high accuracy composite methods, G3 (G3B3)
and CBS (CBS-QB3). The main goal has been to resolve the following
issue: Are there any stable ion π-electron complexes of the
type C6H6/C3H7+ or C6H7+/C3H6 in this system? Two minimum points on the potential energy surface
(PES) corresponding to benzenium ion/propene complexes were found.
Due to free internal rotation, they represent only one species. The
barrier for forming an isopropylbenzenium ion from the complex is
low, so the lifetime will be short. Computation of the IR spectrum
of the complex shows that there is a very intense absorption line
due to C–H stretching, in an otherwise empty region, that may
be used to identify the complex, if present. No stable C6H6/C3H7+ complex was
found, but a quasi-stable species structurally corresponding to the
earlier described stable C6H6/C4H9+ complex was observed. A simplistic explanation
why the benzene/isopropyl cation, in contrast to benzene/tert-butyl cation, does not form a stable ion/π-electron complex
is given.