EXPLORATION OF THE POTENTIAL-ENERGY SURFACE OF C9H9+ BY AB-INITIO METHODS .2. IS THE 1,4-BISHOMOTROPYLIUM CATION A BISHOMOAROMATIC PROTOTYPE

The potential energy surface (PES) of C9H9+ has been investigated in the region of the 1,4-bishomotropylium cation (1) and its classical analogue, the cis-8,9-dihydro-1-indenyl cation (2) using second-, third-, and fourth-order Moller-Plesset perturbation theory (MP2, MP3, MP4(SDQ)) in connection with a 6-31G(d) basis set. Calculations show that 1 rather than 2 corresponds to a minimum-energy structure, contrary to recent Hartree-Fock results. Ion 1 is clearly bishomoaromatic with interaction distances of 2.1 angstrom. Its homoaromatic character is reflected by bond and charge equalization in its seven-membered ring, by a similar equalization of calculated IGLO/6-31G(d) C-13 chemical shifts, and by MP2/6-31G(d) CC bond orders typical of aromatic delocalization of 6pi electrons. Since the PES is rather flat in the vicinity of 1 and since the homoaromatic stabilization energy is (with ca. 3 kcal/mol) rather small, perturbations due to substituents, counterions, or media may easily weaken homoaromaticity in 1. A detection of the homoaromatic character of 1 and its derivatives is best carried out by C-13 NMR measurements while energy-related measurements are probably not sensitive enough to confirm homoaromaticity for 1.