The C9H9+ potential energy surface studied by MINDO/3, MNDO, and ab initio-SCF(STO-3G) calculations

Abstract

Controversies about the C₉H₉⁺ cations (1)–(5) and their reaction mechanisms have been resolved. The ions studied are barbaral-9-yl cation (1), bicyclo[3.2.2]nona-3,6,8-trienyl cation (2), a related ion having D_3h symmetry (3) and bicyclo[4.3.0]nonatrienyl cations with (5) and without (4) 1,4-bishomointeractions. All these ions are found to be represented by potential energy minima and the following energy ordering is obtained (STO-3G)(3) > (2) > (1) > (4) > (5). Since the controversial ion (3) is found to have too high an energy it is suggested not to participate significantly in the partial and totally degenerate rearrangements of (1). Most likely the totally degenerate rearrangement of (1) proceeds via ion (2). The computational results are in agreement with recent experimental results.