EXPLORATION OF THE POTENTIAL-ENERGY SURFACE OF C9H9+ BY AB-INITIO METHODS .1. THE BARBARALYL CATION

The potential energy surface (PES) of C9H9+ has been explored in the region of the 9-barbaralyl cation (3) at MP2/-, MP3/-, and MP4(SDQ)/6-31G(d). Calculations show that 3 is 6.9 and 4.6 kcal/mol more stable than the nonclassical barbaralyl cation with D3h symmetry (5) and the bicyclo [3.2.2] nona-3,6,8-trien-2-yl cation (4), respectively. The PES in the vicinity of 3 is rather flat and characterized by a spider network of reaction paths that connect 181 440 different forms of 3 with 90 720 different forms of 4 and 30 240 forms of 5. Both 4 and 5 will not be accessible to experiment, since they sit either at a transition state (TS) or in a very shallow minimum surrounded by TSs of similar energy. This is in line with the McIver-Stanton symmetry rules for TSs. The energetically most favorable reactions of 3 are sixfold degenerate rearrangements via C2 symmetrical TSs 8, representing an activation energy of just 3.6 kcal/mol. Completely degenerate rearrangements of 3, that lead to an equilibration of all C atoms of 3, proceed via a novel type of mechanism that is characterized by double-bifurcation reactions with three directly connected first-order TSs, namely TS 8, TS 4 as the barrier-determining TS, and another TS 8. According to calculated geometrical parameters, IGLO/6-31G(d) C-13 chemical shifts, and MP2/6-31G(d) response densities, the chemical behavior of 3 is dominated by the stereocomposition of a cyclopropylcarbinyl cation unit in conjugation with two vinyl groups. Four of the nine formal single bonds of 3 are partial bonds with bond orders of just 0.8 and 0.5.