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

The potential energy surface (PES) of C9H9+ has been explored in the r egion of the 9-barbaralyl cation (3) at MP2/-, MP3/-, and MP4(SDQ)/6-3 1G(d). Calculations show that 3 is 6.9 and 4.6 kcal/mol more stable th an the nonclassical barbaralyl cation with D3h symmetry (5) and the bi cyclo [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 netw ork of reaction paths that connect 181 440 different forms of 3 with 9 0 720 different forms of 4 and 30 240 forms of 5. Both 4 and 5 will no t be accessible to experiment, since they sit either at a transition s tate (TS) or in a very shallow minimum surrounded by TSs of similar en ergy. 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 e nergy 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 nove l type of mechanism that is characterized by double-bifurcation reacti ons with three directly connected first-order TSs, namely TS 8, TS 4 a s the barrier-determining TS, and another TS 8. According to calculate d geometrical parameters, IGLO/6-31G(d) C-13 chemical shifts, and MP2/ 6-31G(d) response densities, the chemical behavior of 3 is dominated b y the stereocomposition of a cyclopropylcarbinyl cation unit in conjug ation 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.