Exploration of the potential energy surface of C9H9+ by ab initio methods. 1. The barbaralyl cation

The potential energy surface (PES) of C 9 H 9 + 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 D 3h 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