Revisiting a classic carbocation - DFT, coupled-cluster, and ab initio molecular dynamics computations on barbaralyl cation formation and rearrangements.

Density functional theory computations were used to model the formation and rearrangement of the barbaralyl cation (C 9 H + 9 ). Two highly delocalized minima were located for C 9 H + 9 , one of C s symmetry and the other of D 3h symmetry, with the former having lower energy. Quantum chemistry-based NMR predictions affirm that the lower energy structure is the best match with experimental spectra. Partial scrambling was found to proceed through a C 2 symmetric transition structure associated with a barrier of only 2.3 kcal mol -1 . The full scrambling was found to involve a C 2v symmetric transition structure associated with a 5.0 kcal mol -1 barrier. Ab initio molecular dynamics simulations initiated from the D 3h C 9 H + 9 structure revealed its connection to six minima, due to the six-fold symmetry of the potential energy surface. The effects of tunneling and boron substitution on this complex reaction network were also examined.