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

Abstract

Density functional theory computations were used to model the formation and rearrangement of the barbaralyl cation (C₉H⁺₉). Two highly delocalized minima were located for C₉H⁺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 C₂ 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 C₉H⁺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.