X-ray Crystal Structures of a Benzonorbornenyl Cation and of a Protonated Benzonorbornenol^†

The crystal structure of the 9-methylbenzonorbornenyl cation Me-1⁺ shows a relatively strong interaction between the sp²-hybridized carbon atom C9 and the aromatic ring (C4a−C9 ≡ C8a−C9 = 1.897(10) Å). The anion Sb₂F₁₁^- is refined as rotationally disordered along the Sb···Sb axis. In sharp contrast to the findings about Me-1⁺, the protonated anti-benzonorbornenol 5⁺ is essentially an oxonium ion with only weak interaction between the C9 bridge and the aromatic ring despite the fact that it is already a positively charged ion, which upon loss of a water molecule is expected to give the parent cation H-1⁺. The hydrogen atoms on the oxonium O atom are involved in strong hydrogen bonds to chlorosulfonate anions and probably partially disordered despite the large estimated pK_a differences between the corresponding acid−base pairs. The experimentally determined cation structures are compared with structures computed by DFT methods. Detailed experimental procedures are given.