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 sp(2)-hybridized carbon atom C9 and the aromatic ring (C4a-C9 identical with C8a-C9 = 1.897(10) A). The anion Sb(2)F(11)(-) 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.