Synthesis of salts of the hydrogen dichloride anion in aromatic solvents. 2. Syntheses and crystal structures of [K.cntdot.18-crown-6][Cl-H-Cl], [Mg.cntdot.18-crown-6][Cl-H-Cl]2, [H3O.cntdot.18-crown-6][Cl-H-Cl], and the related [H3O.cntdot.18-crown-6][Br-H-Br] - Simon Bott (Inorganic Chemistry, 1990)