NMR Spectroscopic Evidence for the Structure of Iminium Ion Pairs

The NMR spectroscopic investigation of solutions of the iminium ions Me 2 N + C(H)Cl ( 1 + ), Me 2 N + C(H)Ph ( 2a + ), and Et 2 N + CH 2 ( 3 + ) in aprotic solvents shows that the replacement of the weakly nucleophilic counterions SbCl 6 -, AlCl 4 -, BF 4 -, or CF 3 SO 3 - by halide ions (I -, Br -, Cl - ) causes a deshielding of the proton at the iminium carbon atom (Δδ max = +2.8 for 2a + in CD 2 Cl 2 ) while the chemical shifts of all other 1 H and 13 C nuclei remain almost unaffected. The solvent dependence of these effects, 14 N NMR investigations, and ab initio MO calculations suggest that iminium halides form CH···Y - hydrogen bonds (Y = Cl, Br, and I) and thus exist as contact ion pairs in acetonitrile, chloroform, dichloromethane, and dimethyl sulfoxide solutions. In liquid sulfur dioxide, specific anion−cation interactions cannot be observed due to the superior solvation of the anions by SO 2 .