Interaction of strong acids with a conjugated imine

Abstract

The state of protonation of a Schiff base, trans-trans-heptadienylidene tert-butylamine (HBA) was evaluated by uv and 1H nmr spectroscopies. Using three strong acids, HCl, HBr, HI and by taking spectra at low temperatures, it was found that in a non-polar solvent like chloroform (or CDCl3), no proton exchange occurred between the iminium ion and the halide anion. In contrast, CD3OH induced a leveling effect and some proton transfer occurred between the iminium ion and the solvent for the HBr and HI salts. No proton exchange was present for the HBA:HCl salt. The interaction of trichloroacetic acid with HBA was also studied: full proton exchange occurred between the iminium ion and CD3OH. In CDCl3, proton transfer was also present between the iminium ion and the carboxylate anion. These facts are discussed in relation with the controversy surrounding the true state of protonation of rhodopsin.