INFLUENCE OF THE ANION AND SOLVENT ON THE ION-PAIR STABILITY FOR (CH3)(2)CHCH=NH-DOT-CENTER-DOT-CENTER-DOT-X(-) IMINIUM SALTS (X(-)=CL- OR BF4-) - VIBRATIONAL STUDY(CH(CH3)(2)CENTER)

Protonation of the (CH3)(2)(CHCH)-H-2'-H-1'=(NCH)-H-2(CH3)(2) non-conj ugated aldimine by two different proton donors HCl and HBF4 is extensi vely studied by vibrational spectroscopy. The assignments are consiste nt with a parallel analysis of deuterated derivatives at N, C-1' and C -2' sites. There is evidence for an iminium/enaminium equilibrium. Pro tonation influences the conformation on the C-1' and N sides of the al diminium depending on the anion and the physical state of the salts an d, for solutions, on the polarity of the solvents. Protonation apparen tly yields an electronic effect on the C=N bond increasing the double bond character more for BF4- than for the Cl- salts, but in general le ss for aldiminium than for cetiminium species. These phenomena are rel ated to the strength of the hydrogen bond in which the NH+ function is involved. Depending on the anion electronegativity, organic polar fun ctions may form alternative hydrogen bonds with the aldiminium or on t he contrary strengthen the stability of the ion pair. Such programmed discrete changes on the iminium local environment may influence the st ructure and the stability of the protonated Schiff base in the dynamic al bionatural cycles involving the rhodopsin and bacteriorhodopsin pho topigments.