NUCLEAR-MAGNETIC-RESONANCE STUDY OF COMPLEXES FORMED BETWEEN CONJUGATED SCHIFF-BASES AND CARBOXYLIC-ACIDS

The NMR spectra of the proton located in a N+-H-O-- bond formed in the reaction of a conjugated Schiff base (SB) with a carboxylic acid (HA) in a non-polar solvent indicate that this proton is rapidly exchangin g with other protons present in the system. For this study, two SB wer e used: all-trans-retinylidene tert-butylamine (RBA) and trans, trans- 2,4-heptadienylidene tert-butylamine (HBA), and five carboxylic acids having pK(a)'s between 0.66 and 4.87. (1)HNMR spectra were taken at 29 3 and 233 K in CDCl3 of systems of SB:HA using different ratios of aci d to SB, Results indicate that the rapid proton exchanges are temperat ure and concentration dependent. Finally, the results are related to t he natural pigments rhodopsin and bacteriorhodopsin.